Meningitis Due to Haemophilus influenzae Type b Resistant to Both Ampicillin and Chloramphenicol

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Abstract
A strain of Haemophilus influenzae type b with considerable resistance 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 µg/ml or greater with a bacterial challenge of 105 colony-forming units (CFU)/ml), but it was sensitive to both streptomycin and rifampin (MIC 1.6 µg/ml and 0.5 µg/ml, respectively). Isobolograms constructed from the results of testing various concentrations of ampicillin and chloramphenicol showed additive effects with high bacterial inocula (105 or 107 CFU/ml), but antagonism with low inocula (102 or 104 CFU/ml).