Non-motile subpopulations of Pseudomonas aeruginosa repress flagellar motility in motile cells through a type IV pili- and Pel-dependent mechanism

The downregulation of P. aeruginosa flagellar motility is a key event in biofilm formation, host-colonization, and the formation of microbial communities, but the external factors that repress motility are not well understood. Here, we report that under swarming conditions, motility can be repressed by cells that are non-motile due to the absence of a flagellum or flagellar rotation. Non-motile cells, due to mutations that prevent either flagellum biosynthesis or rotation, present at 5% of the total population suppressed swarming of wild-type cells under the conditions tested in this study. Non-motile cells required functional type IV pili and the ability to produce the Pel exopolysaccharide to suppress swarming by the motile wild type. In contrast, motile cells required only type IV pili, but not Pel production, in order for swarming to be repressed by non-motile cells. We hypothesize that interactions between motile and non-motile cells may enhance the formation of sessile communities including those involving multiple genotypes, phenotypically-diverse cells, and perhaps other species. Significance Our study shows that, under the conditions tested, a small population of non-swarming cells can impact the motility behavior of the larger population. The interactions that lead to the suppression of swarming motility require type IV pili and a secreted polysaccharide, two factors with known roles in biofilm formation. These data suggest that interactions between motile and non-motile cells may enhance the transition to sessile growth in populations and promote interactions between cells with different genotypes. ### Competing Interest Statement The authors have declared no competing interest.