Effect of Ampicillin and Chloramphenicol Alone and in Combination on Ampicillin-Susceptible and -ResistantHaemophilus influenzaeType B

Abstract

To evaluate ampicillin (Amp) and chloramphenicol (Cm) alone and in combination againstHaemophilus influenzaetype b, we examined the viability of 5 log10colony-forming units (CFU) of early-log-phase organisms per ml after 4 and 8 h of incubation with the drug(s). Nine Amp-susceptible (Amps) and five Amp-resistant (Ampr) 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 AmpsH. influenzaetype b after 8 h of incubation (1.36 and 1.47 log10CFU/ml, respectively; bothp< 0.01); only Cm at its MIC significantly reduced the number of viable organisms after 4 h (0.91 log10CFU/ml;P< 0.001). With Amprorganisms, 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 log10CFU/ml, respectively; bothP< 0.02); smaller but significant reductions were noted after 4 and 8 with Cm at its MIC (0.56 and 0.87 log20CFU/ml, respectively; bothP< 0.025). Antagonism with Ampsor Amprstrains was not seen. We conclude that combinations of Amp and Cm have indifferent effects on Ampsand AmprH. influenzaetype b.