Colonization resistance is the mechanism whereby the intestinal microbiota protects itself against incursion by new and often harmful

microorganisms A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

. Colonization resistance was first identified in 1967, and it was initially referred to as antibiotic associated susceptibility. It was observed that animals being treated with the antibiotic streptomycin were susceptible to '' Salmonella enterica'' at doses 10,000 fold lower than the standard minimal infectious dose. This led to investigations about the mechanisms utilized by endogenous microbial populations that conferred protection against exogenous pathogens attempting to colonize the

gut flora Gut microbiota, gut microbiome, or gut flora, are the microorganisms, including bacteria, archaea, fungi, and viruses that live in the digestive tracts of animals. The gastrointestinal metagenome is the aggregate of all the genomes of the gut mi ...

. It has been observed that colonization resistance can occur within the host in a 'direct' or 'indirect' manner. 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 organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or may ...

in the intestinal lumen) or by producing growth inhibitors (e.g. ''Bacteroides thuringiensis'' can secrete bacteriocin that directly targets spore-forming ''

Clostridium difficile ''Clostridioides difficile'' (syn. ''Clostridium difficile'') is a bacterium that is well known for causing serious diarrheal infections, and may also cause colon cancer. Also known as ''C. difficile'', or ''C. diff'' (), is Gram-positive spec ...

'', thus inhibiting its growth through an unknown mechanism), that directly inhibits the colonizing pathogen. Indirect colonization resistance is thought to be mediated through the induction of immune responses in the host that concomitantly 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 Regenerating islet-derived protein 3 gamma (also Regenerating islet-derived protein III-gamma) is a protein that in humans is encoded by the ''REG3G'' gene. Intestinal paneth cells produce REG3G (or REG3 gamma) via stimulation of toll-like recep ...

and REGIIIβ, both anti-microbial peptides that target gram-positive bacteria. Scientists found that gut infections increase its microbiota's resistance to subsequent infections, that taurine is used in as a nutrient to nourish and train the microbiota for this by potentiating its production of sulfide and that the exogenous supply of taurine can induce this microbiota alteration.

References

{{DEFAULTSORT:Colonization resistance Bacteriology Digestive system Gut flora Environmental microbiology