Pseudomonas aeruginosa is a prevalent, Gram-negative bacterium and opportunistic pathogen that employs several different methods of motility to colonize surfaces, including swarming. Swarming is defined as a flagella-mediated community movement of cells through a thin liquid on a surface and is highly variable depending on environmental conditions and available nutrients. P. aeruginosa is capable of modifying its environment by producing rhamnolipid, a surfactant that lowers the surface tension of the thin liquid layer. Provided with glutamate as the sole carbon source rather than glucose, P. aeruginosa can swarm in a rhamnolipid-independent manner, but the extent of this motility depends upon composition of the environment, including surface moisture and nutrients. Noble agar and the agar substitute Gelzan were evaluated for this rhamnolipid-independent swarming (RIS) phenotype and a consistent protocol for investigating RIS was developed.
The RIS phenotype was observed under a variety of environmental and nutritional combinations to determine a causative action. In this work, I show that a ΔrhlAB mutant can swarm when growing on compounds associated with the tricarboxylic acid cycle. I also assert that this rhamnolipid-independent swarming phenotype is not a surfactant-mediated motility. Additionally, it is not related to any other obvious biochemical agent potentially used to promote swarm motility. Two regulatory genes identified to be involved in swarming, dipA and PA1728, were found not to specifically regulate this rhamnolipid-independent swarm behavior. A proteome analysis comparing ΔrhlAB swarming cells with wt did not suggest a clear regulator element that controls rhamnolipid-independent swarming, however, these results do provide some targeted direction for further study.
The phosphodiesterase DipA was identified as potentially involved in RIS, but was found to affect swarming motility through its role in regulation of cyclic-di-GMP. I show that doubling the nutrient composition of rich media (e.g. tryptone) leads to a significant increase in swarming and swimming, while doubling glucose or glutamate in minimal medium does not uniformly increase motility. Correspondingly, I have found that DipA is necessary to regulate intracellular levels of c-di-GMP in a nutritionally dependent-manner and swarming can occur when c-di-GMP levels are elevated on some nutrient media.