Combined action of chloramphenicol and ampicillin on chloramphenicol-resistant Haemophilus influenzae
Mackenzie, A.M.; Chan, F.T.
Antimicrobial Agents and ChemoTherapy 29(4): 565-569
ISSN/ISBN: 0066-4804 PMID: 3486629 DOI: 10.1128/aac.29.4.565
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.