The objectives of the study were to investigate and compare the susceptibility profiles of preoperative and postoperative intestinal Bacteroides fragilis and Bacteroides thetaiotaomicron strains against antimicrobials to evaluate their resistance development patterns due to prophylactic antibiotic administration. Preoperative and postoperative stool samples were obtained from patients undergoing elective abdominal surgery with a “clean-contaminated” wound categorization. All patients received cefoxitin 2 g as prophylaxis. Isolates of Bacteroides species were identified. Susceptibility tests against ampicillin, piperacillin, cefoxitin, clindamycin, chloramphenicol, metronidazole, imipenem, and amoxicillin-clavulanate were performed. Stool samples were collected from 40 patients at two settings: before and after the operation. Most of the patients (53%) were operated on because of colorectal carcinoma and underwent surgery that included colectomy or anterior resection. Bacteroides
fragilis and Bacteroides thetaiotaomicron were isolated in both samples obtained from 22 and 34 patients, respectively. In the present study 95% to 100%, 50% to 74%, 46% to 64%, and 18% to 35% of preoperatively isolated Bacteroides species were resistant to ampicillin, cefoxitin, clindamycin, and piperacillin, respectively. Although there were no statistically significant difference, postoperative strains were more resistant than preoperative isolates, and the resistance rates were found to be 95% to 100%, 55% to 82%, 46% to 86%, and 37% to 41%, respectively. Both Bacteroides species isolated from all specimens before and after the surgery were uniformly susceptible to amoxicillin-clavulanate, imipenem, metronidazol, and chloramphenicol.Our data revealed that Bacteroides strains revealed high resistance to ampicillin, cefoxitin, piperacillin, and clindamycin. Metronidazole, imipenem, and amoxicillin-clavulanate seem highly effective against both Bacteroides species. Cefoxitin prophylaxis did not significantly increase the resistance patterns against antimicrobial agents.