SUMMARY: Penicillin-binding protein (PBP) alterations have been associated with non-β-lactamase-mediated ampicillin resistance in We evaluated the PBP profiles of several ampicillin-susceptible and -resistant clinical isolates of to determine how consistently the described alterations occurred, and to document the reproducibility of the PBP profiles for this species. The MIC of ampicillin ranged from 0.06 to 0.13 μg ml for the susceptible isolates at an inoculum of 100000 c.f.u. when tested by broth dilution, and was 0.5 μg ml for all four isolates when tested by agar dilution. The MIC for the resistant isolates ranged from 4 to 8 μg ml when tested by broth dilution, and from 1.5 to 16 μg ml when tested by agar dilution. At least eight distinct PBPs with molecular masses ranging from 27 to 90 kDa were detected both in cell membrane preparations and whole cell () binding assays done on cells in the exponential growth phase. PBP variability was evident both in the ampicillin-susceptible and -resistant isolates; however, much greater variability existed within the four resistant strains. The differences in PBP patterns included (1) electrophoretic mobility, (2) binding capacity for the antibiotic and (3) the presence of additional PBPs in two of the resistant isolates. However, decreased binding capacity was consistently demonstrated in PBP 5 (56 kDa) of all of the resistant isolates. Saturation curves with both penicillin and ampicillin indicated that PBP 5 had decreased affinity for the antibiotics. These results suggest () that care should be taken in interpreting changes in PBP profiles for species that demonstrate variability such as , and () that the decreased binding affinity of PBP 5 is a consistent finding associated with multiple ampicillin-resistant wild-type isolates.


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