Editors' ChoiceMicrobiology

To Swim or Float?

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Sci. Signal.  13 Sep 2011:
Vol. 4, Issue 190, pp. ec251
DOI: 10.1126/scisignal.4190ec251

Bacteria have evolved several mechanisms for movement, including “swimming” of individuals with flagella or pili, “swarming” through the formation of a cooperative community, and “floating” through the formation of gas-filled intracellular proteinaceous vacuoles. Ramsay et al. report that in strains of Serratia, types of pathogenic enterobacteria that can cause infections in hospital settings, the signal to form gas vesicles involves a quorum-sensing molecule (a specific N-acyl homoserine lactone) and the RNA-binding protein RsmA, which inhibits flagellar-based movement. A screen for mutants that affected the opaque colonial phenotype (opacity is due to the gas vesicles) identified a 16.6-kb region that included 11 genes encoding proteins involved in gas vesicle synthesis, as well as genes encoding candidate regulators of gas vesicle synthesis—a putative transcriptional regulator (GvrA), a sensor histidine kinase (GvrB), and a response regulator (GvrC). Reporter gene experiments with the promoter of the vesicle structural protein–encoding gene GvpA1 indicated that expression was highest in stationary phase cultures or cultures in which exposure to oxygen was limited (sealed vessels covered with mineral oil to reduce aeration). A GvrA reporter suggested that expression of this gene increased during the transition into stationary phase, but before the induction of GvpA1, and in a gvrA mutant, induction of the GvpA1 reporter by reduced aeration was compromised. Genetic disruption of the RsmA pathway compromised expression of the GvpA1 and GvrA reporters, stimulated swarming, and inhibited production of gas vesicles and flotation. Genetic disruption of the quorum-sensing pathway also compromised induction of the GvpA1 and GvrA reporters, gas vesicle formation, and bacterial buoyancy. Transfer of the gas vesicle gene cluster into Escherichia coli promoted the formation of gas vesicles, and this did not require reduced exposure to oxygen, suggesting that E. coli lack the negative regulatory elements of this pathway. This work suggests that under specific conditions, the quorum-sensing pathway can function as a morphogenic signal and that motility mechanisms can be optimized for specific conditions through inverse regulation of flagellar and flotation motility mechanisms.

J. P. Ramsay, N. R. Williamson, D. R. Spring, G. P. C. Salmond, A quorum-sensing molecule acts as a morphogen controlling gas vesicle organelle biogenesis and adaptive flotation in an enterobacterium. Proc. Natl. Acad. Sci. U.S.A. 108, 14932–14937 (2011). [Abstract] [Full Text]

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