Combined action of chloramphenicol and ampicillin on chloramphenicol-resistant Haemophilus influenzae.

The interaction of ampicillin and chloramphenicol on three ampicillin-susceptible, chloramphenicol-resistant strains of Haemophilus influenzae was studied by checkerboard testing with subcultures, time-kill experiments, and a disk method. In all three strains there was inhibition of the bactericidal action of ampicillin by chloramphenicol at concentrations close to the MIC (10 micrograms/ml). This chloramphenicol concentration was close to that which might be achieved in cerebrospinal fluid during treatment for meningitis and was in the bactericidal range for chloramphenicol-susceptible organisms. It is suggested however that in the initial treatment of meningitis caused by ampicillin-susceptible, chloramphenicol-resistant strains, inhibition of the action of ampicillin by chloramphenicol may represent a clinical risk.