R-062. Matrix Production Is Driven by Both Conflict and Cooperation within Biofilms

W. Kim1,2, K. R. Foster2, S. B. Levy1;
1Tufts Univ., Boston, MA, 2Harvard Univ., Cambridge, MA.

A defining feature of biofilm life is that bacteria encase themselves in a matrix of various extracellular polymeric substances (EPS). The prevailing explanation for matrix production is that it cooperatively protects the biofilm community from environmental stresses, including antimicrobials. However, this view presents a serious logical problem: what prevents the emergence of “cheater” strains that use the matrix but do not contribute, leading to the eventual collapse of matrix production. One possibility is that matrix, and indeed biofilms, are not so cooperative after all. A recent individual-based simulation predicted that the secret lies in competition over oxygen: EPS producers push their lineage up into better oxygenic conditions and simultaneously suffocate the neighboring lineages. We have isolated a mucoid variant (MV) that consistently emerged within colonies of Pseudomonas fluorescens Pf0-1 (WT). Carbohydrate analyses revealed that MV overproduces a succinoglycan-like EPS, and its genetic loci were mapped by transposon mutagenesis to a previously uncharacterized cluster sharing significant homology to the succinoglycan biosynthesis genes from other soil and rhizospheric bacteria. MV did not emerge from colonies of EPS deficient-mutants, suggesting that the emergence of MV from WT was solely linked to overproduction of EPS. Utilizing colonies on the agar surface as a model of biofilm, we set up a series of pair-wise competition experiments between MV and WT. Even when initially outnumbered by six orders of magnitude, MV dominated and eventually draped over the WT cells. In contrast, under oxygen-limiting conditions, emergence of MV from WT colonies was not readily observed and MV was no longer as dominant in competition experiments. These observations strongly support the simulation predictions and the working hypothesis that matrix production emerges from the tension between within-strain cooperation and between-strain competition.