Chloramphenicol-resistant Salmonella typhi was detected in Saigon in September 1971. Subsequently, 163 strains of S typhi were isolated, 46% of which were resistant to choramphenicol by the agar-disk method. Sixty-two strains were studied by the broth-dilution method; 37% had minimal inhibitory concentrations for chloramphenicol greater than 250μg/ml, but all strains were susceptible to 0.4μg/ml of ampicillin and to a disk of a combination of trimethoprim and sulfamethoxazole (Bactrim). Persons infected with chloramphenicol-resistant strains of S typhi responded poorly to chloramphenicol alone, but ampicillin or the combination drug was effective.
(JAMA 231:162-166, 1975)