Effect of Antibiotic Treatment on Growth of and Toxin Production by Clostridium difficile in the Cecal Contents of Mice

Abstract

In mice, subcutaneous administration of antibiotics that disrupt the anaerobic microflora (i.e., clindamycin, piperacillin-tazobactam, and ceftriaxone) facilitated in vitro growth of and toxin production by Clostridium difficile in cecal contents, whereas antibiotics that cause minimal disruption of the anaerobic microflora (i.e., levofloxacin, cefepime, and aztreonam) did not.

Antibiotics play a crucial role in the pathogenesis of Clostridium difficile-associated diarrhea (CDAD), presumably by disrupting the indigenous microflora of the colon, thereby allowing C. difficile to grow to high concentrations with the production of toxin (2, 7). Antibiotics that have inhibitory activity against C. difficile may suppress the organism during treatment; however, C. difficile overgrowth and disease may develop after completion of therapy during the period of recovery of the indigenous microflora (2, 4, 7). Antibiotics that disrupt the anaerobic component of the microflora may be particularly likely to cause CDAD (1, 2, 21). However, antibiotics which cause relatively little disruption of the anaerobic microflora, such as ciprofloxacin and trimethoprim-sulfamethoxazole, have also been associated with CDAD (7, 23). In this study, we used an in vitro mouse model to examine the effect of antibiotic treatment on growth of C. difficile in cecal contents. We hypothesized that antibiotics that disrupt the anaerobic microflora would facilitate growth and toxin production by C. difficile, whereas antibiotics that do not disrupt the anaerobic microflora would not.

Three strains of C. difficile were studied. Strain 1 was ATCC 9689, strain 2 was a clinical isolate from Cleveland, and strain was ATCC 43593. Strains 1 and 2 produced toxin, whereas strain 3 did not. Broth dilution MICs of the test antibiotics for the three strains were determined using standard methods for susceptibility testing of anaerobic bacteria (11). The in vitro model that was used was adapted from the in vitro model of colonization resistance to C. difficile infection developed by Borriello et al. (1). These investigators demonstrated that antibiotics that promoted in vitro growth and toxin production by C. difficile in cecal emulsions of hamsters also caused C. difficile disease in hamsters, whereas antibiotics that did not promote in vitro growth and toxin production did not cause disease (1). The experimental protocol was approved by the Animal Care Committee of the Cleveland Veterans Affairs Medical Center.

Female CF-1 mice weighing 25 to 30 g (Harlan Sprague-Dawley, Indianapolis, Indiana) were housed in individual cages with plastic filter tops to prevent cross-contamination among animals. We studied three antibiotics that we have previously shown to cause marked disruption of the anaerobic stool microflora of mice (clindamycin, piperacillin-tazobactam, and ceftriaxone), and three antibiotics that inhibit facultative gramnegative bacilli but cause minimal disruption of the anaerobic microflora (levofloxacin, cefepime, and aztreonam) (12, 13, 16). Mice received daily subcutaneous injections (0.2-ml total volume) of saline, levofloxacin (0.375 mg/day), cefepime (2.0 mg/day), aztreonam (3.0 mg/day), ceftriaxone (2.0 mg/day), clindamycin (1.4 mg/day), or piperacillin-tazobactam (8 mg/day) for 4 days. Three days after the final antibiotic dose, mice were euthanized by CO2 asphyxiation. To evaluate the potential for piperacillin-tazobactam to inhibit growth of C. difficile during the course of treatment, one group of mice received daily treatment with this agent as described above but were euthanized 2 h after administration of the final antibiotic dose. The doses of antibiotics were equal to the usual human doses administered over a 24-hour period (milligrams of antibiotic per gram of body weight).