Identification of a streptococcal penicillin-binding protein that reacts very slowly with penicillin

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Abstract
Penicillin-binding protein (PBP) 5 of Streptococcus faecium ATCC 9790 has an unusually low affinity for penicillin (50% binding occurred at a penicillin level of 8 .mu.g/ml after 60 min of incubation, and the protein only became labeled after 20 min of incubation with high concentrations of radioactive penicillin). PBP with similar properties are carried by strains of S. durans, S. faecalis and S. lactis but not by strains of groups A, B, C and G streptococci or S. pneumoniae. The strains carrying the slow-reacting PBP demonstrated a sensitivity to penicillin that was several hundred times lower than that of strains not carrying it. Spontaneous mutants with minimal inhibitory concentrations of penicillin of 20, 40 and 80 .mu.g/ml were isolated from S. faecium ATCC 9790. They all showed a dramatic increase in the amount of slow-reacting PBP produced. Mutants with increased penicillin resistance were also isolated from wild-type strains of S. durans, S. faecalis and S. faecium. All carried a greater amount of the slow-reacting PBP than that carried by the parent. Resistant S. faecium ATCC 9790 mutants grew normally in the presence of penicillin concentrations that were far above that saturating all PBP except PBP 5. Cell growth was inhibited by a penicillin concentration that saturated the slow-reacting PBP by 90%. This penicillin dose was equal to the minimal inhibitory concentration.