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 < or = 0.25 microgram of ceftriaxone per ml, < or less 0.5 microgram of cefixime per ml, < or = 0.125 microgram of ciprofloxacin per ml, and < or = 0.06 microgram of ofloxacin per ml. Erythromycin was active against all isolates (MIC for 90% of isolates tested, 0.25 microgram/ml), as was azithromycin (MIC, < or = micrograms/ml). In contrast, all but one isolate were resistant to > or = 8.0 micrograms of tetracycline per ml, 11.1% of the isolates were resistant to and 40.9% of the isolates were resistant to trimethoprim-sulfamethoxazole (MIC, > or = 4/76 microgram/ml.)