Swarming is a specialized form of flagella-driven surface motility displayed by several bacterial genera, which shares features with other surface phenomenon such as biofilm formation and host invasion. Swarmer cells generally make more flagella, turn down cell division, and excrete surfactants and polysaccharides that aid their advancement across the surface.

Our studies with swarming-defective mutants in the chemotaxis signaling pathway of Salmonella typhimurium have led to some surprising discoveries. When propagated on the surface of an agar plate, the chemotaxis (che) mutants have shorter flagella and are less hydrated than wild-type cells. It seems that the flagella sense external wetness, and when external hydration is limiting, the flagella inhibit their own growth by blocking the secretion of flagellin subunits and the export of the transcriptional inhibitor FlgM, thereby switching off the synthesis of further flagellum components. We have found that the specialized secretion systems responsible for the export and assembly of flagella and for the secretion of bacterial virulence factors are jointly regulated by this sensing system. Interesting in this regard is the structural resemblance between the flagellar Basal Body and the virulence organelle referred to as the Needle Complex (see below). These two structures have homologous secretion systems for delivering proteins to the outside. Ongoing experiments are aimed at understanding how the flagellum acts as a sensor, how this signal is conveyed to the virulence system, and whether there are shared secretion components between the two systems.