Relative penicillin G resistance in Neisseria meningitidis and reduced affinity of penicillin-binding protein 3

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Abstract
We examined clinical isolates of Neisseria meningitidis relatively resistant to penicillin G (mean MIC, 0.3 .mu.g/ml; range, 0.1 to 0.7 .mu.g/ml), which were isolated from blood and cerebrospinal fluid for resistance mechanisms, by using susceptible isolates (mean MIC, .ltoreq. 0.06 .mu.g/ml) for comparison. The resistant strains did not produce detectable .beta.-lactamase activity, otherwise modify penicillin G, or bind less total penicillin. Penicillin-binding protein (PBP) 3 of the six resistant isolates tested uniformly bound less penicillin G in comparison to the same PBP of four susceptible isolates. Reflecting the reduced binding affinity of PBP 3 of the two resistant strains tested, the amount of 3H-labeled penicillin G required for half-maximal binding was increased in comparison with that of PBP 3 of the two susceptible isolates. We conclude that the mechanism of resistance in these meningococci relatively resistant to penicillin G was decreased affinity of PBP 3.