Characterization of clinical isolates of Haemophilus influenzae type b for heterogeneous populations of susceptibility to ampicillin.

In this study 80 clinical isolates of Haemophilus influenzae type b from 60 patients were used to analyze if heterogeneous populations of ampicillin resistant and sensitive cells were simultaneously present within each strain and to determine how common this phenomenon was among clinical isolates. A total of 50 ampicillin sensitive clinical isolates were screened for resistance to this antibiotic. It was observed that 32 ampicillin sensitive strains did not contain resistant subpopulations. Furthermore, even with the inducement of resistant subgroups to proliferate under antibiotic-mediated selection using maximum subinhibitory concentrations of ampicillin, no subpopulations of resistant cells were discovered among 18 additional strains. In order to determine whether ampicillin resistance was stable in beta-lactamase-producing H. influenzae clinical isolates, 20 strains from 16 patients were examined. No tendency to segregate into a heterogeneous population of sensitive and resistant clones was found. Furthermore, ampicillin resistance was still uniformly expressed after the treatment of ten strains with the curing agent acridine orange. These results suggest that after extensive evaluation no heterogeneous populations existed with ampicillin resistant and sensitive H. influenzae clinical isolates, indicating that this phenomenon is not a prevalent one.