Flow environment and matrix structure interact to shape spatial competition in Pseudomonas aeruginosa biofilms

Abstract

Bacteria commonly live in biofilms, which are dense microbial communities surrounded by a secreted extracellular matrix. The matrix can mediate the outcome of competition in biofilms, with matrix producers displaying growth advantages over non-producing strains under simple flow conditions. In Pseudomonas aeruginosa, a pathogen that is notorious for causing persistent respiratory infections in cystic fibrosis patients, complex flow regimes initiate a structural change in the biofilm matrix. Specifically, heterogeneous flow, which is characteristic of natural environments, induces formation of biofilm streamers that act to clog local flow. Whether this phenomenon influences the matrix-mediated competitive outcomes observed in simple flow regimes has not been investigated. Here, we employ microfluidics and standard fluorescent microscopy techniques to demonstrate an interaction between hydrodynamic flow, matrix organization, and biofilm population dynamics in P. aeruginosa. Under simple flow conditions, wild type P. aeruginosa produces more robust biofilms than isogenic matrix non-producing mutants. Irrespective of initial frequency in co-culture, wild type cells always increase in relative abundance in straight-chamber microfluidic devices bearing simple flow regimes. By contrast, in chambers with complex topographical features and irregular flow profiles that mimic natural habitats of P. aeruginosa, matrix-producing and non-producing strains coexist. This result stems from local clogging of flow by wild type biofilms, which generates regions of zero flow speed that favor occupation by matrix non-producers. The present findings connect the evolutionary stability of matrix production with the rate and spatial complexity of flow patterns, providing a potential explanation for the phenotypic variation in biofilm matrix secretion observed among bacteria in natural and artificial environments. Our results have relevance in the context of medicine and global health, in which links are emerging between biofilm growth and the acquisition of multidrug resistance in pathogens. Recent discoveries concerning the identification of the polysaccharide biofilm matrix components offer future areas of exploration, including the localization and timing of matrix production and how these particular components prevent non-matrix-producers from exploiting matrix secretion. Finally, the findings of the current study are limited to competition between strict matrix secretors and non-secretors within soil-like microenvironments. Future studies could incorporate bacterial isolates displaying a range of matrix production levels, and could investigate the influence of other complex microenvironments on biofilm spatial competition.