Swarming in a Group. Swarming is the flagella-dependent translocation of a bacterial population across a solid surface. Swarming migration necessitates the cohesion of cells sharing the same shape, density, and flagellar mechanics, called rafting. Cells recruited to a raft participate in swarming migration while cells lost from the raft are immobile. Additionally, a reduction of surface tension mediated by damp substrates or surfactant synthesis is required for swarming. A final feature of swarming bacteria is the “swarming lag” where swimming cells are immobile for a period of time following surface contact likely to undergo physiological rearrangements required for swarming. Moving as a collective group provides swarming bacteria with benefits not shared by their swimming counterparts. Swarming bacteria exhibit increased resistance to antibiotics which is attributed to altered metabolic activity and outer membrane composition which leads to an increased oxidative response and reduced membrane permeability, respectively (187-192). As well, the multicellular nature of swarming promotes altruism, as only a subset of the population dies upon antibiotic exposure which provides a protective barrier for survivors. This phenomenon is dependent on cell density as individual surviving swarmer cells undergo cell death when exposed to antibiotics. Movement at higher velocities also promotes increased resistance. This raises the question as to whether swarming is altruistic or selfish behavior – as cell death would be inevitable for slower cells. Overall, the behavior promotes survival of the group at the cost of some individuals (188). Some bacteria couple the expression of virulence factors with swarming motility. Increased expression of hemolysin, IgA protease, and urease are coupled to swarming motility in P. mirabilis (64, 75, 78). The operon encoding phospholipase in S. marcescens is a part of the flagellar regulon (193). Additionally, the flagellar T3SS has been shown to act as a secretion apparatus for virulence proteins (194). Overall, swarming would provide a bacterial pathogen with the machinery to migrate to, exploit, and disperse from a site of infection. Swarming motility has been identified under laboratory conditions in the (i) firmicutes, (ii) α-proteobacteria, and (iii) γ-proteobacteria. These bacteria swarm under different conditions to form distinct patterns on agar plates as a result of unique regulatory mechanisms. Swarming in each of these groups is briefly discuss...
