Chloramphenicol inhibition of the bactericidal effect of ampicillin against Haemophilus influenzae

The effects of combinations of ampicillin and chloramphenicol against seven strains of Haemophilus influenzae type b (five beta-lactamase-negative and two beta-lactamase-positive strains) were evaluated by killing-kinetic methods. Growth of strains was assessed in modified Levinthal broth against an inoculum of 10(5) organisms per ml; colony counts were performed immediately and at 6 and 20 h postinoculation. Ampicillin and chloramphenicol were completely bactericidal at 20 h. reducing bacterial densities to 0 colony-forming units per ml at concentrations equivalent to the ampicillin inhibitory concentration and twofold the chloramphenicol inhibitory concentrations. Chloramphenicol at its inhibitory concentration or at one-half of its inhibitory concentration prevented the normally bactericidal activity of ampicillin at 20 h incubation, but not at 6 h.