FIG. 1.

Effect of antibiotic treatment on growth of (A) and toxin production by (B) Clostridium difficile in the cecal contents of mice. Strains 3 and 4 were epidemic BI strains. Mice received daily subcutaneous antibiotic treatment for 5 days. Two days after the final antibiotic dose, cecal contents were collected and inoculated with 10⁴ CFU/ml of the C. difficile test strains. Samples were incubated anaerobically for 48 h, and then serial dilutions were plated onto selective medium for quantification of C. difficile and assayed for toxin production. Mean toxin titers are expressed as the reciprocal of the highest serial 10-fold dilution that gave positive results. For ciprofloxacin and levofloxacin, 12× and 20× refer to 12 and 20 times the initial dosage, respectively. Error bars represent standard errors.