Meningitis due to Haemophilus influenzae type b resistant to both ampicillin and chloramphenicol

A strain of Haemophilus influenzae type b with considerable resistant to both ampicillin and chloramphenicol was recovered from a South Dakota child with meningitis. There was an initial lack of response to conventional doses but the child improved after a brief period of 200 mg/kg/day of chloramphenicol. The organism showed in vitro resistance to ampicillin, carbenicillin, tetracycline, and chloramphenicol (for each antibiotic the minimal inhibitory concentration (MIC) was 8 micrograms/ml or greater with a bacterial challenge of 10(5) colony-forming units (CFU)/ml), but it was sensitive to both streptomycin and rifampin (MIC 1.6 micrograms/ml, respectively). Isobolograms constructed from the results of testing various concentrations of ampicillin and chloramphenicol showed additive effects with high bacterial inocula (10(5) or 10(7) CFU/ml), but antagonism with low inocula (10(2) or 10(4) CFU/ml).