Fluctuations in the composition and diversity of the gut microbial community can affect organismal health in various ways. Low microbial diversity in the gut can increase a person's susceptibility to potential pathogens, such as Salmonella spp. And changes in the balance between the two bacterial taxa most prominent in the gut, Bacteroidetes and Firmicutes, are associated with conditions like obesity, diabetes, chronic inflammatory bowel disease, gastrointestinal cancer and stress. Medical researchers have therefore suggested that the ability to regulate microbial community composition might provide opportunities to treat these and other diseases.

One way of regulating community composition is by manipulating interactions between community members. Microbes use many mechanisms to detect and respond to the presence of other microbes. For example, bacteria produce small molecules called autoinducers in amounts proportional to the size of the population. By detecting these autoinducers, bacteria can regulate certain behaviors at the population level, including attachment, biofilm formation, motility and virulence. Such activities are relevant in the gut environment, leading researchers to believe that quorum sensing probably has a role in gut microbial community interactions.

Whereas many autoinducers are species-specific, autoinducer-2 (AI-2) functions across the bacterial kingdom; AI-2 produced by one species can influence others. This feature makes AI-2 an excellent candidate for mediating interactions in the mammalian gut microbial community, which includes hundreds of species. To investigate this potential role of AI-2, Karina Bivar Xavier (Instituto Gulbenkian de Ciência, Oeiras, Portugal) and colleagues manipulated AI-2 levels in the mouse gut using specially engineered Escherichia coli mutants and examined how these manipulations affected the composition of the gut microbiota.

The mice were first treated with streptomycin, which disturbed the normal microbiota, allowing E. coli to colonize the gut. Streptomycin reduced total bacterial load, decreased microbial diversity and substantially altered the relative proportions of Bacteroidetes and Firmicutes (from 48% and 43%, respectively, to 90% and 0.7%, respectively, after 28 d of treatment). Introduction of E. coli mutants that overproduce AI-2 reversed the effects of streptomycin by restoring the ratio of Bacteroidetes and Firmicutes bacteria (Cell Rep. doi:10.1016/j.celrep.2015.02.049; published online 19 March 2015). The results indicate that AI-2 can modulate the composition of the gut microbial community.

Xavier's group concluded, “This work highlights the potential gain from understanding and manipulating the bacterial chemical repertoire operating within the bacterial community inhabiting the gut, towards the aim of tailoring the composition of the microbiota to our benefit.”