Swarming of <i>Pseudomonas aeruginosa</i> Is Dependent on Cell-to-Cell Signaling and Requires Flagella and Pili

TLDR

The study defines swarming as a third surface translocation mode in Pseudomonas aeruginosa, distinct from swimming and twitching. Swarming is triggered on semisolid agar (0.5–0.7%) under nitrogen limitation and by specific amino acids. Glutamate, aspartate, histidine, and proline support swarming, whereas arginine, asparagine, and glutamine do not; swarm edge cells are twice as long, flagella are present, and mutations in flagella, pili, or the las/rhl signaling systems reduce or abolish swarming, with rhamnolipids identified as the surfactant.

Abstract

We describe swarming in Pseudomonas aeruginosa as a third mode of surface translocation in addition to the previously described swimming and twitching motilities. Swarming in P. aeruginosa is induced on semisolid surfaces (0.5 to 0.7% agar) under conditions of nitrogen limitation and in response to certain amino acids. Glutamate, aspartate, histidine, or proline, when provided as the sole source of nitrogen, induced swarming, while arginine, asparagine, and glutamine, among other amino acids, did not sustain swarming. Cells from the edge of the swarm were about twice as long as cells from the swarm center. In both instances, bacteria possessing two polar flagella were observed by light and electron microscopy. While a fliC mutant of P. aeruginosa displayed slightly diminished swarming, a pilR and a pilA mutant, both deficient in type IV pili, were unable to swarm. Furthermore, cells with mutations in the las cell-to-cell signaling system showed diminished swarming behavior, while rhl mutants were completely unable to swarm. Evidence is presented for rhamnolipids being the actual surfactant involved in swarming motility, which explains the involvement of the cell-to-cell signaling circuitry of P. aeruginosa in this type of surface motility.