Cholera stool bacteria repress chemotaxis to increase infectivity

Effect of phage on the infectivity of Vibrio cholerae and emergence of genetic variants

Seasonal epidemics of cholera in Bangladesh are self-limited in nature, presumably due to phage predation of the causative Vibrio cholerae during the late stage of an epidemic, when cholera patients excrete large quantities of phage in their stools. To further understand the mechanisms involved, we studied the effect of phage on the infectivity and survival of V. cholerae shed in stools. The 50% infectious dose of stool vibrios in infant mice was approximately 10-fold higher when the stools contained a phage (1.8 x 10(3) to 5.7 x 10(6) PFU/ml) than when stools did not contain a detectable phage. In competition assays in mice using a reference strain and phage-negative cholera stools, the infectivity of biofilm-like clumped cells was 3.9- to 115.9-fold higher than that of the corresponding planktonic cells. However, the difference in infectivity of these two cell populations in phage-positive stools was significantly less than that in phage-negative stools (P = 0.0006). Coculture of a phage and V. cholerae or dilutions of phage-positive cholera stools in nutrient medium, but not in environmental water, caused rapid emergence of phage-resistant derivatives of the bacteria, and these derivatives lost their O1 antigen. In cholera stools and in intestinal contents of mice prechallenged with a mixture of V. cholerae and phage, the bacteria remained completely phage susceptible, suggesting that the intestinal environment did not favor the emergence of phage-resistant derivatives that lost the O1 antigen. Our results indicate that phages lead to the collapse of epidemics by modulating the required infectious dose of the bacteria. Furthermore, the dominance of phage-resistant variants due to the bactericidal selective mechanism occurs rarely in natural settings, and the emerging variants are thus unable to sustain the ongoing epidemic.

Genes induced late in infection increase fitness of Vibrio cholerae after release into the environment

The facultative pathogen Vibrio cholerae can exist in both the human small bowel and in aquatic environments. While investigation of the infection process has revealed many factors important for pathogenesis, little is known regarding transmission of this or other water-borne pathogens. Using a temporally controlled reporter of transcription, we focus on bacterial gene expression during the late stage of infection and identify a unique class of V. cholerae genes specific to this stage. Mutational analysis revealed limited roles for these genes in infection. However, using a host-to-environment transition assay, we detected roles for six of ten genes examined for the ability of V. cholerae to persist within cholera stool and/or aquatic environments. Furthermore, passage through the intestinal tract was necessary to observe this phenotype. Thus, V. cholerae genes expressed prior to exiting the host intestinal tract are advantageous for subsequent life in aquatic environments.

Exploiting cholera vaccines as a versatile antigen delivery platform

The development of safe, immunogenic and protective cholera vaccine candidates makes possible their use as a versatile antigen delivery platform. Foreign antigens can be delivered to the immune system with cholera vaccines by expressing heterologous antigens in live attenuated vectors, as fusion proteins with cholera toxin subunits combined with inactivated Vibrio cholerae whole cells or by exposing them on the surface of V. cholerae ghosts. Progress in our understanding of the genes expressed by V. cholerae during infection creates unprecedented opportunities to develop an improved generation of vaccine vectors to induce immune protection against a broad range of pathogenic organisms.

Complexity of rice-water stool from patients with Vibrio cholerae plays a role in the transmission of infectious diarrhea

At the International Centre for Diarrhoeal Disease Research, Bangladesh, one-half of the rice-water stool samples that were culture-positive for Vibrio cholerae did not contain motile V. cholerae by standard darkfield microscopy and were defined as darkfield-negative (DF(-)). We evaluated the host and microbial factors associated with DF status, as well as the impact of DF status on transmission. Viable counts of V. cholerae in DF(-) stools were three logs lower than in DF(+) stools, although DF(-) and DF(+) stools had similar direct counts of V. cholerae by microscopy. In DF(-) samples, non-V. cholerae bacteria outnumbered V. cholerae 10:1. Lytic V. cholerae bacteriophage were present in 90% of DF(-) samples compared with 35% of DF(+) samples, suggesting that bacteriophage may limit culture-positive patients from producing DF(+) stools. V. cholerae in DF(-) and DF(+) samples were found both planktonically and in distinct nonplanktonic populations; the distribution of organisms between these compartments did not differ appreciably between DF(-) and DF(+) stools. This biology may impact transmission because epidemiological data suggested that household contacts of a DF(+) index case were at greater risk of infection with V. cholerae. We propose a model in which V. cholerae multiply in the small intestine to produce a fluid niche that is dominated by V. cholerae. If lytic phage are present, viable counts of V. cholerae drop, stools become DF(-), other microorganisms bloom, and cholera transmission is reduced.

An in vivo expression technology screen for Vibrio cholerae genes expressed in human volunteers

In vivo expression technology (IVET) has been widely used to study gene expression of human bacterial pathogens in animal models, but has heretofore not been used in humans to our knowledge. As part of ongoing efforts to understand Vibrio cholerae pathogenesis and develop improved V. cholerae vaccines, we have performed an IVET screen in humans for genes that are preferentially expressed by V. cholerae during infection. A library of 8,734 nontoxigenic V. cholerae strains carrying transcriptional fusions of genomic DNA to a resolvase gene was ingested by five healthy adult volunteers. Transcription of the fusion leads to resolvase-dependent excision of a sacB-containing cassette and thus the selectable phenotype of sucrose resistance (Suc(R)). A total of approximately 20,000 Suc(R) isolates, those carrying putative in vivo-induced fusions, were recovered from volunteer stool samples. Analysis of the fusion junctions from >7,000 Suc(R) isolates from multiple samples from multiple volunteers identified 217 candidate genes for preferential expression during human infection. Of genes or operons induced in three or more volunteers, the majority of those tested (65%) were induced in an infant mouse model. VC0201 (fhuC), which encodes the ATPase of a ferrichrome ABC transporter, is one of the identified in vivo-induced genes and is required for virulence in the mouse model.

Processes controlling the transmission of bacterial pathogens in the environment

Many pathogens in the environment can be transmitted to human populations and cause outbreaks and epidemics. Transmission is a multifactorial process influenced by the physiology of the pathogen as it exits its initial host, the mechanisms it uses for surviving outside the host, the physiology of the pathogen as it enters the next susceptible host and its ability to establish a successful infection. Few studies so far have focused on the processes responsible for modulating microbial survival in non-host environments and the transmission dynamics between infected and susceptible hosts, as well as the interplay between hosts. A better understanding of these mechanisms is thus necessary for predicting and preventing future outbreaks.

RpoS controls the Vibrio cholerae mucosal escape response

Vibrio cholerae causes a severe diarrhoeal disease by secreting a toxin during colonization of the epithelium in the small intestine. Whereas the initial steps of the infectious process have been intensively studied, the last phases have received little attention. Confocal microscopy of V. cholerae O1-infected rabbit ileal loops captured a distinctive stage in the infectious process: 12 h post-inoculation, bacteria detach from the epithelial surface and move into the fluid-filled lumen. Designated the “mucosal escape response,” this phenomenon requires RpoS, the stationary phase alternative sigma factor. Quantitative in vivo localization assays corroborated the rpoS phenotype and showed that it also requires HapR. Expression profiling of bacteria isolated from ileal loop fluid and mucus demonstrated a significant RpoS-dependent upregulation of many chemotaxis and motility genes coincident with the emigration of bacteria from the epithelial surface. In stationary phase cultures, RpoS was also required for upregulation of chemotaxis and motility genes, for production of flagella, and for movement of bacteria across low nutrient swarm plates. The hapR mutant produced near-normal numbers of flagellated cells, but was significantly less motile than the wild-type parent. During in vitro growth under virulence-inducing conditions, the rpoS mutant produced 10- to 100-fold more cholera toxin than the wild-type parent. Although the rpoS mutant caused only a small over-expression of the genes encoding cholera toxin in the ileal loop, it resulted in a 30% increase in fluid accumulation compared to the wild-type. Together, these results show that the mucosal escape response is orchestrated by an RpoS-dependent genetic program that activates chemotaxis and motility functions. This may furthermore coincide with reduced virulence gene expression, thus preparing the organism for the next stage in its life cycle.

Vibrio cholerae, a pathogenic microbe, causes a severe diarrhoeal disease mainly in Third World countries. Although the pathogenicity of this organism has been intensively studied for more than a century, most research has focused on the initial stages of the infection, especially colonization of the intestine and virulence gene expression. However, the last stages of the infectious process have received very little attention. In the present manuscript, the authors use the rabbit ileal loop model of cholera to show how this organism, late in the infection, detaches from the epithelial surface and migrates into the luminal fluid, a process the authors termed the “mucosal escape response.” This study identifies, for the first time, how the alternative starvation sigma factor RpoS regulates this process. Features of this genetic program include the dramatic induction of genes involved in motility and chemotaxis functions. This study furthermore identifies RpoS as an important regulator of virulence gene expression and shows that the mucosal escape response may coincide with diminished virulence gene expression. This work is essential for understanding a key and under-appreciated step in the life cycle of this important human pathogen: its exit from the intestine and how this serves to prepare it for transmission into environmental reservoirs or to new human hosts.