Effect of ampicillin and chloramphenicol alone and in combination on ampicillin-susceptible and -resistant Haemophilus influenzae type B.

To evaluate ampicillin (Amp) and chloramphenicol (Cm) alone and in combination against Haemophilus influenzae type b, we examined the viability of 5 log(10) colony-forming units (CFU) of early-log-phase organisms per ml after 4 and 8 h of incubation with the drug(s). Nine Amp-susceptible (Amp(s)) and five Amp-resistant (Amp(r)) systemic isolates were examined. Antibiotic concentrations included: the minimum inhibitory concentration (MIC) of Amp, 50% of the MIC of Amp, 25% of the MIC of Amp, the MIC of Cm, 50% of the MIC of Cm, 25% of the MIC of Cm, and nine combinations of these concentrations. Both Amp and Cm at their MIC significantly reduced bacterial titers of Amp(s)H. influenzae type b after 8 h of incubation (1.36 and 1.47 log(10) CFU/ml, respectively; both p < 0.01); only Cm at its MIC significantly reduced the number of viable organisms after 4 h (0.91 log(10) CFU/ml; P < 0.001). With Amp(r) organisms, significant reductions in mean bacterial titers occurred after 4 and 8 h of incubation in the presence of Amp at its MIC (1.66 and 2.06 log(10) CFU/ml, respectively; both P < 0.02); smaller but significant reductions were noted after 4 and 8 with Cm at its MIC (0.56 and 0.87 log(20) CFU/ml, respectively; both P < 0.025). Antagonism with Amp(s) or Amp(r) strains was not seen. We conclude that combinations of Amp and Cm have indifferent effects on Amp(s) and Amp(r)H. influenzae type b.