Invasion of the human small intestine by the enteropathogen Vibrio cholerae leads to acute diarrhoeal disease. In a new study, Gordon and colleagues examined the role of the human gut microbiota in V. cholerae infection and showed that one of its members, Ruminococcus obeum, decreases the expression of V. cholerae virulence factors to limit colonization.
The authors used a metagenomics approach to analyse the bacterial composition of faecal samples collected from seven infected Bangladeshi adults and healthy controls. They showed that during the early phases of diarrhoea, the microbiota was divergent from the healthy adult microbiota, with V. cholerae as well as Streptococcus and Fusobacterium species being dominant, while during recovery, a healthy microbial community, including Bacteriodes, Ruminococcus and Prevotella species, was restored. Interestingly, several bacterial species that have previously been shown to be associated with the assembly and maturation of the gut microbiota in healthy Bangladeshi children were present in samples collected during the recovery phase. This suggests that similar processes underlie normal postnatal community assembly and microbiota restoration after disruption.
R. obeum restricted pathogen colonization and reduced the expression of V. cholerae virulence factors
To determine if these recovery-associated species have a role in limiting V. cholerae infection, the authors colonized gnotobiotic mice with an artificial community composed of 14 human gut bacterial species that correlated with recovery from cholera in adults and with normal microbiota maturation in children. Following infection, the levels of V. cholerae in mice with the artificial human gut microbiota were lower compared with control gnotobiotic mice. In addition, the authors found that the abundance of one of the members of the artificial microbiota, R. obeum, was consistently increased following infection and, using mono- and co-colonization experiments, they showed that R. obeum restricted pathogen colonization and reduced the expression of V. cholerae virulence factors.
So, how does R. obeum limit V. cholerae colonization? Expression of V. cholerae virulence factors is repressed by an interspecies quorum sensing pathway that involves the signalling molecule autoinducer 2 (AI-2). Consistent with this, the authors found that expression of R. obeum AI-2 and the AI-2 synthase LuxS were increased following V. cholerae invasion and that R. obeum AI-2 represses V. cholerae virulence factor expression. Finally, the authors demonstrated that R. obeum-mediated modulation of V. cholerae colonization does not depend on the V. cholerae AI-2 sensor LuxP or other downstream regulatory genes that have been implicated in V. cholerae virulence gene expression, which suggests that R. obeum regulates virulence through a novel pathway.
Future studies should identify other members of the human microbiota that use quorum sensing — or possibly other mechanisms — to limit enteropathogen infections.
Hsiao, A. et al. Members of the human gut microbiota involved in recovery from Vibrio cholerae infection. Nature http://dx.doi.org/10.1038/nature13738 (2014)
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Du Toit, A. First aid kit for cholera. Nat Rev Microbiol 12, 726 (2014). https://doi.org/10.1038/nrmicro3366