Our understanding of bacterial cell shape has taken steps forward with the recent discovery of cytoskeletal elements such as cell-shape determinants, but there is still much to learn about how shape is generated and maintained.
The bacterial cell wall, with its peptidoglycan layer, has a primary role in maintaining cell shape. The penicillin-binding proteins (PBPs) carry out the reactions for synthesis and remodelling of peptidoglycan. Different PBPs have specific roles in cell division and elongation, and therefore in cell-shape determination.
Growth of the cell wall is not uniform, but is localized to specific regions. These regions of localized peptidoglycan synthesis vary among bacteria and often change during the cell cycle, reflecting different modes of cell growth (longitudinal, septal or polar).
Bacteria have counterparts of all three eukaryotic cytoskeletal protein classes: FtsZ for tubulin, MreB for actin and crescentin for intermediate filament proteins. FtsZ is essential for cell division, MreB is a rod shape determinant and crescentin is required for the curved-rod shape of Caulobacter crescentus. These proteins form internal structures within cells at locations where they are thought to influence peptidoglycan synthesis or remodelling.
Future research on the nature of the peptidoglycan synthesis machinery and its relationship with the bacterial cytoskeleton is key to our understanding of cell-shape generation.
Bacterial species have long been classified on the basis of their characteristic cell shapes. Despite intensive research, the molecular mechanisms underlying the generation and maintenance of bacterial cell shape remain largely unresolved. The field has recently taken an important step forward with the discovery that eukaryotic cytoskeletal proteins have homologues in bacteria that affect cell shape. Here, we discuss how a bacterium gains and maintains its shape, the challenges still confronting us and emerging strategies for answering difficult questions in this rapidly evolving field.
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Owing to space constraints, we were forced to eliminate references to many papers that we feel have contributed valuable ideas and data to the field and to our review. We extend our sincerest apologies to the authors of these papers. The authors are grateful to members of the Jacobs-Wagner laboratory for critical reading of the manuscript. Research in our laboratory is funded by the National Institutes of Health and by the Pew Scholars Programme in the Biological Sciences, sponsored by the Pew Charitable Trusts.
The authors declare no competing financial interests.
A covalently linked macromolecular structure made up of stiff glycan strands crosslinked by somewhat flexible peptide bridges. It gives the cell wall its strength. Also called 'murein', from Latin murus, wall.
A synonym for the 'sac-like' peptidoglycan molecule that surrounds the cytoplasmic membrane of a bacterium.
A cell in which the cell wall is either absent or disrupted, causing it to adopt a spherical shape.
- PENICILLIN-BINDING PROTEINS
A class of enzymes first discovered by their ability to bind labelled penicillin. They catalyse the reactions that are necessary to synthesize and modify peptidoglycan.
- TEICHOIC ACIDS
Phosphate-rich, anionic polysaccharides that are attached to the peptidoglycan of Gram-positive bacteria. In Bacillus subtilis, most are polyglycerol phosphate or polyribitol phosphate and, in the case of lipoteichoic acids, have lipid modifications that allow association with the cytoplasmic membrane.
An enzyme that catalyses the attachment of a peptidoglycan disaccharide-pentapeptide precursor molecule to an existing glycan strand by a β-1,4 glycosidic bond.
An enzyme that catalyses the formation of a peptide bond between adjacent polypeptide side chains, forming a flexible peptide bridge between glycan strands.
- PEPTIDE INTERBRIDGE
Additional amino acids that bridge the D-alanine in position 4 from one peptide with the dibasic amino acid in position 3 of the adjacent peptide. In the Gram-positive bacterium Staphylococcus aureus, for example, interbridges comprise five glycine residues.
- PEPTIDOGLYCAN HYDROLASES
A class of enzymes that break molecular bonds in peptidoglycan. They are required to allow insertion of new peptidoglycan and to enable cell division, but must be tightly regulated to prevent autolysis.
- ATOMIC FORCE MICROSCOPY
A technique in which a sharp tip is scanned across the surface of a sample, probing sample-tip interaction forces. The resulting 'image' is high resolution and, as no light is required, the sample can be hydrated in aqueous solutions.
The Min system comprises three proteins in Escherichia coli: MinC, MinD and MinE. Mutations in the min genes produce characteristic mini cells. The cooperative action of MinC, MinD and MinE proteins ensures the placement of the division site at the midcell.
- Z RING
The ring-shaped structure that is formed during cell division from FtsZ polymers. The Z ring recruits proteins that are required for septal wall synthesis and cell division.
An antibiotic that binds to the C-terminal D-alanine–D-alanine polypeptide of peptidoglycan precursors, preventing the transpeptidation reaction that is required for peptide crosslinking of glycan strands.
A class of wall-less bacteria that includes acholeplasmas, mycoplasmas and spiroplasmas. They have the simplest genomes of any self-replicating, free-living organisms but can retain defined shapes by virtue of internal cytoskeletons.
- CRYO-ELECTRON TOMOGRAPHY
A technique in which a specimen, embedded in vitreous ice, is imaged from multiple angles using electron microscopy. The resulting images are then combined to reconstruct the 3D structure of the specimen.
Describes multiple genes in an operon, a single transcriptional unit driven by a single promoter. Operons often contain genes encoding protein products that act in the same pathway.
Transcribed as a single gene.
- LYTIC TRANSGLYCOSYLASE
An enzyme that cleaves the bonds between adjacent aminosugar moieties in glycan strands of peptidoglycan, enabling new precursor molecules to be added.
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Cabeen, M., Jacobs-Wagner, C. Bacterial cell shape. Nat Rev Microbiol 3, 601–610 (2005). https://doi.org/10.1038/nrmicro1205
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