Trends in Microbiology
ReviewVibrio biofilms: so much the same yet so different
Section snippets
Vibrios and biofilms
Vibrio species are ubiquitous in aquatic ecosystems. Although many Vibrio species are free living, a small group can form pathogenic or symbiotic interactions with eukaryotic hosts. Indeed, these Vibrio species alternate between growth within their hosts and prolonged survival in aquatic habitats. Vibrio cholerae, for example, causes periodic occurrences of the severe diarrheal disease cholera. These epidemics typically result from consumption of drinking water contaminated with the pathogen.
Flagella are involved in initial stages of biofilm formation by Vibrio spp.
Biofilm formation begins when a bacterium reaches and attaches to a surface. After the initial attachment, subsequent formation of microcolonies and 3D structures is mediated by movement and growth of attached bacteria. In many bacteria, flagella-mediated motility promotes the initial stages of biofilm formation, usually by enhancing movement towards and along the surface [12]. In vibrios, the impact of motility seems to extend beyond attachment.
In V. cholerae, loss of flagellar genes usually
Pili promote cell-surface and/or cell–cell interactions
In V. cholerae, at least three types of pili contribute to biofilm formation: mannose-sensitive haemagglutinin type IV pili (MSHA), toxin co-regulated pili (TCP) and chitin-regulated pili (ChiRP; formerly termed PilA) 19, 20, 21, 22. The relative importance of these pili varies under different conditions, and from strain to strain. MSHA, for example, is crucial for early attachment to abiotic surfaces in V. cholerae O1 El Tor strains, yet initial studies revealed no role in strain O139 [13].
Polysaccharides are the most prevalent component of Vibrio biofilms
Production of mature biofilms requires extracellular matrix components that hold the cells together and keep the biofilm attached to the surface. The capsular polysaccharide (CPS) or exopolysaccharide (EPS, or VPS in V. cholerae) loci involved in biofilm formation have been identified from numerous Vibrio spp. Expression of these loci is frequently correlated with biofilm-associated changes in colony morphology, in particular OP, rugose or wrinkled colonies (Box 1).
Transcriptional control of EPS production genes
In Vibrio species, regulation of exopolysaccharide production and biofilm formation is complex, and involves numerous transcriptional regulators, particularly two-component signal transduction and quorum sensing regulators (Figure 3). In a typical two-component system, a stimulus detected by a sensor histidine kinase (HK) is transformed into a cellular signal by a phosphorelay event that involves autophosphorylation of the HK at a conserved histidine residue. The phosphoryl group is then passed
C-di-GMP signaling and biofilm formation
C-di-GMP is a ubiquitous second messenger that controls the transition from a free-living, motile lifestyle to a biofilm lifestyle in many bacteria (reviewed in Ref. [57]), including vibrios 58, 59, 60, 61, 62. Increased c-di-GMP levels tend to promote biofilm formation and/or inhibit flagellar motility. C-di-GMP production and degradation is controlled by diguanylate cyclases (DGCs) and phosphodiesterases (PDEs), respectively [57] (Figure 4). Overexpression of these regulators tends to cause
Concluding remarks
Biofilm formation, particularly on a biotic, possibly nutritional surface, seems likely to provide a substantial survival advantage to aquatic organisms such as Vibrio species. That these organisms use similar traits and regulators to solve the problem of biofilm formation is not unexpected. That they use such diversity in approaches – the relative importance of the traits and regulators, and even the sense (positive or negative) of control – is surprising and thus has the potential to provide
Acknowledgements
We thank Emily Yip and Cindy DeLoney-Marino for photos of V. fischeri aggregates, Linda McCarter for V. parahaemolyticus pictures, and Kivanc Bilecen, Sinem Beyhan and Ates Gurcan for figure preparation. We also thank members of our laboratories, Karen Ottemann and Alan Wolfe for critical reading of the manuscript. Work in our laboratories investigating biofilm formation in Vibrio species was supported by NIH grants AI055987 to F.H.Y. and GM59690 to K.L.V.
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