Commensal - pathogen interactions in the intestinal tract

In both humans and mice, the intestinal microbiota are essential for development of a mature immune system, and for maintaining immunological homeostasis in the intestinal tract. Although many hundreds of bacterial species colonize the mammalian gut, the balance between these species varies widely between individuals in a population. Importantly, the presence or absence of specific species of bacteria within the microbiota can be instrumental in driving immunological differentiation in the intestinal tract. For example, segmented filamentous bacteria stimulate naïve T cells toward an inflammatory T helper (Th)17 phenotype, while a number of bacterial species promote immunosuppressive T regulatory cell (Treg) differentiation. More broadly, germ-free (GF) mice raised in the absence of commensal microbes show impaired immunological development, and a systemic skewing toward Th2 responses. While GF mice are generally more susceptible to infections with bacterial or viral agents, when they are infected with the gastrointestinal helminth parasite Heligmosomoides polygyrus, helminth survival is markedly reduced in these animals compared with conventionally colonized hosts. H. polygyrus establishes in the anterior small intestine of mice, alongside a substantial microbial community. As the outcome of H. polygyrus infection is likely dependent on the immediate cytokine environment, we postulated that the composition of the microbiota may alter