Coordinated surface activities in Variovorax paradoxus EPS.

Background: Variovorax paradoxus is an aerobic soil bacterium frequently associated with important biodegradative processes in nature. Our group has cultivated a mucoid strain of Variovorax paradoxus for study as a model of bacterial development and response to environmental conditions. Colonies of this organism vary widely in appearance depending on agar plate type.

Results: Surface motility was observed on minimal defined agar plates with 0.5% agarose, similar in nature to swarming motility identified in Pseudomonas aeruginosa PAO1. We examined this motility under several culture conditions, including inhibition of flagellar motility using Congo Red. We demonstrated that the presence of a wetting agent, mineral, and nutrient content of the media altered the swarming phenotype. We also demonstrated that the wetting agent reduces the surface tension of the agar. We were able to directly observe the presence of the wetting agent in the presence and absence of Congo Red, and found that incubation in a humidified chamber inhibited the production of wetting agent, and also slowed the progression of the swarming colony. We observed that swarming was related to both carbon and nitrogen sources, as well as mineral salts base. The phosphate concentration of the mineral base was critical for growth and swarming on glucose, but not succinate. Swarming on other carbon sources was generally only observed using M9 salts mineral base. Rapid swarming was observed on malic acid, d-sorbitol, casamino acids, and succinate. Swarming at a lower but still detectable rate was observed on glucose and sucrose, with weak swarming on maltose. Nitrogen source tests using succinate as carbon source demonstrated two distinct forms of swarming, with very different macroscopic swarm characteristics. Rapid swarming was observed when ammonium ion was provided as nitrogen source, as well as when histidine, tryptophan, or glycine was provided. Slower swarming was observed with methionine, arginine, or tyrosine. Large effects of mineral content on swarming were seen with tyrosine and methionine as nitrogen sources. Biofilms form readily under various culture circumstances, and show wide variance in structure under different conditions. The amount of biofilm as measured by crystal violet retention was dependent on carbon source, but not nitrogen source. Filamentous growth in the biofilm depends on shear stress, and is enhanced by continuous input of nutrients in chemostat culture.

Conclusion: Our studies have established that the beta-proteobacterium Variovorax paradoxus displays a number of distinct physiologies when grown on surfaces, indicative of a complex response to several growth parameters. We have identified a number of factors that drive sessile and motile surface phenotypes. This work forms a basis for future studies using this genetically tractable soil bacterium to study the regulation of microbial development on surfaces.