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

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).