Quorum-regulated biofilms enhance the development of conditionally viable, environmental Vibrio cholerae

Abstract

The factors that enhance the waterborne spread of bacterial epidemics and sustain the pathogens in nature are unclear. The epidemic diarrheal disease cholera caused by Vibrio cholerae spreads through water contaminated with the pathogen. However, the bacteria exist in water mostly as clumps of cells, which resist cultivation by standard techniques but revive into fully virulent form in the intestinal milieu. These conditionally viable environmental cells (CVEC), alternatively called viable but nonculturable cells, presumably play a crucial role in cholera epidemiology. However, the precise mechanism causing the transition of V. cholerae to the CVEC form and this form's significance in the biology of the pathogen are unknown. Here we show that this process involves biofilm formation that is dependent on quorum sensing, a regulatory response that is controlled by cell density. V. cholerae strains carrying mutations in genes required for quorum sensing and biofilm formation displayed altered CVEC formation in environmental water following intestinal infections. Analysis of naturally occurring V. cholerae CVEC showed that organisms that adopt this quiescent physiological state typically exist as clumps of cells that comprise a single clone closely related to isolates causing the most recent local cholera epidemic. These results support a model of cholera transmission in which in vivo-formed biofilms convert to CVEC upon the introduction of cholera stools into environmental water. Our data further suggest that a temporary loss of quorum sensing due to dilution of extracellular autoinducers confers a selective advantage to communities of V. cholerae by blocking quorum-mediated regulatory responses that would break down biofilms and thus interfere with CVEC formation.biofilm formation | conditionally viable environmental cells (CVEC) | quorum sensing | transmissibility of cholera B acterial gene regulation in response to cell density, known as "quorum sensing," is a regulatory response thought to occur in bacterial communities through the sensing of extracellular signal molecules called autoinducers that are produced by members of the community (1, 2). Quorum-sensing systems (Fig. S1) such as those described in Vibrio cholerae, the causative agent of the epidemic diarrheal disease cholera, have been shown to regulate certain phenotypes, including biofilm formation and virulence (1-4). As a waterborne pathogen, V. cholerae is known to transit between the host intestinal milieu and a hypotonic aquatic environment during spreading epidemics of cholera (5), and both biofilm formation and quorum sensing have been proposed to influence transmission of V. cholerae (2-4, 6). Although phage predation (7-9), along with other regulatory changes (10, 11) that have been proposed to occur in vivo and in vitro, might also influence transmissibility, the genetic mechanisms that promote survival of V. cholerae in the natural aquatic environments (and thus support waterborne disease) remain unknown.Although cholera is a waterborne disea...

Nutrient-dependent, rapid transition of Vibrio cholerae to coccoid morphology and expression of the toxin co-regulated pilus in this form

“…It has been described that when V. cholerae enters a VBNC state, the bacteria are not culturable on conventional growth media, although the cells remain viable and display active metabolism (Rahman et al, 1994) and transcription (Lleò et al, 2000), and can divide again upon restoration of favourable environmental conditions (i.e. nutrient supplementation, increased temperature, host environment) (Kamruzzaman et al, 2010;Nilsson et al, 1991;Smith & Oliver, 2006a).…”

“…GbpA, a characterized GlcNAc-binding protein, is an example of a V. cholerae colonization factor utilized in both the host and the aquatic environment . In addition, pathogenic strains of a variety of bacterial species have been characterized to retain and express virulence factors when in the VBNC state (Du et al, 2007;Fischer-Le Saux et al, 2002;Rahman et al, 1996;Smith & Oliver, 2006b), and also maintain antibiotic resistance (Kamruzzaman et al, 2010;Lleò et al, 2003). In the host environment, virulence factors such as TCP are required for colonization and persistence in the small intestine.…”

“…Discoveries in the past 20 years have revealed the existence of a dormant, viable but non-culturable (VBNC) state into which V. cholerae enters in response to nutrient deprivation, decreased temperature, and other environmental signals (reviewed by Colwell, 2000;Oliver, 2005;Rice et al, 2000). Alternatively, these environmental survival forms of bacteria have also been recently referred to as 'conditionally viable environmental cells' (CVEC) (Kamruzzaman et al, 2010). When in this state, the bacteria are not culturable on conventional growth media, although the cells remain viable and display active metabolism (Rahman et al, 1994) and transcription (Lleò et al, 2000), and can divide again upon restoration of favourable environmental conditions (i.e.…”

“…When in this state, the bacteria are not culturable on conventional growth media, although the cells remain viable and display active metabolism (Rahman et al, 1994) and transcription (Lleò et al, 2000), and can divide again upon restoration of favourable environmental conditions (i.e. nutrient supplementation, increased temperature, host environment) (Kamruzzaman et al, 2010;Nilsson et al, 1991;Smith & Oliver, 2006a). In the laboratory, incubating bacterial cells at decreased temperatures and in nutrientlimiting growth media promotes the VBNC state, after which the bacteria fail to form colonies on solid media (Carroll et al, 2001;Chaiyanan et al, 2001Chaiyanan et al, , 2007.…”

“…Culturing V. cholerae from the aquatic environment by conventional methods has been notoriously difficult, although V. cholerae cells are detectable in environmental samples by fluorescent antibodies and other methods Kamruzzaman et al, 2010). Discoveries in the past 20 years have revealed the existence of a dormant, viable but non-culturable (VBNC) state into which V. cholerae enters in response to nutrient deprivation, decreased temperature, and other environmental signals (reviewed by Colwell, 2000;Oliver, 2005;Rice et al, 2000).…”