Colonization resistance of the digestive tract--mechanism and clinical consequences.

Potentially pathogenic bacteria and yeasts meet a number of resistance factors. These colonization resistance (CR) determining factors control the growth density of potentially pathogenic microorganisms in the various parts of the alimentary canal. The CR of the digestive tract is to be seen partially of host origin but to a greater extent due to direct and indirect activity of the (anaerobic) indigenous microflora. This implies that the CR mechanism is vulnerable to antibiotics which affect the indigenous flora during treatment. A decreased CR enhances colonization by microorganisms that are resistant to the antibiotic(s) used for therapy. Increased population densities of potentially pathogenic bacteria ("overgrowth") may correlate with invasion of the mucosal tissues often followed by translocation to remote lymphatic organs. Overgrowth also may correlate with development or acquisition of resistance to the antibiotic applied. By screening in experimental animals antibiotics have been selected which do not affect the CR. Such antimicrobial drugs have successfully been used to selectively eliminate potentially pathogenic bacteria and yeast in neutropenic patients.