Biofilm formation and phenotypic variation enhance predation-driven persistence of Vibrio cholerae

Abstract

Persistence of the opportunistic bacterial pathogen Vibrio cholerae in aquatic environments is the principal cause for seasonal occurrence of cholera epidemics. This causality has been explained by postulating that V. cholerae forms biofilms in association with animate and inanimate surfaces. Alternatively, it has been proposed that bacterial pathogens are an integral part of the natural microbial food web and thus their survival is constrained by protozoan predation. Here, we report that both explanations are interrelated. Our data show that biofilms are the protective agent enabling V. cholerae to survive protozoan grazing while their planktonic counterparts are eliminated. Grazing on planktonic V. cholerae was found to select for the biofilm-enhancing rugose phase variant, which is adapted to the surface-associated niche by the production of exopolymers. Interestingly, grazing resistance in V. cholerae biofilms was not attained by exopolymer production alone but was accomplished by the secretion of an antiprotozoal factor that inhibits protozoan feeding activity. We identified that the cell density-dependent regulator hapR controls the production of this factor in biofilms. The inhibitory effect of V. cholerae biofilms was found to be widespread among toxigenic and nontoxigenic isolates. Our results provide a mechanistic explanation for the adaptive advantage of surface-associated growth in the environmental persistence of V. cholerae and suggest an important contribution of protozoan predation in the selective enrichment of biofilm-forming strains in the out-of-host environment.grazing ͉ resistance ͉ protozoa ͉ quorum sensing E pidemics of cholera, an acute intestinal infection caused by toxigenic strains of the facultative pathogen Vibrio cholerae, are a major public health problem in developing countries around the globe. Both toxigenic and nontoxigenic strains of V. cholerae are natural inhabitants of a wide range of aquatic ecosystems, including estuarine and coastal waters, that provide the environmental reservoir of virulent V. cholerae strains (1). The fact that many environmental nontoxigenic strains carry virulence genes (2, 3) and that the occurrence of epidemics coincides with the increased prevalence of the causative V. cholerae strain in the aquatic environment (4, 5) supports the notion of an environmental origin of toxigenic V. cholerae clones. This view has led to the hypothesis that cholera epidemics are triggered by environmental factors and selective forces governing aquatic microbial communities. In recent years, studies on the ecology of V. cholerae have considerably increased our understanding of physical and biological parameters that influence the persistence of V. cholerae in the environment and hold the potential to predict the outbreak of cholera epidemics (6).As a member of the natural bacterioplankton community, V. cholerae is an integral part of the pelagic microbial food web and is thus constrained in its growth and survival by the predatory action of bacterivorous protists, so...

Extracellular matrix structure governs invasion resistance in bacterial biofilms

“…Biofilm production is strongly implicated in V. cholerae disease transmission and environmental survival on biotic and abiotic surfaces (Watnick et al, 2001;Kierek and Watnick, 2003;Matz et al, 2005;Nielsen et al, 2006;Fong et al, 2010;Tamayo et al, 2010). We find that biofilms exclude immotile planktonic cells from gaining access to the interior and from remaining bound to the biofilm surface.…”

Availability of prey resources drives evolution of predator–prey interaction

“…The formation of clumps and biofilm is another common prey defence mechanism in bacteria, which is known to reduce the risk of predation by particle-feeding ciliates (e.g. Matz et al 2005;Meyer & Kassen 2007). According to our follow-up experiment, predation by T. thermophila can increase the biofilm formation of S. marcescens (V.-P. Friman, J.…”

“…Rapid evolution of prey traits can also have important correlated consequences on other species interactions. For example, predation by free-living protozoa has been suggested to increase survival of pathogenic bacteria outside their host (Matz et al 2005) and to maintain genetic diversity required for evading the host immune system ( Wildschutte et al 2004). Yet, long-term studies where both evolutionary and ecological dynamics of predator-prey interaction are considered simultaneously in different environmental conditions are rare.…”

Architectural differentiation reflects bacterial community structure in stream biofilms

“…Biofilm maturation could induce structural homogeneity due to the increased presence of larger building blocks (for example, algae) that shape the overall architecture (Besemer et al, 2007), and repeatedly observed sloughing may induce a certain turnover and loss of stratification of the community. In addition, protozoan grazing is known to affect bacterial growth and survival in biofilms (Matz et al, 2005) and may differentially shape community structure in the base and streamer.…”