OVERPRODUCTION OF A LOW-AFFINITY PENICILLIN-BINDING PROTEIN AND HIGH-LEVEL AMPICILLIN RESISTANCE IN ENTEROCOCCUS-FAECIUM

Five ampicillin-resistant clinical isolates of Enterococcus faecium we re analyzed for a correlation between overproduction of the low-affini ty penicillin-binding protein (PBP 5) and the level of ampicillin resi stance. Comparison was made with one susceptible clinical isolate and its ampicillin-resistant derivative obtained in the laboratory by sele ction with increasing concentrations of penicillin. Overproduction of the low-affinity PBP relative to the susceptible isolate was noted in moderately resistant strains (MIC, 32 mu g/ml) but not in highly resis tant strains (MIC, 128 mu g/ml). Polyclonal antibodies specifically re acting with the low-affinity PBP of Enterococcus hirae, Enterococcus f aecalis, and Enterococcus faecium (M. Ligozzi, M. Aldegheri, S. C. Pre dari, and R. Fontana, FEMS Microbiol. Lett. 83:335-340, 1991) were use d to determine the amount of this PBP in the E. faecium isolates. In a ll strains, the antibody preparation reacted with a membrane protein o f the same molecular mass as PBP 5. The amount of this protein was ver y small in the susceptible strain but large in all of the resistant st rains. These results suggest that the highly resistant strains also ov erproduced the low-affinity PBP, which, compared with PBP 5 of moderat ely resistant strains, appeared to be modified in its penicillin-bindi ng capability.