The role of flagella in Pseudomonas-Pedobacter social motility across a surface

Bacteria often reside in multi-species communities where many behaviors result from interspecies relationships. In a two-species community, we show that co-culture of Pseudomonas fluorescens and Pedobacter sp. permits motility across a hard agar surface where neither species moves alone. Pseudomonas species engage in surface motility, including swimming and swarming, but these require moist environments. We are exploring the role of the Pseudomonas flagella in social motility. We deleted genes related to flagellar structure and function to study the importance of flagellar elements on the social phenotype. Using microscopy and swimming assays, we evaluate the effects of gene deletions on the presence and function of flagella in the resulting mutants, and evaluate the effect on social motility by observing the phenotype on both hard (2% w/v) and soft agar (1% w/v). Removal of the flagellar filament abolishes social motility, indicating a requirement for flagella in social motility. Removal of the flagellar motor also abolishes social motility, demonstrating that flagella must be functional. However, removal of membrane-spanning structural components, or part of the type III secretion system, results in mutants that lack flagella, but participate in a similar motile behavior with Pedobacter. Here we describe a role for flagella in motility of a two-species consortium across a hard agar surface, an environment considered non-permissive for flagellar motility. The requirement for both bacterial species indicates we are observing motility as a social phenotype, with a contribution from Pedobacter that enables the Pseudomonas flagella to function under conditions relevant in the natural soil environment.