Effects of ampicillin plus sulbactam on bowel flora in patients undergoing colorectal surgery

The influence of a combination of ampicillin and a beta-lactamase inhibitor, sulbactam, on colonic bacterial flora was studied in patients undergoing colorectal surgery. Drug concentrations were determined in serum, intestinal mucosa, and feces. The results of the drug concentrations were correlated with changes in the numbers of aerobic and anaerobic bacterial bowel flora. Ampicillin and sulbactam were given intravenously to 21 patients in 500-mg doses of each drug every 8 h for 2 days. The first dose was given when anesthesia was begun. The concentrations of ampicillin in serum were significantly higher (0.01 greater than P greater than 0.001) than those of sulbactam. In contrast, at all time intervals, the concentrations of sulbactam in intestinal mucosa were significantly higher than those of ampicillin (0.05 greater than P greater than 0.01). The serum half-life of the two drugs was the same at 2.2 h. After one dose of the antibiotics, sulbactam was detectable in the feces of four patients, and ampicillin was detectable in the feces of 11 patients. During the 2 days of ampicillin and sulbactam prophylaxis, no measurable decrease occurred in the anaerobic flora of the bowel; a mean decrease of 3 logs occurred in the anaerobic bowel flora. The results of this study show that ampicillin and sulbactam prophylaxis had a greater effect on the anaerobic bowel flora than on the aerobic bowel flora. In light of the in vitro spectrum of ampicillin and sulbactam activity, it was surprising that bowel aerobic flora was unchanged.