| Swarming is a specialized form of social motility that has been associated with virulence and allows bacteria to move over and colonize surfaces. In many organisms swarming is associated with cell elongation and increased production of flagella. Here we use DNA microarrays to study gene expression in swarmer cells of the marine bacterium and human pathogen Vibrio parahaemolyticus . Because signals that induce swarmer cell differentiation in V. parahaemolyticus have been identified, we could induce differentiation in broth, bypassing complications inherent in comparisons of broth-grown to plate-grown cells. We identified 104 genes (85 induced, 19 repressed) that were differentially regulated in swarmer as compared to swimmer cells. This gene set encodes the lateral flagella (∼40 genes), virulence factors, morphogenesis proteins, regulatory molecules and more than a dozen hypothetical proteins. Additional microarrays probed the LafK regulon and led to some unanticipated findings, including a laf operon that is outside of LafK control and several new LafK-dependent genes, some of which do not appear to encode proteins associated with the lateral flagellum.;Using fluorescence microscopy, we found that the key cell division protein FtsZ fails to assemble into a ring structure at the inner face of the cytoplasmic membrane in filamentous swarmer cells. DNA microarrays revealed a potential explanation for the lack of FtsZ rings---sulA (VP 1264) was induced during growth on a plate. In enteric bacteria SulA has been shown to bind and sequester FtsZ. A V. parahaemolyticus sulA mutant was constructed and characterized. The mutant swarmed at normal rates but microscopy revealed mostly short cells, implying that SulA contributes to swarmer cell elongation but elongation is not necessary for robust swarming motility under laboratory conditions. LafK was also required for elongation, but not for sulA induction. We propose the following model for how SulA and LafK work together to inhibit cell division during swarming. Elevated levels of SulA partially deplete the pool of FtsZ, while LafK-dependent elaboration of hundreds of lateral flagella creates a "forest" of basal bodies with few unobstructed sites in the cytoplasmic membrane where FtsZ can assemble into a ring structure. |
