Vesicles derived from the outer membrane of Gram-negative bacteria, or outer-membrane vesicles (OMVs), are heterogeneous in size and composition, encapsulate soluble periplasmic content and are ubiquitously produced. The difficulty in finding a single molecular or genetic basis for OMV production is probably due to species-dependent differences in envelope architecture, environmental influences on envelope composition and redundancy of OMV-producing pathways.
Mutations that subtly affect envelope crosslinking affect OMV production, whereas bacterial mutants that are unable to crosslink the envelope are typically unstable and form lysis products instead of OMVs. Lipopolysaccharide (LPS) subtypes also affect the levels of OMV production, as well as OMV cargo recruitment.
OMV cargo may be enriched or excluded compared with its abundance in the bacterial envelope, suggesting that cargo recruitment is a regulated rather than stochastic process. Well-characterized cargoes include virulence factors, antibiotic-degrading enzymes, surface adherence factors, proteases and enzymes that are important for nutrient acquisition.
OMVs can serve in bacterial communities as 'public goods' by distributing enzymes that break down extracellular material into nutrients, by recruiting iron, by acting as decoys for bacteriophages or antibiotics and by transferring DNA between cells.
The versatile characteristics of OMVs and their immunomodulatory properties can be exploited for bioengineering applications and vaccine development.
Outer-membrane vesicles (OMVs) are spherical buds of the outer membrane filled with periplasmic content and are commonly produced by Gram-negative bacteria. The production of OMVs allows bacteria to interact with their environment, and OMVs have been found to mediate diverse functions, including promoting pathogenesis, enabling bacterial survival during stress conditions and regulating microbial interactions within bacterial communities. Additionally, because of this functional versatility, researchers have begun to explore OMVs as a platform for bioengineering applications. In this Review, we discuss recent advances in the study of OMVs, focusing on new insights into the mechanisms of biogenesis and the functions of these vesicles.
In all domains of life — Eukarya, Archaea and Bacteria — cells produce and release membrane-bound material, often termed membrane vesicles, microvesicles, exosomes, tolerasomes, agrosomes and virus-like particles. Outer-membrane vesicles (OMVs), which are derived from the cell envelope of Gram-negative bacteria, have been observed and studied for decades. All types of Gram-negative bacteria have been seen to produce OMVs1,2,3 in a variety of environments, including planktonic cultures, fresh and salt water, biofilms, inside eukaryotic cells and within mammalian hosts4,5,6,7,8,9. Over the years, the study of OMVs has generally focused on the function of these vesicles, particularly as it relates to bacterial pathogenesis. Only recently have genetic and biochemical analyses led researchers to begin to elucidate mechanistic aspects of OMV production, as well as to appreciate aspects of OMV production by non-pathogenic bacteria. Notably, OMVs from non-pathogenic bacteria mediate functions similar to those mediated by other extracellular vesicles, such as cellular communication, surface modifications and the elimination of undesired components10. Furthermore, it is becoming clear that multiple mechanisms can lead to the production of OMVs in bacteria.
As OMVs are derived from the cell envelope of Gram-negative bacteria, it is important to consider the unique architecture of this bacterial component in order to understand the mechanisms that are involved in OMV budding and detachment (Fig. 1). The Gram-negative envelope consists of two membranes, the outer membrane and the cytoplasmic membrane, and the periplasmic space in between, which contains a layer of peptidoglycan (PG)11. The outer membrane is a fairly unusual outermost cell barrier, being composed of an interior leaflet of phospholipids and an exterior leaflet of lipopolysaccharide (LPS; also known as endotoxin). The cytoplasmic membrane consists of a typical phospholipid bilayer that serves as an electrochemical barrier11. The periplasm is an oxidative environment that promotes protein folding but does not contain nucleotide sources of energy, such as ATP or GTP12. The net-like PG layer within the periplasm gives bacteria their shape and imparts protection from osmotic changes and sheer stress. Envelope proteins are either soluble (periplasmic proteins), membrane-associated, integral or anchored into the leaflet of either membrane via covalently attached lipid appendages (lipoproteins) (Fig. 1).
For most Gram-negative bacteria, envelope stability comes from different envelope crosslinks: the covalent crosslinking of Braun's lipoprotein (Lpp) in the outer membrane with the PG sacculus13,14,15; the non-covalent interactions between the PG and outer-membrane protein A (OmpA), which is an outer-membrane porin; and the non-covalent interactions between the PG and the Tol–Pal (peptidoglycan-associated lipoprotein) complex, which spans the envelope from the outer membrane across the periplasm to the cytoplasmic membrane16,17. As OMVs are spherical portions of the outer membrane, ∼20–250 nm in diameter, that contain periplasmic luminal components and that bud and detach from the cell during active growth, and not as a by-product of cell lysis18, OMV biogenesis presumably must rely on the dissociation of the outer membrane from the underlying PG in areas devoid or depleted of attachments, followed by fission without compromising envelope integrity (see below).
Although initial publications that demonstrated the presence of vesicular material or 'blebs' were actually reporting on lysed bacterial debris, several subsequent analyses revealed that particular OMV contents (such as lipids or proteins) can be enriched or depleted as compared with their prevalence in the bacterial envelope, and that OMV production occurs in the absence of lysis or cell leakiness. The selectivity of OMV cargo revealed that OMV biogenesis is a deliberate process, rather than a stochastic event. Furthermore, vesiculation levels can be altered by factors such as temperature, nutrient availability, oxidation, quorum sensing and envelope-targeting antibiotics19,20,21,22,23,24. In addition, subpopulations of OMVs with distinct compositions may exist within a bacterial culture, although OMV population heterogeneity is a facet of the field that has yet to be explored in much depth. Importantly, although the field has suffered from scepticism, often as a result of unreliable terminology — initially, vesicles that were the result of bacterial lysis were not differentiated from intact OMVs — careful experimentation on OMV production and cargo, and on the mechanisms regulating these processes, now supports the concept that OMV production is a bona fide bacterial secretion process.
Functionally, OMVs have been determined to have diverse roles, depending on the OMV-producing species and the culture conditions. This is unsurprising, as bacterial gene expression, and consequently envelope composition, is highly variable between species and is influenced by the bacterial environment. Owing to the diversity of functions mediated by OMVs, it has become important to focus on the question of why bacteria produce OMVs. In general, it appears that bacteria can utilize OMVs to improve their chances for survival and to induce changes within their environment2,19,25,26,27. For example, OMVs can deliver virulence factors and modulate the host immune system during pathogenesis; they can aid in nutrient acquisition and ecological niche protection; and they can help provide structural support in multispecies environments such as biofilms1,28,29.
In this Review, we discuss recent advances in the field regarding OMV biogenesis and cargo selection, as well as the functions of OMVs during nutrient and iron acquisition, interbacterial communication, stress relief and pathogenesis. It has become apparent that these multifaceted particles are challenging to analyse but that the outcome is worthwhile, contributing to our understanding of basic bacterial physiology as well as our ability to engineer vesicles to carry out desirable functions.
In principle, for an OMV to form, the outer membrane must be liberated from the underlying PG and bulge outwards until the budding vesicle membrane undergoes fission and detaches. Therefore, understanding how, when, where and why covalent crosslinks in the bacterial envelope change without causing membrane instability (thus preserving bacterial viability) is essential if we are to understand OMV biogenesis. In addition, biophysical properties of the outer-membrane lipids and their interaction with proteins or other molecules that influence membrane bending are likely to have fundamental roles in OMV biogenesis. Here, we discuss novel contributions to the current models for the mechanism of OMV biogenesis (reviewed in more detail in Ref. 2). Progress in this area has resulted from the careful evaluation of bacterial mutants that carry modifications or deletions in genes encoding envelope components. Strains with mutations in envelope components often have lytic phenotypes, which make comparisons with wild-type bacteria challenging; additionally, genetic complementation often alters the expression levels of these genes, which can affect the envelope by activating stress pathways that alter OMV levels (see below).
Modulation of envelope crosslinks. The crosslinks bridging the outer membrane to PG have been extensively studied in Escherichia coli, although little is known about the dynamics of their formation and destruction or the homogeneity and dynamics of their distribution. OmpA is an outer-membrane porin that contains a periplasmic binding site for diaminopimelic acid (DAP), a component of PG30. The Tol–Pal complex is a cell-division component that aids in invagination of the outer membrane and in membrane stability, and also interacts with PG16,31,32. Lpp is an extremely abundant outer-membrane lipoprotein, one-third of which is covalently crosslinked to PG33. Lpp is evenly distributed throughout the entire cell wall, whereas Pal is preferentially located at the cell poles31,32.
E. coli, Salmonella spp. and Acinetobacter baumannii mutants lacking OmpA display increased OMV production34,35,36,37, most probably as a consequence of decreased crosslinking between PG and the outer membrane30. Interestingly, although modulation of this type of envelope crosslink also seemed to lead to hypervesiculation in Pseudomonas aeruginosa mutants lacking OprF (an OmpA homologue38), in this case hypervesiculation is actually a consequence of an increase in the levels of Pseudomonas quinolone signal (PQS), which stimulates OMV production in P. aeruginosa39(see below).
To examine the role of the covalent and highly abundant Lpp–PG crosslinks in vesiculation, past studies have characterized the effects of null mutations that cause a complete lack of these crosslinks, but such null mutations can also cause problems with membrane integrity. Indeed, deletions and mutations eliminating either the Lpp membrane anchor or the covalent crosslink between PG and Lpp lead to membrane instability associated with cellular leakage16,35,37,40,41. During OMV formation, either a temporary decrease in overall crosslink abundance or a localized displacement of crosslinks is thought to occur1,18,38,42. It is worth noting that periplasmic enzymes that liberate Lpp from its covalent bond with PG have not yet been found.
New studies correlating vesiculation levels with more subtle changes in covalent envelope crosslinking have revealed distinct, and hopefully more physiological, envelope conditions that control OMV production. In some cases, the overall number of Lpp–PG crosslinks inversely correlates with OMV production. For instance, the amount of Lpp crosslinked to PG in the hypervesiculating nlpI mutant was approximately 40% lower than that in wild-type E. coli43. NlpI is an outer-membrane lipoprotein that participates in cell division44 and was recently discovered to control the activity of Spr (also known as MepS), a PG endopeptidase that cleaves peptide crosslinks in PG43,44,45. Therefore, it is postulated that an altered balance of PG breakdown and synthesis in the nlpI mutant prevents the formation of proper crosslinks between PG and Lpp and indirectly leads to increased OMV production. By contrast, the loss of the minor DAP–DAP peptide crosslinks increased the levels of PG–Lpp crosslinking and resulted in hypovesiculation46. Production of OMVs in Neisseria meningitidis also seems to be affected by PG architecture, as OMVs from this bacterium were found to contain lower levels of the three lytic transglycosylases MltA, MltB and Slt47. Together, these data support a model in which OMVs bud off at sites with locally decreased levels of crosslinks between the outer membrane and PG, and with locally reduced PG hydrolase activity (Fig. 2a,b). Therefore, wild-type cells may modulate OMV production by controlling the number of Lpp–PG crosslinks through the regulation of PG remodelling.
Although Lpp seems to play a crucial part in regulating OMV biogenesis, it was revealed that the overall amount of Lpp–PG crosslinks did not change in all cases of increased OMV production in E. coli. The data suggested the existence of a second route of OMV biogenesis, in which vesicle production is independent of the total level of PG-bound Lpp. This mechanism was discovered from investigating mutants that hypervesiculate as a consequence of a general stress response to misfolded proteins or owing to high concentrations of envelope proteins, PG fragments and/or aberrant LPS46. These findings led to a model in which these envelope components accumulate in nanoterritories that are relatively free of bound Lpp, although the overall level of Lpp–PG crosslinks throughout the envelope as a whole remains constant. Following accumulation of this cargo in these regions, the outer membrane could bulge outwards and bud off, effectively removing the undesirable envelope components from the cell (Fig. 2c).
Lipids and lipid-binding molecules. The biophysical characteristics of membrane lipids dictate membrane curvature and fluidity and thus probably have a key role in OMV biogenesis. However, defining the role of lipids in OMV production is complicated by the technical challenges involved in generating liposomes that simulate the asymmetrical leaflet composition of the outer membrane for in vitro studies. Furthermore, when the lipid content of the outer membrane is altered via genetic modifications, there is an indirect but considerable and often overlooked effect on the composition, organization and biophysical properties of the membrane and membrane-associated proteins.
Temperature-dependent differences in OMV production are likely to reflect the modulation of membrane lipid dynamics, but these effects are species specific. For example, in E. coli, as temperature increases, so does the amount of vesiculation19, presumably owing to increasing membrane fluidity. However, in P. aeruginosa, neither a 12 °C nor a 14 °C increase in temperature affected vesiculation levels20, whereas in the cold-adapted bacterium Shewanella livingstonensis, the soil bacterium Serratia marcescens and the pathogen Bartonella henselae, lower temperatures resulted in increased OMV production21,22,23. Biophysical analyses and assessments of protein composition will be necessary in order to conclude whether the observed temperature-dependent effects are protein mediated or lipid mediated.
The involvement of particular lipid species in OMV biogenesis has been proposed on the basis of enriched and excluded OMV components. For example, in the Antarctic bacterium Pseudomonas syringae, even-numbered carbon chain fatty acids were highly enriched in OMVs (making up more than 80% of the fatty acids present in the OMVs)48. Accordingly, it was hypothesized that increased membrane flexibility from areas of enrichment of these lipids may have a role in promoting OMV biogenesis. Furthermore, protein components of the LPS machinery were found very rarely in OMVs, suggesting that mature LPS, rather than nascent LPS, is preferentially shed in OMVs48 (Fig. 2d). Additionally, the presence of unsaturated and branched-chain fatty acids in the OMVs from P. syringae suggests that lipids which increase membrane fluidity are shed via OMV production49. By contrast, analyses of the fatty acid composition of OMVs from P. aeruginosa revealed that these OMVs were enriched in longer and more saturated fatty acids compared with the outer membrane, suggesting that the more rigid regions of the outer membrane are prone to forming OMVs50. These relationships between fatty acid composition, membrane fluidity and propensity to form OMVs seem contradictory; however, it should be noted that the differences in lipid enrichment in OMVs from these bacteria may reflect differences in the habitats of these species, and thus these hypotheses require further testing.
LPS subtypes can be enriched in OMVs and can directly or indirectly influence OMV composition and outer-membrane curvature. For instance, P. aeruginosa expresses two different forms of LPS: a neutral form and a charged polysaccharide form that is enriched in OMVs51. Alterations in LPS affect both the size and protein profile of P. aeruginosa OMVs; cells that express only neutral LPS produce smaller OMVs with a protein composition that is more distinct from wild type than the protein composition of OMVs from cells expressing only charged LPS52. P. aeruginosa lacking the ability to express either polysaccharide produces larger OMVs but, unexpectedly, these have a composition more similar to the wild type. Porphyromonas gingivalis also contains two forms of LPS, neutral and negatively charged anionic LPS (A-LPS). Gingipains, which are extracellular proteases and virulence factors of P. gingivalis, are processed and anchored by anionic polysaccharide53; thus, it was not surprising that a mutation that disrupts the synthesis of anionic polysaccharide and therefore eliminates A-LPS from P. gingivalis was reported to cause decreased levels of OMV-associated gingipains54. However, the lack of A-LPS also caused an increase in OMV incorporation of envelope proteins such as RagA and RagB, which are normally excluded from the OMVs of wild-type P. gingivalis by an as-yet-unidentified mechanism54,55. These findings suggest that microdomains of charged LPS can influence OMV size, and that certain proteins predominantly localize to these regions and thereby become enriched in OMVs, whereas proteins that associate with uncharged LPS are retained in the outer membrane (Fig. 2d,f).
The impact of changes in the curvature of the outer membrane in OMV biogenesis has been explored in detail with the OMV-promoting and LPS-binding molecule PQS, which is produced by P. aeruginosa. Exogenously added PQS promotes the generation of OMVs by P. aeruginosa as well as other Gram-negative organisms, and also perturbs the ultrastructure of pure preparations of LPS56,57,58; PQS incorporation into and/or fusion with LPS aggregates was shown to depend on the alkyl side chain and third-position hydroxyl of PQS58. MgCl2 represses E. coli OMV production in either the absence or the presence of PQS, suggesting that PQS enhances anionic repulsion on the surface of E. coli and that this leads to OMV production57. These data support a model of bilayer budding by outer-leaflet expansion, in which the concentration of an amphiphilic molecule in one membrane leaflet causes that leaflet to expand relative to the other, resulting in curvature of the whole membrane39 (Fig. 2e). Notably, however, PQS can act on membranes independently of its ability to bind LPS, as PQS is also incorporated into phospholipid liposomes made from zwitterionic phosphatidylethanolamine and negatively charged phosphatidylglycerol58, and it induces both membrane vesicle formation in Gram-positive organisms57 and membrane curvature of erythrocyte membranes39 (neither of which contain LPS). Indeed, magnesium chloride did not repress vesicle production from Bacillus subtilis, suggesting that PQS has a different mode of action on Gram-positive membranes57. Furthermore, as OMVs are still produced by P. aeruginosa grown in conditions that inhibit PQS synthesis and by P. aeruginosa mutants that cannot produce PQS20,24,27, an alternative pathway (or pathways) must allow OMV production not only by non-PQS-producing Gram-negative organisms, but also by wild-type P. aeruginosa. OMV populations from the same strain that are generated by different pathways would be expected to have distinct compositions, and future work to identify these differences could provide substantial insight into their distinct biogenesis mechanisms.
OMV protein cargo
Some of the most important, but reasonably rare, investigations in the field focus on how cargo is selected for enrichment in OMVs. These studies can provide critical mechanistic and functional insight into OMV biogenesis. However, our ability to draw meaningful conclusions from these data is complicated by the fact that OMV proteome reports are often inaccurate, typically owing to inexact purification methods (see Ref. 1 for a comprehensive review covering aspects of OMV purification). In most cases, in addition to outer-membrane components, OMV proteomes include inner-membrane and cytoplasmic proteins52,59,60,61,62,63; in fact, it is exceptional if no inner-membrane and cytoplasmic components are found64. Importantly, even in those cases in which the samples were rigorously prepared, purified and analysed, some inner-membrane and cytoplasmic components were still detected in OMVs65,66. It is still unclear how and why these components enter and/or associate with OMVs, although this is of particular interest, both functionally and mechanistically.
Virulence factors. The selective export of virulence factors into OMVs is thought to have evolved as a benefit for pathogenic bacterial species. Intriguingly, both positive and negative selection of virulence factors into OMVs has been reported (Fig. 2f). For example, in Helicobacter pylori, the type IV secretion system component VirD4 was completely excluded from OMVs67, suggesting that this selectivity benefits the parent cell, in which an intact secretion system is needed for virulence. Notably, these OMVs were enriched in the protease HtrA, which is critical for bacterial survival under conditions of misfolded-protein accumulation42,67; moreover, in its secreted form, HtrA plays a part in pathogenesis in enteropathogenic E. coli, Shigella flexneri and Campylobacter jejuni by mediating E-cadherin cleavage and thereby disrupting the epithelial barrier68. The selectivity of envelope protein cargo was also investigated in the human pathogen N. meningitidis47, and outer-membrane proteins that were enriched in OMVs included almost all the N. meningitidis autotransporters, regulatory proteins involved in iron and zinc acquisition, and two-partner secretion systems. By contrast, OMVs showed reduced levels of the outer-membrane porins PorA and PorB, the PG-binding protein RmpM, the efflux pump channel MtrE and the pilus pore protein PilQ. The functional consequences of this selectivity are still unknown.
It has been postulated that virulence factors involved in adhesion would be excluded from OMVs, as this could be important for pathogens that specifically benefit from direct contact with the host cells: if OMVs from these bacteria also carry the same adhesins, they would compete with the bacteria themselves for direct bacterium–host interactions. This notion is supported by the observation that in H. pylori, blood group antigen-binding adhesin (BabA) and sialic acid-binding adhesin (SabA), two adhesins that contribute to bacterial colonization, were found to be less abundant in OMVs than in the outer membrane67.
In a few cases, specific bacterial factors have been identified which contribute to the incorporation of virulence factors into OMVs or to the association of soluble virulence determinants with the external leaflet of OMVs. For example, in meningitis-causing E. coli, the targeting of cytotoxic necrotizing factor 1 (CNF1) beyond the periplasm and into OMVs somehow depends on YgfZ69,70,71,72. YgfZ is a folate-binding protein that is predominantly located in the periplasmic space and may associate with the inner-membrane iron sulphur protein ferredoxin. The ability of YgfZ to target CNF1 to OMVs may be indirect, as the two proteins could not be co-precipitated. As discussed above, LPS can also contribute to the selective recruitment of virulence factors to OMVs. For example, numerous P. gingivalis virulence factors that are enriched in OMVs, including gingipains, are anchored extracellularly to the outer leaflet of the outer membrane via interactions with A-LPS54,55,73.
Sorting of soluble proteins into OMVs. OMV cargo selectivity is also observed in non-pathogenic species, and identifying and understanding how these cargoes are enriched or excluded will shed light on the basic physiological mechanics and functions of OMVs. The relative abundance of macromolecules in OMVs compared with the bacterial envelope reflects their abundance at sites of OMV budding, the ability of bacteria to selectively export damaged or toxic molecules and/or the ability of bacteria to conserve resources by excluding from OMVs those macromolecules that are energetically costly or scarce and are needed in the OMV-producing cell.
As is the case for virulence factors, the sorting of soluble bacterial proteins into OMVs may result from interactions with the periplasmic face of particular outer-membrane proteins, outer-membrane-associated proteins or lipids (Fig. 2f). For example, a selective packaging signal for soluble cargo was identified in E. coli: when the carboxy-terminal sequence that triggers the envelope σE heat shock response was appended onto a soluble protein, this not only stimulated OMV production, but also led to an approximately tenfold enrichment of the chimeric soluble cargo in OMVs19. As discussed in more detail below, misfolded outer-membrane proteins and mislocalized LPS in the periplasm activate the σE heat shock response to manage the potential toxic biological consequences of these damaged envelope components74. An enrichment of misfolded native proteins in OMVs has also been observed; misfolded luminal outer-membrane proteins were enriched in OMVs produced by a strain lacking the envelope stress-inducible periplasmic chaperone–protease DegP42, although the molecular interactions promoting this enrichment remain unknown.
OMV functions in bacterial physiology
The contribution of OMVs to bacterial pathogenesis obviously remains a topic of great interest (see below); however, the benefit of OMV production for non-pathogenic bacteria has also recently begun to be appreciated. Despite the energetic cost that is required for the secretion of these large macromolecular complexes, under many circumstances OMV production seems to offer advantages for the producing bacteria. Here, we summarize studies of OMVs in bacterial responses to stress conditions, in regulating complex bacterial communities, and in lipid acquisition and exchange (Fig. 3).
OMVs in stress responses. As mentioned above, vesiculation functions as an envelope stress response, and increasing OMV production aids in bacterial survival under stress conditions19,20,22,24,25,26,27,42,75,76 (Fig. 3a,b). For example, a study showing that increased OMV production correlated with mutations in the σE heat shock response in E. coli19 resulted in a hypothesis that OMV production increases when misfolded toxic products or highly overexpressed proteins accumulate in the periplasm (Fig. 3a). The utility of OMVs in promoting envelope homeostasis and preventing toxicity appears to be supplemental to the other transcriptionally controlled stress response pathways. More recently, it was discovered that increased levels of AlgU (also known as σH), which is the σE (also known as RpoE) homologue in P. aeruginosa, correlated with increased OMV production, and that the loss of AlgU resulted in a hypervesiculation phenotype, which is consistent with a role for OMVs in providing envelope stress relief20.
The hypothesis that vesiculation serves as an important means to dispose of envelope 'garbage' was first evaluated with regards to proteinaceous waste accumulation using the E. coli ΔdegP strain. Misfolded proteins are not degraded in this strain because it lacks the chaperone–protease DegP, and such substrates can cause lethality at high temperatures, when protein misfolding is more likely to occur77. Notably, reduced vesiculation in the context of high levels of periplasmic protein waste impaired bacterial growth, and the lumen of OMVs produced by the ΔdegP strain contained misfolded outer-membrane proteins, which are DegP substrates42. Subsequently, a similar response was observed for other types of envelope garbage: the accumulation of both PG fragments (anhydrous tri- and tetrapeptides)78 and LPS resulted in hypervesiculation, and in the case of LPS, accumulation was lethal in a hypovesiculating mutant46,79,80. Interestingly, unassembled protein and LPS components of the electron-dense surface layer (EDSL) accumulate in the periplasm in EDSL maturation mutants of P. gingivalis54,73. The EDSL is a 20 nm wide extracellular layer composed of 33 proteins that contain a C-terminal signal and are covalently anchored to the cell surface by anionic LPS. Based on the effects discussed above for the accumulation of other types of envelope components, it is predicted that the observed accumulation of unassembled EDSL components could drive hypervesiculation in these mutants.
Finally, vesiculation has been shown to play a part in the response to oxidative stress. For example, OMV production in P. aeruginosa increases on treatment with ciprofloxacin, an antibiotic that leads to DNA damage and results in the activation of the SOS response26. Notably, vesicle production was impaired in an antibiotic-treated SOS response mutant, suggesting a link between the SOS response genes and the vesiculation machinery (Fig. 3b). In another study, OMV production was found to significantly increase after hydrogen peroxide treatment20. This oxidation-induced increase in vesiculation by P. aeruginosa was found to be dependent on the ability of the bacterium to synthesize B-band LPS; this type of LPS carries the longer and highly charged form of O antigen and had previously been shown to be enriched in constitutively produced P. aeruginosa OMVs51. Furthermore, these data provided some insight into the reason for the previously observed hypersensitivity of O antigen mutants to oxidative stress: as these mutants are unable to synthesize B-band LPS, their ability to produce OMVs is impaired, resulting in increased sensitivity to oxidative stress81.
OMVs and bacterial communities. Evidence supporting versatile roles for OMVs in promoting nutrient acquisition in bacterial communities include the preferential packing of glycosidases and proteases into OMVs (see above)64, the use of OMVs to prey on competitors within mixed bacterial communities82 and the ability of OMVs to serve as nutrient sources6 (Fig. 3c). For example, the ability of Myxococcus xanthus OMVs to lyse E. coli and the almost exclusive packaging of alkaline phosphatase into these OMVs suggest that M. xanthus OMVs are important to liberate phosphate — a vital nutrient during the development of a multicellular community — following E. coli lysis82. It is anticipated that the hydrolytic and proteolytic enzymes that were detected within M. xanthus OMVs, as well as the secondary metabolites with antibiotic activities (cittilin A, myxovirescin A, myxochelins and myxalamids), contribute to the killing of M. xanthus microbial prey66. Similarly, OMVs from the marine cyanobacterium Prochlorococcus were shown to support growth of the heterotrophs Alteromonas and Halomonas as the sole carbon source6. These data suggest that OMV-associated DNA and proteins function as a source of nitrogen and phosphorous for bacterial growth, but further work is needed in order to establish a general role for OMVs as a nutrient source in the biosphere. In addition, the secretion of OMVs carrying enolase by Borrelia burgdorferi suggests that OMVs can provide bacterial nutrients for pathogens during colonization of a host83. The catalytic product of the enolase is phosphoenolpyruvate, a receptor for the host glycoprotein plasminogen, which is proteolytically activated into the protease plasmin; this protease can aid the pathogen by degrading matrix proteins, enabling the spreading of the pathogen and possibly also generating nutrients that can be used for growth by the bacterium.
OMVs also seem to be involved in bacterial iron acquisition (Fig. 3d). Iron is an essential metal for nearly all organisms, and bacteria have evolved three mechanisms to obtain iron from sequestered iron stores within the host environment: siderophores, haem-scavenging proteins and haem acquisition systems. OMVs from a variety of species contain iron acquisition proteins and bacterial cell surface receptors that recognize haem groups. For example, OMVs from N. meningitidis are enriched in iron acquisition proteins, such as the iron-transporter components FetA and FetB47. Similarly, OMVs from P. gingivalis are enriched in HmuY, iron haem transport B (IhtB; also known as FetB) and gingipains55, which have all been shown to play a part in haem or iron acquisition from haemoglobin84,85. Similarly, the surface receptors transferrin-binding protein B (TbpB) and CopB, along with the haem chaperone CcmE, are involved in iron acquisition by OMVs from Moraxella catarrhalis86,87,88. These data, along with studies of vesicles from Gram-positive bacteria (Box 1), suggest a crucial role for vesicles in iron acquisition in a wide variety of bacterial species. This is not necessarily surprising, as vesicles are smaller and more readily diffusible than cells and are thereby able to cover larger areas to bind the vital metal for the bacteria. Although these findings imply that OMVs are able to capture iron and deliver it back to the bacteria that need it, thus far, this has only been shown to occur for Mycobacterium tuberculosis vesicles (Box 1). Notably, the OMVs from N. meningitidis are also enriched in the zinc acquisition proteins ZnuA and ZnuD47, suggesting that the role of OMVs in metal acquisition is not restricted to iron (Fig. 3d).
Considering the dispersive and functional aspects of OMVs, it can be postulated that OMVs act as 'public goods' that benefit both the producing bacteria and bystander, non-OMV-producing bacteria. Indeed, the addition of E. coli OMVs increased the survival of an E. coli population that was challenged with antimicrobial peptides and bacteriophages, as the OMVs acted as 'decoy' targets (see below; Fig. 4a)25. Similarly, OMVs produced by species of the Bacteroides genus (members of which are some of the main components of the human gut microbiota) were identified as vehicles that can distribute, at a cost to the producer, hydrolases and polysaccharide lyases, which serve as public goods that allow bacteria that do not produce these enzymes (termed cheaters) to metabolize polysaccharides for nutrient acquisition89 (Fig. 3c).
The potential for OMVs to mediate bacterial transformation has been studied for at least two decades, and the data implicate OMVs in the spread of antibiotic resistance within heterogeneous bacterial communities90 (Fig. 4b). Recent studies have shown that A. baumannii OMVs can mediate the transfer of carbapenem resistance through both vesicle-associated antibiotic resistance-encoding DNA and enzymatic antibiotic resistance activity91,92. Typically, OMV-associated DNA is thought to be indiscriminately and externally bound to the vesicle. However, in a study investigating the capacity of Acinetobacter baylyi OMVs to enable horizontal gene transfer to A. baylyi and E. coli cells93, the location of DNA was carefully tracked using an antibody against double-stranded DNA. This analysis revealed that the DNA translocated from the cytoplasm into the periplasm and subsequently into OMVs, and that successful gene transfer required competence proteins in the recipient cell, suggesting that the OMVs lysed before DNA uptake by the recipient. Further work is required to generate a more molecular, mechanistic basis to support these observations, but these data suggest that A. baylyi is able to regulate the DNA content of OMVs via an unusual translocation process.
Bacterium-associated OMVs. In some cases, OMVs remain associated with the producer cell, such as in the case of nanopods and nanowires. For example, Delftia acidovorans Cs1-4 benefits from the production of nanopods, which are proteinaceous surface-layer (S-layer) extensions filled with OMVs94. An increase in the number of nanopods and, consequently, in OMV production was observed when the bacteria were cultured in the presence of phenanthrene, a polycyclic aromatic hydrocarbon composed of three fused benzene rings. Bacteria with a substantial reduction in their production of nanopods and OMVs were unable to grow effectively in the presence of phenanthrene. The nanopods contain proteins that are encoded by genes close to the phenanthrene biodegradation locus, although the activity of these proteins in phenanthrene biodegradation has not yet been directly assessed. These data suggest that the OMVs enable the biodegradation of phenanthrene and that the OMV-containing nanopods are a mechanism to counteract the problems of limited diffusion in solid (that is, non-fluid) environments. Additionally, nanowires have been implicated in the ability of bacteria to scavenge and transport electrons — which serve as an energy source — from the environment over long distances, as demonstrated for the metal-reducing marine bacterium Shewanella oneidensis95. Bacterial nanowires, which are composed of outer-membrane lipids, multihaem cytochrome envelope protein complexes and periplasm, are thought to be an extracellular electron transport (EET) pathway linking bacteria to the external solid-phase iron and manganese minerals that can serve as terminal electron acceptors for respiration. The production of nanowires coincided with the formation of OMVs and may be the result of OMV extension and/or fusion. Indeed, microscopy studies suggest that chains of OMVs from S. oneidensis can smooth into tubes and form intercellular connections, indicating that these filaments could facilitate cell–cell electron signalling, similar to the connectivity observed for M. xanthus96 and Chlorochromatium aggregatum97.
OMVs in bacteria–host lipid exchange. Some bacteria require cholesterol in the outer membrane for optimal fitness, but lack the biosynthetic pathway for cholesterol synthesis and instead acquire cholesterol from the culture medium or their animal host. Interestingly, cholesterol exchange seems to be bidirectional, as labelled cholesterol and cholesterol glycolipids that had been incorporated into B. burgdorferi membranes were transferred to host cells, both via direct contact and via OMVs98. Furthermore, clustering of cholesterol into lipid rafts was observed in the outer membrane of B. burgdorferi. As cholesterol promotes curvature in asymmetrical bilayers99, these raft regions could be favoured for OMV biogenesis. Similar observations have been made for H. pylori100, but it remains to be determined whether the involvement of OMVs in this two-way lipid exchange is frequently used by other bacteria.
OMVs in pathogenesis
Compared with soluble secretion methods, OMVs provide a uniquely beneficial secretion option for pathogens, as OMVs can protect virulence determinants from host proteases and concentrate them for host cell delivery (Fig. 4c,d). In addition, OMVs can simultaneously deliver multiple virulence factors, OMV-associated adhesins can provide tissue-tropic delivery of OMV content, and OMVs can confer antibiotic resistance (Fig. 4). It should be appreciated that other basic physiological functions of OMVs (see above) and their association with biofilms (Box 2) also contribute to bacterial pathogenesis by increasing bacterial adaptation and survival in the hostile host environment. Intriguingly, both harmful and beneficial effects have been linked to OMV production by bacteria present in the host gut microbiota.
Resistance to antimicrobials. OMVs can help bacteria to battle antibiotics in ways beyond the spread of antibiotic resistance genes (see above; Fig. 4b). For example, OMVs can provide immediate protection for the bacteria well before the bacteria can adapt by modifying or mutating antibiotic targets, because the OMVs act as decoys that bind or absorb antibiotics and toxins (Fig. 4a). In E. coli, the addition of OMVs or the use of a hypervesiculating mutant increased immediate resistance to the antimicrobial peptides polymixin B and colistin25. Similarly, OMVs from P. syringae reduced the levels of colistin and melittin (another antimicrobial peptide) in solution by sequestering these compounds49. In a mechanism that promotes cross-resistance, pre-incubation of Vibrio cholerae with sublethal amounts of polymixin B was found to result in the generation of OMVs with a modified protein composition, and these modified OMVs had an increased capacity to bind another antimicrobial peptide, defensin LL-37 (Ref. 101).
Phages are also a type of antimicrobial, and both the addition of OMVs and a hypervesiculation mutation increased the viability of E. coli cultured with the lytic T4 phage, as OMVs irreversibly bound and inactivated the phage25 (Fig. 4a). This in vitro observation is likely to reflect interactions that occur in the biosphere, as phage–OMV complexes have been observed in marine samples6.
Finally, OMV-mediated absorption was also shown to mediate the resistance of B. henselae against haem toxicity. B. henselae resides in its haematophagous arthropod vector, the cat flea, and B. henselae OMVs containing hemin-binding protein C (HbpC) can sequester haem, which increases bacterial resistance to haem toxicity. This physiological role for B. henselae OMVs is further supported by data showing that OMV production by the bacterium was higher at 28 °C, the temperature in arthropods, than at 37 °C, the temperature in mammalian hosts21.
Beyond reducing the effective antibiotic concentration in culture by adsorption, OMVs can also carry enzymes that mediate antibiotic protection (Fig. 4b). It was demonstrated about a decade ago and recently confirmed for Gram-positive bacteria (Box 1) that β-lactamase can be packaged into OMVs and that co-incubation of these vesicles with β-lactam-sensitive species improves resistance to these antibiotics102,103. Recently, OMVs from amoxicillin-resistant M. catarrhalis were found to carry active β-lactamase and to protect amoxicillin-sensitive M. catarrhalis from antibiotic-induced killing. Furthermore, OMVs from amoxicillin-resistant M. catarrhalis also improved the amoxicillin resistance of non-typeable Haemophilus influenzae and Streptococcus pneumoniae, two bacteria that typically co-infect the respiratory tract104.
Delivery of virulence factors. It has long been appreciated that OMVs can also work as a toxin shuttle for pathogens (reviewed in Ref. 105) (Fig. 4c). Furthermore, recent reports have started to elucidate how OMV-transported factors manipulate the host cell trafficking machinery. For example, M. catarrhalis OMVs act as immunomodulatory molecules, delivering the outer-membrane-bound superantigen Moraxella immunoglobulin D-binding protein (MID) into B cells. M. catarrhalis OMVs are internalized via receptor clustering in lipid rafts, and they activate the B cells via OMV-associated DNA, which induces Toll-like receptor 9 (TLR9) signalling106. OMV-mediated delivery of MID increases the survival of M. catarrhalis, as B cell activation leads to polyclonal immunoglobulin M (IgM) production, potentially delaying the production of specific antibodies. In P. aeruginosa, the OMV-associated toxin CFTR-inhibitory factor (Cif) controls the host deubiquitylating enzyme USP10 to cause increased ubiquitylation of cystic fibrosis transmembrane conductance regulator (CFTR), which is involved in mucus production. Ubiquitylation of CFTR leads to its degradation in lysosomes and results in increased bacterial survival owing to decreased chloride secretion by the host cells and decreased mucociliary clearance of P. aeruginosa107.
In addition, some OMVs have now been identified as genotoxins. For example, H. pylori OMVs cause the formation of micronuclei (in which chromosomes are not correctly distributed), as well as alterations in iron metabolism and oxidative stress associated with genomic damage in gastric epithelial cells. These effects are dependent on the OMV-associated cytotoxin VacA108. VacA was also found to increase OMV association with host cells, most probably allowing OMVs to gain access to multiple internalization pathways109. VacA somehow increases the rate of cholesterol-independent OMV uptake, possibly by interacting with cell surface receptors and increasing the likelihood for secondary adhesin–receptor interactions to occur. Purified VacA cytotoxin reduces glutathione, and as glutathione peroxidases utilize glutathione in the breakdown of hydrogen peroxide, the VacA associated with OMVs may increase the opportunity for hydrogen peroxide-promoted DNA damage, either alone or in combination with redox-active iron. Additionally, OMVs isolated from three different E. coli strains (avirulent DH5α, pathogenic adherent-invasive E. coli (AIEC), and enterohaemorrhagic E. coli (EHEC)) were discovered to be genotoxic to human enterocyte-like cells, causing double-stranded DNA breaks, increased proliferation and multinucleation of the enterocyte-like cells110. However, the particular component (or components) of E. coli OMVs that are responsible for this genotoxicity remain unidentified.
Microbiota-produced OMVs. New health-related properties of OMVs are also being uncovered in the context of the human microbiome. Delivery of an OMV-associated antigen in a sulfatase-dependent manner was recently reported to occur in the intestinal tract of a mouse model that is genetically prone to colitis4. Notably, orally gavaged Bacteroides thetaiotaomicron produced OMVs that were found to traverse the gut mucosal barrier and access the gut epithelial cells and the underlying intestinal macrophages in a sulfatase-dependent manner, initiating intestinal inflammation (Fig. 4d). By contrast, OMV-associated capsular polysaccharide from Bacteroides fragilis was reported to modulate the immune system to prevent colitis111. Therefore, microbiota-derived OMVs seem to both promote and prevent intestinal inflammation, and these effects appear to depend on host susceptibility factors as well as specific OMV components.
The field of bacterial vesicle research has long experienced a wealth of information on the individual aspects of OMVs generated by a large variety of bacteria, but in-depth studies focused on identifying molecules that are crucial for the biogenesis or function of OMVs have been lacking. This is both exciting and frustrating. Excitement is generated by the ubiquitous nature of OMV production and the unique capabilities of each type of OMV, suggesting that there are even more widely diverse functions of these particles than is currently appreciated. However, frustration has mounted because there is not a single complete picture that explains either functional or mechanistic aspects of OMV production. Nevertheless, it is important to step back and consider whether such expectations for this field are just. As neither bacterial composition nor even the composition of the bacterial envelope is conserved among Gram-negative species, and these factors further depend on growth and environmental conditions, it is naive to consider that a single mechanism exists or should exist that can generate OMVs. In a close parallel to the study of OMVs, the field of exosomes and microvesicles is challenged with defining both the origin and composition of these ubiquitous and multifaceted extracellular particles, as these factors depend on the cell type, the status of the cell (for example, whether it is stressed, infected, cancerous, and so on) and the cell environment112,113. Nevertheless, understanding how diverse bacteria achieve a common mechanistic outcome involving vesicle budding, cargo selection, vesicle detachment and vesicle stability will benefit both the OMV field and biology in general. Likewise, common as well as distinguishing functional features of OMVs will enlighten our understanding of how bacterial products influence their environment.
Vesicle biogenesis, its mechanisms of regulation and the factors influencing cargo selection used to be a 'black box'. However, common mechanisms are starting to emerge, and these insights will need to be extended over the coming years. Numerous basic unanswered questions are still left in the field. Which envelope factors lead to outer-membrane fission and OMV release? What signals and pathways regulate OMV biogenesis? How is LPS-independent enrichment and exclusion of soluble cargo achieved? Which processes are conserved across different Gram-negative species? An understanding of these and other concepts will be critical for the future development of OMVs as therapeutically potent delivery tools (Box 3). Future studies will hopefully begin to address these fundamental questions as well as increase our appreciation of the unique capabilities of these complex and multifaceted entities.
Kulp, A. & Kuehn, M. J. Biological functions and biogenesis of secreted bacterial outer membrane vesicles. Annu. Rev. Microbiol. 64, 163–184 (2010).
Schwechheimer, C., Sullivan, C. J. & Kuehn, M. J. Envelope control of outer membrane vesicle production in Gram-negative bacteria. Biochemistry 52, 3031–3040 (2013). This review highlights the challenges of elucidating mechanistic details in OMV production.
Zhou, L., Srisatjaluk, R., Justus, D. E. & Doyle, R. J. On the origin of membrane vesicles in Gram-negative bacteria. FEMS Microbiol. Lett. 163, 223–228 (1998).
Hickey, C. A. et al. Colitogenic Bacteroides thetaiotaomicron antigens access host immune cells in a sulfatase-dependent manner via outer membrane vesicles. Cell Host Microbe 17, 672–680 (2015).
Beveridge, T. J. Structures of gram-negative cell walls and their derived membrane vesicles. J. Bacteriol. 181, 4725–4733 (1999).
Biller, S. J. et al. Bacterial vesicles in marine ecosystems. Science 343, 183–186 (2014).
Beveridge, T. J., Makin, S. A., Kadurugamuwa, J. L. & Li, Z. Interactions between biofilms and the environment. FEMS Microbiol. Rev. 20, 291–303 (1997).
Brandtzaeg, P. et al. Meningococcal endotoxin in lethal septic shock plasma studied by gas chromatography, mass-spectrometry, ultracentrifugation, and electron microscopy. J. Clin. Invest. 89, 816–823 (1992).
Hellman, J. et al. Release of gram-negative outer-membrane proteins into human serum and septic rat blood and their interactions with immunoglobulin in antiserum to Escherichia coli J5. J. Infect. Dis. 181, 1034–1043 (2000).
Deatherage, B. L. & Cookson, B. T. Membrane vesicle release in bacteria, eukaryotes, and archaea: a conserved yet underappreciated aspect of microbial life. Infect. Immun. 80, 1948–1957 (2012).
Silhavy, T. J., Kahne, D. & Walker, S. The bacterial cell envelope. Cold Spring Harb. Perspect. Biol. 2, a000414 (2010).
Kojer, K. & Riemer, J. Balancing oxidative protein folding: the influences of reducing pathways on disulfide bond formation. Biochim. Biophys. Acta 1844, 1383–1390 (2014).
Braun, V. & Wolff, H. Attachment of lipoprotein to murein (peptidoglycan) of Escherichia coli in the presence and absence of penicillin FL 1060. J. Bacteriol. 123, 888–897 (1975).
Braun, V. Covalent lipoprotein from the outer membrane of Escherichia coli. Biochim. Biophys. Acta 415, 335–377 (1975).
Huang, Y. X., Ching, G. & Inouye, M. Comparison of the lipoprotein gene among the Enterobacteriaceae. DNA sequence of Morganella morganii lipoprotein gene and its expression in Escherichia coli. J. Biol. Chem. 258, 8139–8145 (1983).
Cascales, E., Bernadac, A., Gavioli, M., Lazzaroni, J. C. & Lloubes, R. Pal lipoprotein of Escherichia coli plays a major role in outer membrane integrity. J. Bacteriol. 184, 754–759 (2002).
Wang, Y. The function of OmpA in Escherichia coli. Biochem. Biophys. Res. Commun. 292, 396–401 (2002).
McBroom, A. J., Johnson, A. P., Vemulapalli, S. & Kuehn, M. J. Outer membrane vesicle production by Escherichia coli is independent of membrane instability. J. Bacteriol. 188, 5385–5392 (2006). The research identifies random transposon mutants with OMV production phenotypes, and their characterization shows that vesiculation is not a by-product of bacterial lysis.
McBroom, A. J. & Kuehn, M. J. Release of outer membrane vesicles by Gram-negative bacteria is a novel envelope stress response. Mol. Microbiol. 63, 545–558 (2007). This report presents genetic and biochemical evidence to support a model whereby OMV release can rid the envelope of misfolded proteins and affect antibiotic resistance.
Macdonald, I. A. & Kuehn, M. J. Stress-induced outer membrane vesicle production by Pseudomonas aeruginosa. J. Bacteriol. 195, 2971–2981 (2013).
Roden, J. A., Wells, D. H., Chomel, B. B., Kasten, R. W. & Koehler, J. E. Hemin binding protein C is found in outer membrane vesicles and protects Bartonella henselae against toxic concentrations of hemin. Infect. Immun. 80, 929–942 (2012).
McMahon, K. J., Castelli, M. E., Vescovi, E. G. & Feldman, M. F. Biogenesis of outer membrane vesicles in Serratia marcescens is thermoregulated and can be induced by activation of the Rcs phosphorelay system. J. Bacteriol. 194, 3241–3249 (2012).
Frias, A., Manresa, A., de Oliveira, E., López-Iglesias, C. & Mercade, E. Membrane vesicles: a common feature in the extracellular matter of cold-adapted Antarctic bacteria. Microb. Ecol. 59, 476–486 (2010).
Toyofuku, M. et al. Membrane vesicle formation is associated with pyocin production under denitrifying conditions in Pseudomonas aeruginosa PAO1. Environ. Microbiol. 16, 2927–2938 (2014).
Manning, A. J. & Kuehn, M. J. Contribution of bacterial outer membrane vesicles to innate bacterial defense. BMC Microbiol. 11, 258 (2011).
Maredia, R. et al. Vesiculation from Pseudomonas aeruginosa under SOS. ScientificWorldJournal 2012, 402919 (2012). This work shows the long-range effect of vesiculation and the importance of OMVs in bacterial physiology.
Tashiro, Y. et al. Outer membrane machinery and alginate synthesis regulators control membrane vesicle production in Pseudomonas aeruginosa. J. Bacteriol. 191, 7509–7519 (2009).
Ellis, T. N. & Kuehn, M. J. Virulence and immunomodulatory roles of bacterial outer membrane vesicles. Microbiol. Mol. Biol. Rev. 74, 81–94 (2010).
Berleman, J. & Auer, M. The role of bacterial outer membrane vesicles for intra- and interspecies delivery. Environ. Microbiol. 15, 347–354 (2013).
Park, J. S. et al. Mechanism of anchoring of OmpA protein to the cell wall peptidoglycan of the gram-negative bacterial outer membrane. FASEB J. 26, 219–228 (2011).
Yeh, Y. C., Comolli, L. R., Downing, K. H., Shapiro, L. & McAdams, H. H. The caulobacter Tol–Pal complex is essential for outer membrane integrity and the positioning of a polar localization factor. J. Bacteriol. 192, 4847–4858 (2010).
Gerding, M. A., Ogata, Y., Pecora, N. D., Niki, H. & de Boer, P. A. The trans-envelope Tol–Pal complex is part of the cell division machinery and required for proper outer-membrane invagination during cell constriction in E. coli. Mol. Microbiol. 63, 1008–1025 (2007).
Nikaido, H. in Escherichia coli and Salmonella: Cellular and Molecular Biology (eds Neidhardt, F. C. et al.) 29–47 (ASM Press, 1996).
Moon, D. C. et al. Acinetobacter baumannii outer membrane protein A modulates the biogenesis of outer membrane vesicles. J. Microbiol. 50, 155–160 (2012).
Deatherage, B. L. et al. Biogenesis of bacterial membrane vesicles. Mol. Microbiol. 72, 1395–1407 (2009).
Song, T. et al. A new Vibrio cholerae sRNA modulates colonization and affects release of outer membrane vesicles. Mol. Microbiol. 70, 100–111 (2008).
Sonntag, I., Schwarz, H., Hirota, Y. & Henning, U. Cell envelope and shape of Escherichia coli: multiple mutants missing the outer membrane lipoprotein and other major outer membrane proteins. J. Bacteriol. 136, 280–285 (1978).
Wessel, A. K., Liew, J., Kwon, T., Marcotte, E. M. & Whiteley, M. Role of Pseudomonas aeruginosa peptidoglycan-associated outer membrane proteins in vesicle formation. J. Bacteriol. 195, 213–219 (2013).
Schertzer, J. W. & Whiteley, M. A bilayer-couple model of bacterial outer membrane vesicle biogenesis. mBio 3, e00297-11 (2012). This article presents a biophysical analysis describing how the binding and insertion of secreted small molecular metabolites into the outer leaflet of the outer membrane can induce curvature.
Torti, S. V. & Park, J. T. Lipoprotein of Gram-negative bacteria is essential for growth and division. Nature 263, 323–326 (1976).
Suzuki, H. et al. Murein-lipoprotein of Escherichia coli: a protein involved in the stabilization of bacterial cell envelope. Mol. Gen. Genet. 167, 1–9 (1978).
Schwechheimer, C. & Kuehn, M. J. Synthetic effect between envelope stress and lack of outer membrane vesicle production in Escherichia coli. J. Bacteriol. 195, 4161–4173 (2013).
Schwechheimer, C., Rodriguez, D. L. & Kuehn, M. J. NlpI-mediated modulation of outer membrane vesicle production through peptidoglycan dynamics in Escherichia coli. MicrobiologyOpen 4, 375–389 (2015).
Ohara, M., Wu, H. C., Sankaran, K. & Rick, P. D. Identification and characterization of a new lipoprotein, NlpI, in Escherichia coli K-12. J. Bacteriol. 181, 4318–4325 (1999).
Singh, S. K., SaiSree, L., Amrutha, R. N. & Reddy, M. Three redundant murein endopeptidases catalyse an essential cleavage step in peptidoglycan synthesis of Escherichia coli K12. Mol. Microbiol. 86, 1036–1051 (2012).
Schwechheimer, C., Kulp, A. & Kuehn, M. J. Modulation of bacterial outer membrane vesicle production by envelope structure and content. BMC Microbiol. 14, 324 (2014). This study demonstrates that there are at least two distinct envelope architectures that can lead to the formation of OMVs in E. coli.
Lappann, M., Otto, A., Becher, D. & Vogel, U. Comparative proteome analysis of spontaneous outer membrane vesicles and purified outer membranes of Neisseria meningitidis. J. Bacteriol. 195, 4425–4435 (2013).
Chowdhury, C. & Jagannadham, M. V. Virulence factors are released in association with outer membrane vesicles of Pseudomonas syringae pv. tomato T1 during normal growth. Biochim. Biophys. Acta 1834, 231–239 (2013).
Kulkarni, H. M., Swamy, Ch. V. & Jagannadham, M. V. Molecular characterization and functional analysis of outer membrane vesicles from the Antarctic bacterium Pseudomonas syringae suggest a possible response to environmental conditions. J. Proteome Res. 13, 1345–1358 (2014).
Tashiro, Y. et al. Characterization of phospholipids in membrane vesicles derived from Pseudomonas aeruginosa. Biosci. Biotechnol. Biochem. 75, 605–607 (2011).
Li, Z., Clarke, A. J. & Beveridge, T. J. A major autolysin of Pseudomonas aeruginosa: subcellular distribution, potential role in cell growth and division and secretion in surface membrane vesicles. J. Bacteriol. 178, 2479–2488 (1996).
Murphy, K. et al. Influence of O polysaccharides on biofilm development and outer membrane vesicle biogenesis in Pseudomonas aeruginosa PAO1. J. Bacteriol. 196, 1306–1317 (2014). These findings highlight a critical role for O-saccharide LPS in vesicle composition and size.
Yamaguchi, M. et al. A Porphyromonas gingivalis mutant defective in a putative glycosyltransferase exhibits defective biosynthesis of the polysaccharide portions of lipopolysaccharide, decreased gingipain activities, strong autoaggregation, and increased biofilm formation. Infect. Immun. 78, 3801–3812 (2010).
Haurat, M. F. et al. Selective sorting of cargo proteins into bacterial membrane vesicles. J. Biol. Chem. 286, 1269–1276 (2011). This work establishes that the human oral pathogen P. gingivalis selectively incorporates and excludes proteins into OMVs.
Veith, P. D. et al. Porphyromonas gingivalis outer membrane vesicles exclusively contain outer membrane and periplasmic proteins and carry a cargo enriched with virulence factors. J. Proteome Res. 13, 2420–2432 (2013).
Mashburn, L. M. & Whiteley, M. Membrane vesicles traffic signals and facilitate group activities in a prokaryote. Nature 437, 422–425 (2005).
Tashiro, Y., Ichikawa, S., Nakajima-Kambe, T., Uchiyama, H. & Nomura, N. Pseudomonas quinolone signal affects membrane vesicle production in not only Gram-negative but also Gram-positive bacteria. Microbes Environ. 25, 120–125 (2010).
Mashburn-Warren, L. et al. Interaction of quorum signals with outer membrane lipids: insights into prokaryotic membrane vesicle formation. Mol. Microbiol. 69, 491–502 (2008).
Ayalew, S., Confer, A. W., Shrestha, B., Wilson, A. E. & Montelongo, M. Proteomic analysis and immunogenicity of Mannheimia haemolytica vesicles. Clin. Vaccine Immunol. 20, 191–196 (2013).
Choi, D. S. et al. Proteomic analysis of outer membrane vesicles derived from Pseudomonas aeruginosa. Proteomics 11, 3424–3429 (2011).
Lee, J. C. et al. Klebsiella pneumoniae secretes outer membrane vesicles that induce the innate immune response. FEMS Microbiol. Lett. 331, 17–24 (2012).
Pierson, T. et al. Proteomic characterization and functional analysis of outer membrane vesicles of Francisella novicida suggests possible role in virulence and use as a vaccine. J. Proteome Res. 10, 954–967 (2011).
Zielke, R. A., Wierzbicki, I. H., Weber, J. V., Gafken, P. R. & Sikora, A. E. Quantitative proteomics of the Neisseria gonorrhoeae cell envelope and membrane vesicles for the discovery of potential therapeutic targets. Mol. Cell Proteom. 13, 1299–1317 (2014).
Elhenawy, W., Debelyy, M. O. & Feldman, M. F. Preferential packing of acidic glycosidases and proteases into Bacteroides outer membrane vesicles. mBio 5, e00909-14 (2014).
Pérez-Cruz, C. et al. New type of outer membrane vesicle produced by the Gram-negative bacterium Shewanella vesiculosa M7T: implications for DNA content. Appl. Environ. Microbiol. 79, 1874–1881 (2013).
Berleman, J. E. et al. The lethal cargo of Myxococcus xanthus outer membrane vesicles. Front. Microbiol. 5, 474 (2014).
Olofsson, A. et al. Biochemical and functional characterization of Helicobacter pylori vesicles. Mol. Microbiol. 77, 1539–1555 (2010).
Hoy, B. et al. Distinct roles of secreted HtrA proteases from Gram-negative pathogens in cleaving the junctional protein and tumor suppressor E-cadherin. J. Biol. Chem. 287, 10115–10120 (2012).
Davis, J. M., Carvalho, H. M., Rasmussen, S. B. & O'Brien, A. D. Cytotoxic necrotizing factor type 1 delivered by outer membrane vesicles of uropathogenic Escherichia coli attenuates polymorphonuclear leukocyte antimicrobial activity and chemotaxis. Infect. Immun. 74, 4401–4408 (2006).
Yu, H. & Kim, K. S. YgfZ contributes to secretion of cytotoxic necrotizing factor 1 into outer-membrane vesicles in Escherichia coli. Microbiology 158, 612–621 (2012).
Yu, H. & Kim, K. S. Ferredoxin is involved in secretion of cytotoxic necrotizing factor 1 across the cytoplasmic membrane in Escherichia coli K1. Infect. Immun. 78, 838–844 (2010).
Kouokam, J. C. et al. Active cytotoxic necrotizing factor 1 associated with outer membrane vesicles from uropathogenic Escherichia coli. Infect. Immun. 74, 2022–2030 (2006).
Chen, Y. Y. et al. The outer membrane protein LptO is essential for the O-deacylation of LPS and the co-ordinated secretion and attachment of A-LPS and CTD proteins in Porphyromonas gingivalis. Mol. Microbiol. 79, 1380–1401 (2011).
Lima, S., Guo, M. S., Chaba, R., Gross, C. A. & Sauer, R. T. Dual molecular signals mediate the bacterial response to outer-membrane stress. Science 340, 837–841 (2013).
Henry, R. et al. Precipitation of iron on the surface of Leptospira interrogans is associated with mutation of the stress response metalloprotease HtpX. Appl. Environ. Microbiol. 79, 4653–4660 (2013).
Shao, P., Comolli, L. & Bernier-Latmani, R. Membrane vesicles as a novel strategy for shedding encrusted cell surfaces. Minerals 4, 74–88 (2014).
Strauch, K. L., Johnson, K. & Beckwith, J. Characterization of degP, a gene required for proteolysis in the cell envelope and essential for growth of Escherichia coli at high temperature. J. Bacteriol. 171, 2689–2696 (1989).
Uehara, T. & Park, J. T. An anhydro-N-acetylmuramyl-l-alanine amidase with broad specificity tethered to the outer membrane of Escherichia coli. J. Bacteriol. 189, 5634–5641 (2007).
Mahalakshmi, S., Sunayana, M. R., Saisree, L. & Reddy, M. yciM is an essential gene required for regulation of lipopolysaccharide synthesis in Escherichia coli. Mol. Microbiol. 91, 145–157 (2013).
Klein, G., Kobylak, N., Lindner, B., Stupak, A. & Raina, S. Assembly of lipopolysaccharide in Escherichia coli requires the essential LapB heat shock protein. J. Biol. Chem. 289, 14829–14853 (2014).
Berry, M. C., McGhee, G. C., Zhao, Y. & Sundin, G. W. Effect of a waaL mutation on lipopolysaccharide composition, oxidative stress survival, and virulence in Erwinia amylovora. FEMS Microbiol. Lett. 291, 80–87 (2009).
Evans, A. G. et al. Predatory activity of Myxococcus xanthus outer membrane vesicles and properties of their hydrolase cargo. Microbiology 158, 2742–2752 (2012).
Toledo, A., Coleman, J. L., Kuhlow, C. J., Crowley, J. T. & Benach, J. L. The enolase of Borrelia burgdorferi is a plasminogen receptor released in outer membrane vesicles. Infect. Immun. 80, 359–368 (2011).
Dashper, S. G. et al. Characterization of a novel outer membrane hemin-binding protein of Porphyromonas gingivalis. J. Bacteriol. 182, 6456–6462 (2000).
Smalley, J. W. et al. HmuY haemophore and gingipain proteases constitute a unique syntrophic system of haem acquisition by Porphyromonas gingivalis. PLoS ONE 6, e17182 (2011).
Schaar, V. et al. Multicomponent Moraxella catarrhalis outer membrane vesicles induce an inflammatory response and are internalized by human epithelial cells. Cell. Microbiol. 13, 432–449 (2011). This proteomic and functional study highlights the interaction of OMVs with immune cells and the ability of OMVs to stimulate a pro-inflammatory immune response in vitro and in vivo.
Aebi, C. et al. Expression of the CopB outer membrane protein by Moraxella catarrhalis is regulated by iron and affects iron acquisition from transferrin and lactoferrin. Infect. Immun. 64, 2024–2030 (1996).
Myers, L. E. et al. The transferrin binding protein B of Moraxella catarrhalis elicits bactericidal antibodies and is a potential vaccine antigen. Infect. Immun. 66, 4183–4192 (1998).
Rakoff-Nahoum, S., Coyne, M. J. & Comstock, L. E. An ecological network of polysaccharide utilization among human intestinal symbionts. Curr. Biol. 24, 40–49 (2014). The data presented in this paper demonstrate how OMV-associated hydrolases are utilized as public goods within bacterial communities.
Klieve, A. V. et al. Naturally occurring DNA transfer system associated with membrane vesicles in cellulolytic Ruminococcus spp. of ruminal origin. Appl. Environ. Microbiol. 71, 4248–4253 (2005).
Jin, J. S. et al. Acinetobacter baumannii secretes cytotoxic outer membrane protein A via outer membrane vesicles. PLoS ONE 6, e17027 (2011).
Sahu, P. K., Iyer, P. S., Oak, A. M., Pardesi, K. R. & Chopade, B. A. Characterization of eDNA from the clinical strain Acinetobacter baumannii AIIMS 7 and its role in biofilm formation. ScientificWorldJournal 2012, 973436 (2012).
Fulsundar, S. et al. Gene transfer potential of outer membrane vesicles of Acinetobacter baylyi and effects of stress on vesiculation. Appl. Environ. Microbiol. 80, 3469–3483 (2014).
Shetty, A. & Hickey, W. J. Effects of outer membrane vesicle formation, surface-layer production and nanopod development on the metabolism of phenanthrene by Delftia acidovorans Cs1-4. PLoS ONE 9, e92143 (2014).
Pirbadian, S. et al. Shewanella oneidensis MR-1 nanowires are outer membrane and periplasmic extensions of the extracellular electron transport components. Proc. Natl Acad. Sci. USA 111, 12883–12888 (2014). This study develops a new aspect of OMV physiology: how OMVs attached to bacteria can play a part in energy acquisition.
Remis, J. P. et al. Bacterial social networks: structure and composition of Myxococcus xanthus outer membrane vesicle chains. Environ. Microbiol. 16, 598–610 (2014).
Wanner, G., Vogl, K. & Overmann, J. Ultrastructural characterization of the prokaryotic symbiosis in “Chlorochromatium aggregatum”. J. Bacteriol. 190, 3721–3730 (2008).
Crowley, J. T. et al. Lipid exchange between Borrelia burgdorferi and host cells. PLoS Pathog. 9, e1003109 (2013).
Yesylevskyy, S. O., Demchenko, A. P., Kraszewski, S. & Ramseyer, C. Cholesterol induces uneven curvature of asymmetric lipid bilayers. ScientificWorldJournal 2013, 965230 (2013).
Wang, H. J., Cheng, W. C., Cheng, H. H., Lai, C. H. & Wang, W. C. Helicobacter pylori cholesteryl glucosides interfere with host membrane phase and affect type IV secretion system function during infection in AGS cells. Mol. Microbiol. 83, 67–84 (2012).
Duperthuy, M. et al. Role of the Vibrio cholerae matrix protein Bap1 in cross-resistance to antimicrobial peptides. PLoS Pathog. 9, e1003620 (2013).
Ciofu, O., Beveridge, T. J., Kadurugamuwa, J., Walther-Rasmussen, J. & Hoiby, N. Chromosomal β-lactamase is packaged into membrane vesicles and secreted from Pseudomonas aeruginosa. J. Antimicrob. Chemother. 45, 9–13 (2000).
Lee, J. et al. Staphylococcus aureus extracellular vesicles carry biologically active β-lactamase. Antimicrob. Agents Chemother. 57, 2589–2595 (2013).
Schaar, V., Nordstrom, T., Morgelin, M. & Riesbeck, K. Moraxella catarrhalis outer membrane vesicles carry β-lactamase and promote survival of Streptococcus pneumoniae and Haemophilus influenzae by inactivating amoxicillin. Antimicrob. Agents Chemother. 55, 3845–3853 (2011).
Bonnington, K. E. & Kuehn, M. J. Protein selection and export via outer membrane vesicles. Biochim. Biophys. Acta 1843, 1612–1619 (2013).
Vidakovics, M. L. et al. B cell activation by outer membrane vesicles — a novel virulence mechanism. PLoS Pathog. 6, e1000724 (2010).
Bomberger, J. M. et al. Pseudomonas aeruginosa Cif protein enhances the ubiquitination and proteasomal degradation of the transporter associated with antigen processing (TAP) and reduces major histocompatibility complex (MHC) class I antigen presentation. J. Biol. Chem. 289, 152–162 (2014).
Chitcholtan, K., Hampton, M. B. & Keenan, J. I. Outer membrane vesicles enhance the carcinogenic potential of Helicobacter pylori. Carcinogenesis 29, 2400–2405 (2008).
Parker, H., Chitcholtan, K., Hampton, M. B. & Keenan, J. I. Uptake of Helicobacter pylori outer membrane vesicles by gastric epithelial cells. Infect. Immun. 78, 5054–5061 (2010).
Tyrer, P. C., Frizelle, F. A. & Keenan, J. I. Escherichia coli-derived outer membrane vesicles are genotoxic to human enterocyte-like cells. Infect. Agent Cancer 9, 2 (2014).
Shen, Y. et al. Outer membrane vesicles of a human commensal mediate immune regulation and disease protection. Cell Host Microbe 12, 509–520 (2012).
Robbins, P. D. & Morelli, A. E. Regulation of immune responses by extracellular vesicles. Nat. Rev. Immunol. 14, 195–208 (2014).
De Toro, J., Herschlik, L., Waldner, C. & Mongini, C. Emerging roles of exosomes in normal and pathological conditions: new insights for diagnosis and therapeutic applications. Front. Immunol. 6, 203 (2015).
Brown, L., Wolf, J. M., Prados-Rosales, R. & Casadevall, A. Through the wall: extracellular vesicles in Gram-positive bacteria, mycobacteria and fungi. Nat. Rev. Microbiol. 13, 620–630 (2015).
Lee, E. Y. et al. Gram-positive bacteria produce membrane vesicles: proteomics-based characterization of Staphylococcus aureus-derived membrane vesicles. Proteomics 9, 5425–5436 (2009).
Rivera, J. et al. Bacillus anthracis produces membrane-derived vesicles containing biologically active toxins. Proc. Natl Acad. Sci. USA 107, 19002–19007 (2010).
Dorward, D. W. & Garon, C. F. DNA is packaged within membrane-derived vesicles of Gram-negative but not Gram-positive bacteria. Appl. Environ. Microbiol. 56, 1960–1962 (1990).
Gurung, M. et al. Staphylococcus aureus produces membrane-derived vesicles that induce host cell death. PLoS ONE 6, e27958 (2011).
Chernov, V. M. et al. Extracellular vesicles derived from Acholeplasma laidlawii PG8. ScientificWorldJournal 11, 1120–1130 (2011).
Medvedeva, E. S. et al. Adaptation of mycoplasmas to antimicrobial agents: Acholeplasma laidlawii extracellular vesicles mediate the export of ciprofloxacin and a mutant gene related to the antibiotic target. ScientificWorldJournal 2014, 150615 (2014).
Yamaguchi, Y., Takei, M., Kishii, R., Yasuda, M. & Deguchi, T. Contribution of topoisomerase IV mutation to quinolone resistance in Mycoplasma genitalium. Antimicrob. Agents Chemother. 57, 1772–1776 (2013).
Prados-Rosales, R. et al. Mycobacteria release active membrane vesicles that modulate immune responses in a TLR2-dependent manner in mice. J. Clin. Invest. 121, 1471–1483 (2011).
Rath, P. et al. Genetic regulation of vesiculogenesis and immunomodulation in Mycobacterium tuberculosis. Proc. Natl Acad. Sci. USA 110, E4790–E4797 (2013).
Prados-Rosales, R. et al. Role for Mycobacterium tuberculosis membrane vesicles in iron acquisition. J. Bacteriol. 196, 1250–1256 (2014).
Schooling, S. R., Hubley, A. & Beveridge, T. J. Interactions of DNA with biofilm-derived membrane vesicles. J. Bacteriol. 191, 4097–4102 (2009).
Schooling, S. R. & Beveridge, T. J. Membrane vesicles: an overlooked component of the matrices of biofilms. J. Bacteriol. 188, 5945–5957 (2006).
Yonezawa, H. et al. Outer membrane vesicles of Helicobacter pylori TK1402 are involved in biofilm formation. BMC Microbiol. 9, 197 (2009).
Yonezawa, H. et al. Analysis of outer membrane vesicle protein involved in biofilm formation of Helicobacter pylori. Anaerobe 17, 388–390 (2011).
Baumgarten, T. et al. Membrane vesicle formation as a multiple-stress response mechanism enhances Pseudomonas putida DOT-T1E cell surface hydrophobicity and biofilm formation. Appl. Environ. Microbiol. 78, 6217–6224 (2012).
Zhu, Y. et al. Porphyromonas gingivalis and Treponema denticola synergistic polymicrobial biofilm development. PLoS ONE 8, e71727 (2013).
Ito, R., Ishihara, K., Shoji, M., Nakayama, K. & Okuda, K. Hemagglutinin/adhesin domains of Porphyromonas gingivalis play key roles in coaggregation with Treponema denticola. FEMS Immunol. Med. Microbiol. 60, 251–260 (2010).
Kamaguchi, A. et al. Effect of Porphyromonas gingivalis vesicles on coaggregation of Staphylococcus aureus to oral microorganisms. Curr. Microbiol. 47, 485–491 (2003).
Kamaguchi, A. et al. Adhesins encoded by the gingipain genes of Porphyromonas gingivalis are responsible for co-aggregation with Prevotella intermedia. Microbiology 149, 1257–1264 (2003).
Nokleby, H. et al. Safety review: two outer membrane vesicle (OMV) vaccines against systemic Neisseria meningitidis serogroup B disease. Vaccine 25, 3080–3084 (2007).
McCaig, W. D., Koller, A. & Thanassi, D. G. Production of outer membrane vesicles and outer membrane tubes by Francisella novicida. J. Bacteriol. 195, 1120–1132 (2013). The purification and characterization of OMVs from Francisella tularensis subsp. novidica demonstrates the coordinated regulation of spherical OMVs and tube-shaped vesicles that extend out from the bacterial surface.
Nieves, W. et al. A Burkholderia pseudomallei outer membrane vesicle vaccine provides protection against lethal sepsis. Clin. Vaccine Immunol. 21, 747–754 (2014).
Bishop, A. L. et al. Immunization of mice with Vibrio cholerae outer-membrane vesicles protects against hyperinfectious challenge and blocks transmission. J. Infect. Dis. 205, 412–421 (2012).
Park, M., Sun, Q., Liu, F., Delisa, M. P. & Chen, W. Positional assembly of enzymes on bacterial outer membrane vesicles for cascade reactions. PLoS ONE 9, e97103 (2014). The authors demonstrate a novel use of engineered OMVs as a platform for a three-enzyme cascade assembly pathway.
Baker, J. L., Chen, L., Rosenthal, J. A., Putnam, D. & Delisa, M. P. Microbial biosynthesis of designer outer membrane vesicles. Curr. Opin. Biotechnol. 29, 76–84 (2014).
Gujrati, V. et al. Bioengineered bacterial outer membrane vesicles as cell-specific drug-delivery vehicles for cancer therapy. ACS Nano 8, 1525–1537 (2014).
This work was supported by US National Institutes of Health grants R01GM099471 and R01AI079068 and by the Duke University Medical Center.
The authors declare no competing financial interests.
- Outer-membrane vesicle
(OMVs). Spherical portions (approximately 20–250 nm in diameter) of the outer membrane of Gram-negative bacteria, containing outer-membrane lipids and proteins, and soluble periplasmic content. OMVs are not the products of cell lysis.
Heterogeneous bacterial communities that are adherent to a surface and often resistant to antibiotics and other chemical disruptants. The attachments between biofilm bacteria and their substrates are typically mediated by extracellular proteins, DNA, polymeric fibres and carbohydrates.
(LPS). A glycolipid found exclusively in the outer leaflet of the outer membrane of Gram-negative bacteria. LPS has a phosphorylated diglucosamine backbone that is typically hexa-acylated and modified with a variable core oligosaccharide and a highly variable O antigen oligosaccharide or polysaccharide chain.
- Envelope crosslinks
Covalent and non-covalent links between the peptidoglycan layer and the outer membrane of Gram-negative bacteria.
- OMV cargo
(Outer-membrane vesicle cargo). Molecules that are within or associated with outer-membrane vesicles.
The process of producing vesicles. This process may be upregulated (hypervesiculation) or downregulated (hypovesiculation).
- Pseudomonas quinolone signal
(PQS). 2-heptyl-3-hydroxy-4-quinolone, an extracellular, hydrophobic quorum sensing signalling molecule that is produced by aerobically grown Pseudomonas aeruginosa. PQS production starts during early stationary phase and is maximal during late stationary phase.
- Peptide crosslinks
In the context of this Review: common peptide bonds between the third and fourth residues of two peptide tails in peptidoglycan, catalysed by D,D-transpeptidases and generating D-Ala–meso-diaminopimelic acid crosslinks.
- DAP–DAP peptide crosslinks
(Diaminopimelic acid–diaminopimelic acid peptide crosslinks). Fairly uncommon peptide bonds between the third residues of two peptide tails in peptidoglycan, catalysed by L,D-transpeptidases and generating meso-DAP–meso-DAP linkages.
- Lytic transglycosylases
Muramidases which cleave the β-(1,4) glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine and participate in cell wall turnover.
Portions of the cell envelope with distinct compositions or biophysical characteristics owing to their specific protein and/or lipid compositions.
Portions of the cell membrane with distinct compositions or biophysical characteristics owing to their specific protein and/or lipid compositions.
- Type IV secretion system
A multicomponent protein and DNA translocation complex that traverses the cell envelope and is evolutionarily related to the conjugation system.
Outer-membrane proteins composed of a carboxy-terminal β-barrel translocator domain and an amino-terminal passenger domain that passes through the interior of the barrel to face the external environment.
Surface-associated bacterial molecules that act as ligands or receptors for receptors or ligands on the mammalian host cell, respectively. Typically, adhesins are lectins that bind specific carbohydrate moieties of mammalian glycoproteins and glycolipids.
- σE heat shock response
A transcriptional cascade involved in the maintenance, adaptation and protection of the bacterial envelope. The pathway is induced by envelope stress and is mediated by the activation of the σ-factor σE on degradation of the anti-σ-factor in the cytoplasmic membrane, RseA.
- SOS response
The coordinated DNA repair pathways that are induced by bacteria in response to DNA damage.
A community of microorganisms that inhabit a particular site.
- Bacterial transformation
The stable genetic modification of bacteria with foreign DNA.
- Carbapenem resistance
The efflux or enzymatic destruction of carbapenems, a class of broad-spectrum β-lactam antibiotics that inhibit cell wall synthesis.
(S-layer). An outermost envelope layer that commonly occurs in archaea and is also found in bacteria. This layer is composed of a single type of protein or glycoprotein that self-assembles into a crystalline or lattice monomolecular structure.
- Haem toxicity
The generation of highly reactive hydroxyl radicals owing to the production of ferrous iron from haem within the reducing environment of cells.
Substances that induce cellular damage through interactions with DNA.
About this article
Cite this article
Schwechheimer, C., Kuehn, M. Outer-membrane vesicles from Gram-negative bacteria: biogenesis and functions. Nat Rev Microbiol 13, 605–619 (2015). https://doi.org/10.1038/nrmicro3525
The role of TolA, TolB, and TolR in cell morphology, OMVs production, and virulence of Salmonella Choleraesuis
AMB Express (2022)
Nature Protocols (2022)
Antigen-bearing outer membrane vesicles as tumour vaccines produced in situ by ingested genetically engineered bacteria
Nature Biomedical Engineering (2022)
The anti-inflammatory effects of Akkermansia muciniphila and its derivates in HFD/CCL4-induced murine model of liver injury
Scientific Reports (2022)
Chlamydia trachomatis inhibits apoptosis in infected cells by targeting the pro-apoptotic proteins Bax and Bak
Cell Death & Differentiation (2022)