In vitro susceptibilities of isolates of Haemophilus ducreyi from Thailand and the United States to currently recommended and newer agents for treatment of chancroid

We determined the in vitro susceptibilities of 54 isolates of Haemophilus ducreyi from Thailand (29 isolates) and San Francisco (25 isolates) to penicillin G, tetracycline, amoxicillin-clavulanic acid, ceftriaxone, cefixime, erythromycin, azithromycin, ciprofloxacin, ofloxacin, and trimethoprim-sulfamethoxazole. Isolates were susceptible to ≤0.25 μg of ceftriaxone per ml, ≤0.5 μg of cefixime per ml, ≤0.125 μg of ciprofloxacin per ml, and ≤0.06 μg of ofloxacin per ml. Erythromycin was active against all isolates (MIC for 90% of isolates tested, 0.25 μg/ml), as was azithromycin (MIC, ≤0.06 μg/ml). In contrast, all but one isolate were resistant to ≥8.0 μg of tetracycline per ml, 11.1% of the isolates were resistant to ≥8.0 μg of amoxicillin-clavulanic acid per ml, and 40.9% of the isolates were resistant to trimethoprim-sulfamethoxazole (MIC, ≥4/76 μg/ml).