Immune-Microbiota Interplay and Colonization Resistance in Infection

Commensal microbial communities inhabit biological niches in the mammalian host, where they impact the host's physiology through induction of "colonization resistance'' against infections by a multitude of molecular mechanisms. These colonization-regulating activities involve microbe-microbe and microbe-host interactions, which induce, through utilization of complex bacterial networks, competition over nutrients, inhibition by antimicrobial peptides, stimulation of the host immune system, and promotion of mucus and intestinal epithelial barrier integrity. Distinct virulent pathogens overcome this colonization resistance and host immunity as part of a hostile takeover of the host niche, leading to clinically overt infection. The following review provides a mechanistic overview of the role of commensal microbes in modulating colonization resistance and pathogenic infections and means by which infectious agents may overcome such inhibition. Last, we outline evidence, unknowns, and challenges in developing strategies to harness this knowledge to treat infections by microbiota transfer, phage therapy, or supplementation by rationally defined bacterial consortia.