Using gas as a raft

Proc. Natl. Acad. Sci. USA., published online 22 August 2011, doi:10.1073/pnas.1109169108

Credit: JOSHUA RAMSAY

The production of intracellular vesicles permeable to gases such as H₂, O₂, CO₂ or CH₄ has been shown to be important for buoyancy in various bacterial and archaeal species, but it has been mostly seen in planktonic microorganisms, whose physiology favors life at air-liquid interfaces. Ramsay et al. now show that the virulent Gram-negative bacteria Serratia 39906 also produces gas vesicles that serve a potentially important role in mobility to air-liquid interfaces. Using a transposon-based mutagenesis strategy, the authors found a gene cluster containing 19 open reading frames that is required to maintain an opaque colony phenotype, a typical measure of gas vesicle formation. Whereas a wild-type strain was buoyant and densely packed with gas vesicles, a strain lacking the gene cluster sedimented. The genes in the cluster were largely homologous to known gas vesicle proteins, such as a key structural protein GvpA, and to putative regulatory proteins. Monitoring the temporal expression patterns of gvpA1 indicated that gas vesicle formation was stimulated under conditions of low O₂. Finally, the authors found that gas vesicle formation required the quorum sensing molecule N-butanoyl-L-homoserine lactone and RsmA, an RNA-binding protein that simultaneously represses flagellar motility. These results suggest that these bacteria use oxygen- and quorum-sensing–dependent gas vesicles to control mobility when an air-liquid interface lifestyle is desirable but flagellar-based motion is too energetically costly.