Fig. 1.

Commensal gut microbiota-mediated colonisation resistance. (1) Gut commensals compete with pathogens for nutrients that are essential to support their growth. (2) Some gut commensals produce small antimicrobial peptides (‘bacteriocins’), which can inhibit pathogen growth. (3) Metabolism of bile acids by gut commensals can reduce germination in spore-forming pathogens and inhibit vegetative growth. (4) Gut commensals produce SCFAs that inhibit pathogen growth through intracellular acidification. (5) Gut commensals produce butyrate and valerate, which maintain epithelial membrane barrier integrity and prevent pathogen translocation from the gut to the bloodstream. (6) Gut commensals produce butyrate, which is utilized by colonic epithelial cells as an energy source in a process that consumes oxygen. This creates an environment that is less supportive of the growth of facultatively anaerobic pathogens.