Effect on colonization resistance: an important criterion in selecting antibiotics.

Infections in humans are most often caused by aerobic microorganisms colonizing the digestive tract. Aerobic microorganisms are constantly entering the digestive tract with food, but colonization is resisted by autochthonous anaerobic flora (microbial colonization resistance) and by host-related factors (physiologic colonization resistance). Antibiotics to which the autochthonous anaerobic flora are sensitive and that achieve sufficiently high concentrations at the sites of colonization will reduce colonization resistance. Consequently, resistant aerobic flora of the digestive tract may reach high concentrations, increasing the risk of superinfection. Therefore, when choosing antimicrobial agents for therapy, the effect on colonization resistance should be taken into account. Immunosuppressed hosts and acutely ill patients undergoing mechanical ventilation can be protected from serious infections by eliminating the most dangerous species of the aerobic endogenous flora, leaving colonization resistance intact. This is called selective decolonization. This article summarizes the effects of antimicrobial agents on colonization resistance.