Effect of antimicrobial therapy on bowel flora.

The known patterns and functions of gastrointestinal flora and antimicrobial effects on flora are described, and the clinical implications of altered bowel flora are described. A major role of the normal endogenous intestinal flora is to inhibit gastrointestinal colonization and overgrowth by potentially pathogenic bacteria. Serious disruption of this protective mechanism may occur after the administration of various antimicrobial agents. Alteration of bacterial flora by antimicrobials is often the result of incomplete absorption after oral administration or of high concentrations in saliva, bile, or secretions from the intestinal flora. Suppression of endogeneous microflora after antimicrobial administration may permit overgrowth of pathogenic strains in the gastrointestinal tract, development or selection of antimicrobial-agent-resistant strains, and disruption of colonization resistance. The disruption of normal bowel flora can result in diarrhea, pseudomembranous colitis, and systemic infections or sepsis in immunocompromised patients. Agents that are well absorbed and have minimal biliary excretion should minimize the detrimental effects from altered flora. The choice of an antimicrobial agent that has a narrow spectrum, is specific for the infection, is not appreciably excreted in the bile, and is completely absorbed if given orally appears to be the most appropriate practice for preventing or minimizing alteration of bowel flora.