Physical and Environmental Factors Affecting the Swarm Motility of Pseudomonas aeruginosa

Pseudomonas aeruginosa is a ubiquitous, Gram-negative bacterium that utilizes several different modes of motility to colonize surfaces, including swarming, which is the coordinated movement of cells over surfaces in groups. Swarming facilitates surface colonization and biofilm development for P. aeruginosa, and it is known that swarming behavior is influenced by changes in surface moisture and nutrient composition. Variations in assay preparation factors, such as nutrient-agar medium volume, curing time, incubation humidity, agar variety and carbon source, can greatly influence swarming behavior once cells are added to these assays. A range of environmental and preparation factors were investigated to determine their effect(s) on P. aeruginosa swarm motility, and a set of general guidelines for increasing swarm assay reproducibility were developed. While swarming requires a functional flagellum, the involvement of type IV pili (TFP) appendages in Pseudomonas aeruginosa swarming is unclear. The influence of TFP on swarming motility was investigated using a combination of iterative laboratory experiments and in silico computer simulations. It was determined that TFP preferentially interact with other TFP during P. aeruginosa swarming, and that these interactions promote cell-cell associations that allow for moderation of swarm motility when needed, such as in the presence of a toxic antibiotic. While previous studies have shown surface-attached bacterial biofilms to be highly resistant to heavy metal toxicity, little is known about the influence of heavy metals upon surface motile bacteria and developing biofilms. A variety of laboratory assays were utilized to examine differences in bacterial behavior in response to two metals, cadmium and nickel. Metal resistance was shown to vary with motility mode (swimming vs. swarming), but did not correlate with rhamnolipid production, as was originally hypothesized; while cell products such as rhamnolipid were shown to bind heavy metals (as expected) and should limit metal bioavailability, the results suggest at least one additional mechanism (as yet undetermined) that promotes cell survival during swarming in the presence of these heavy metals.