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Description: TITLE: SOCIAL MOTILITY: EXPLORING INTERACTIONS AMONG SOIL MICROBES

Abstract:The soil bacterium Pseudomonas fluorescens Pf0-1 is a model for soil survival, persistence, and biocontrol. When co-cultured with the soil bacterium Pedobacter sp. V48, a novel social motility phenotype emerges, allowing the co-culture to move on a hard agar surface, an environment where the mono-cultures of both species are immotile. We have taken molecular genetic and experimental evolution approaches to investigate mechanisms underlying the interaction in this model community.

With our collaborators, we have used transposon mutagenesis screens to generate mutants with altered social motility. Among Pf0-1 mutants, there are several with hits in genes related to flagellum assembly and function. Flagella normally function in "swimming" locomotion of P. fluorescens in moist environments, distinct from the social motility requiring the Pedobacter partner. Phenotypes of flagellum gene mutants indicate a role for flagella in social motility, in addition to their role under swimming conditions. We are testing the hypothesis that flagella mediate contact and adhesion between the two strains. Data suggest a requirement for the flagellum filament protein; however, no clear pattern has yet emerged to provide a comprehensive explanation.

Motile co-cultures frequently spawn faster-moving sectors, in which one species has evolved to facilitate faster migration. From these spontaneous mutants we have identified genes involved in social motility, in a complementary approach to the transposon screens. From over 100 sectors emerged from wild-type interactions, 86% of isolates conferring fast social motility were evolved Pedobacter. Whole-genome sequencing has shown that among the emerged Pedobacter strains, many have mutations in a region encoding genes related to polysaccharide biosynthesis; repeated selection of such mutants identifies this locus as important for enhancing social motility. Many of these strains have a mucoid phenotype, indicating probable overproduction of extracellular polysaccharides, suggesting EPS as a key driver of social motility. Evolved Pf0-1 have mostly emerged as a second evolutionary step during co-culture with evolved Pedobacter strains which already confer faster social motility. While interaction of wild-type Pf0-1 and evolved Pedobacter resulted in social motility two times faster than the wild-type co-culture, interaction of evolved strains of both species resulted in a five-fold increase in social motility. These experiments not only reveal genes important for social motility, they will provide insight into the evolution of an apparently cooperative trait in soil bacteria.

These studies are uncovering novel ways by which these two bacteria communicate and behave, providing new insight into how members of complex soil communities interact. Our research may also lead to development of bacterial communities as biocontrol agents, and approaches to enhance the colonization of biotic surfaces in the rhizosphere.