24-hour systemic antimicrobial prophylaxis with gentamicin and metronidazole, or metronidazole alone, in elective colorectal surgery after mechanical bowel preparation with mannitol and whole gut irrigation.

In a prospective randomized trial, systemic antimicrobial prophylaxis with metronidazole and gentamicin was compared to the administration of metronidazole alone in elective colorectal surgery. In both groups, an identical mechanical large-bowel preparation was performed. In order to reduce the duration of the whole-gut irrigation, a 10% mannitol solution was administered before the irrigation procedure. Although no serious infections were observed in either group, the incidence of superficial wound infections was relatively high: 19% in the group with metronidazole and gentamicin prophylaxis and 25% in the other group. From all these wounds E. coli was isolated, and no obligate anaerobic bacteria were cultured. This high rate of wound infections is most likely caused by an overgrowth of E. coli after irrigation, due to the residues of mannitol in the colon, which serve as a nutrient for E. coli. It is concluded that mannitol should not be used in the preoperative mechanical preparation of the large bowel before elective colorectal surgery.