A ten hospital cooperative study comparing prophylactic oral neomycin and erythromycin base versus placebo demonstrated the clinical efficacy of the antibiotics in preventing septic complications following elective colon operations. The present report concerns microbiological studies accomplished during this trial. Cultures of colon contents during surgery showed the antibiotic preparation reduced concentrations of both aerobes and anaerobes by approximately 105 bacteria/ml. Virtually all major bacterial components of the normal flora were affected. Wound irrigation specimens at the time of closure failed to predict subsequent wound infection, but significantly fewer antibiotic recipients had positive irrigation cultures. Postoperative stool specimens showed that the oral antibiotics did not cause an emergence in resistant forms. Bacteriological studies of postoperative infections indicated that most postoperative infections involved a mixed aerobic-anaerobic flora, and that Bacteroides fragilis accounted for six of eight episodes of bacteremia.
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