Commensal bacteria mediated defenses against pathogens

• Commensal bacteria directly inhibit invasive pathogen colonization.
• The microbiota indirectly inhibit pathogens by stimulating host defense mechanisms.
• Pathogens exploit host and commensal bacteria-derived products to grow.
• Manipulating the microbiota is an effective therapeutic option against infection.

Commensal bacterial communities residing within the intestinal lumen of mammals have evolved to flourish in this microenvironment. To preserve this niche, commensal bacteria act with the host to prevent colonization by invasive pathogens that induce inflammation and disrupt the intestinal niche commensal bacteria occupy. Thus, it is mutually beneficial to the host and commensal bacteria to inhibit a pathogen's ability to establish an infection. Commensal bacteria express factors that support colonization, maximize nutrient uptake, and produce metabolites that confer a survival advantage over pathogens. Further, commensal bacteria stimulate the host's immune defenses and drive tonic expression of anti-microbial factors. In combination, these mechanisms preserve the niche for commensal bacteria and assist the host in preventing infection.