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