Effects of antimicrobial prophylaxis on colonization resistance.

The effect of antimicrobial agents on the intestinal microflora of patients undergoing colorectal surgery was examined. Two narrow spectrum agents, clindamycin and aztreonam, disturbed colonization resistance. This was preserved with the broad spectrum compound, imipenem. Ecological effects are difficult to predict and clinical studies of new antibiotics should include investigations of their impact on the normal human intestinal flora.