Lack of emergence of resistant fecal flora during successful prophylaxis of traveler's diarrhea with norfloxacin.

Norfloxacin, a new quinolone carboxylic acid derivative, was compared with an identical-appearing placebo preparation in a prospective, randomized, double-blind trial for prevention of traveler's diarrhea among 120 U.S. students arriving in Mexico. Prophylaxis was continued for 2 weeks. Diarrhea was defined as four unformed stools in 24 h plus an additional symptom of enteric disease. In the norfloxacin prophylaxis group, 4 of 56 subjects (7%) experienced diarrhea, compared with 36 of 59 subjects (61%) in the placebo group. The difference was significant (P less than 0.0001). In contrast to our previous experience with use of trimethoprim-sulfamethoxazole to prevent traveler's diarrhea, quantitative stool cultures in the norfloxacin-treated group revealed a significant decline of normal aerobic fecal flora during prophylaxis (P less than 0.0005). Among stool samples from norfloxacin-treated subjects, 32 of 38 (84%) cultured on day 7 and 34 of 37 (92%) cultured on day 14 had no gram-negative bacilli. After norfloxacin was discontinued, fecal flora returned to pretreatment levels. No gram-negative aerobic flora resistant to norfloxacin were found during weekly quantitative cultures before, during, or after therapy.