Colonization resistance is the mechanism whereby the microbiome protects itself against incursion by new and often harmful microorganisms.
[1][2][3] Colonization resistance was first identified in 1967, and it was initially referred to as antibiotic-associated susceptibility.
[4] This led to investigations about the mechanisms utilized by endogenous microbial populations that conferred protection against exogenous pathogens attempting to colonize the gut flora.
[5] The former refers to particular components of the microbiota directly competing with exogenous pathogens for nutritional niches (e.g. E. coli directly competes with Citrobacter rodentium for carbohydrates in the intestinal lumen[6]) or by producing growth inhibitors (e.g. Bacteroides thuringiensis can secrete bacteriocin that directly targets spore-forming Clostridioides difficile, thus inhibiting its growth through an unknown mechanism),[7] that directly inhibit the colonizing pathogen.
An example of this has been observed with B. thetaiotaomicron, which can induce the host to produce antimicrobial C-type lectins REGIIIγ and REGIIIβ, both anti-microbial peptides that target gram-positive bacteria.