Surface-Associated Flagellum Formation and Swarming Differentiation inBacillus subtilisAre Controlled by theifmLocus

Abstract

Knowledge of the highly regulated processes governing the production of flagella in Bacillus subtilis is the result of several observations obtained from growing this microorganism in liquid cultures. No information is available regarding the regulation of flagellar formation in B. subtilis in response to contact with a solid surface. One of the best-characterized responses of flagellated eubacteria to surfaces is swarming motility, a coordinate cell differentiation process that allows collective movement of bacteria over solid substrates. This study describes the swarming ability of a B. subtilis hypermotile mutant harboring a mutation in the ifm locus that has long been known to affect the degree of flagellation and motility in liquid media. On solid media, the mutant produces elongated and hyperflagellated cells displaying a 10-fold increase in extracellular flagellin. In contrast to the mutant, the parental strain, as well as other laboratory strains carrying a wild-type ifm locus, fails to activate a swarm response. Furthermore, it stops to produce flagella when transferred from liquid to solid medium. Evidence is provided that the absence of flagella is due to the lack of flagellin gene expression. However, restoration of flagellin synthesis in cells overexpressing D or carrying a deletion of flgM does not recover the ability to assemble flagella. Thus, the ifm gene plays a determinantal role in the ability of B. subtilis to contact with solid surfaces.

Swarming Differentiation and Swimming Motility inBacillus subtilisAre Controlled byswrA, a Newly Identified Dicistronic Operon

“…Plates were incubated for up to 24 to 48 h at 37°C, and swarm cell differentiation was analyzed as previously described (36,37). For microscopic examination of the cells, samples were obtained by slide overlaying on isolated colonies.…”

“…The behavior of a Bacillus subtilis ifmP mutant (PB5249) isolated from the laboratory strain PB1831 was recently described (37). Like other laboratory strains tested for swarming motility, PB1831, although motile, was nonswarming and did not produce flagella when transferred from liquid onto solid media.…”

“…Swarming, therefore, is regarded as a behavioral response to the surface, which provides flagellated bacteria with the ability to act as a multicellular population rapidly colonizing nutrient-rich solid substrates (38). Swarming of many different gram-positive and gram-negative bacteria has been described, and it is increasingly recognized in several soil Bacillus species (9,20,21,36,37).…”

“…Collective differentiation of swimmers into swarmers has been reported to be critically dependent on cell density signals (6,24), and a role for surfactants in swarming differentiation has been proposed, as they may be secreted in response to cell population density signals (24). However, although the forward movement of swarm cells is often encased within a wetting slime of diverse composition (22,39), surfactants are more likely to facilitate migration of swarm cells (13) rather than induce swarming differentiation (4,21,37). Signals evoking a chemotactic response have also been suggested to play a role in swarming differentiation; nevertheless, chemotaxis itself is not required for the outward migration of swarm cells, and it is still not known whether the chemotaxis sensory system plays a role in swarming differentiation (2,36).…”

Comparative Analysis of the Development of Swarming Communities ofBacillus subtilis168 and a Natural Wild Type: Critical Effects of Surfactin and the Composition of the Medium

“…A recent study (20) also demonstrated swarming associated with a mutation leading to hyperflagellation, under conditions where the parental laboratory strain normally failed to swarm. However, other studies have described forms of swarming in B. subtilis apparently independent of flagella or surfactin (6,12), (14).…”