Intestinal colonization resistance

Abstract

Dense, complex microbial communities, collectively termed the microbiota, occupy a diverse array of niches along the length of the mammalian intestinal tract. During health and in the absence of antibiotic exposure the microbiota can effectively inhibit colonization and overgrowth by invading microbes such as pathogens. This phenomenon is called 'colonization resistance' and is associated with a stable and diverse microbiota in tandem with a controlled lack of inflammation, and involves specific interactions between the mucosal immune system and the microbiota. Here we overview the microbial ecology of the healthy mammalian intestinal tract and highlight the microbe-microbe and microbe-host interactions that promote colonization resistance. Emerging themes highlight immunological (T helper type 17/regulatory T-cell balance), microbiota (diverse and abundant) and metabolic (short-chain fatty acid) signatures of intestinal health and colonization resistance. Intestinal pathogens use specific virulence factors or exploit antibiotic use to subvert colonization resistance for their own benefit by triggering inflammation to disrupt the harmony of the intestinal ecosystem. A holistic view that incorporates immunological and microbiological facets of the intestinal ecosystem should facilitate the development of immunomodulatory and microbe-modulatory therapies that promote intestinal homeostasis and colonization resistance.

Figure 1

Indigenous microbiota of the human gastrointestinal tract (GIT). The indigenous microbiota varies in density and composition along the length of the human GIT. The densities of colonizing microbes are shown at different locations within the GIT and some examples of distinct environmental niches available for microbial growth within the colon are highlighted (includes information from Cummings and Macfarlane61). Invading foreign microorganisms such as pathogens must actively compete against these indigenous microbial communities to successfully replicate and cause disease.

Figure 2

Microbial and immunological signatures of intestinal colonization resistance. Schematic of colon illustrating the associations between the intestinal microbiota composition and the immune system status during differing degrees of colonization resistance.