Alterations in Penicillin-Binding Proteins of Clinical and Laboratory Isolates of Pathogenic Streptococcus pneumoniae with Low Levels of Penicillin Resistance

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Abstract
Several recent surveys of clinical isolates have indicated that substantial fractions of naturally occurring populations of Streptococcus pneumoniae have undergone a distinct upward move in the required minimal inhibitory concentration (MIC) of benzylpenicillin (from a range of 0.006–0.008 to 0.03–0.05 µg/ml). Evidence is presented that in clinical pneumococcal isolates, penicillin-binding proteins (PBPs) groups 1 and 2 have a decreased affinity for radioactive benzylpenicillin as compared with penicillin-sensitive isolates from the same locale. Exposure of a penicillin-sensitive type 2 strain (MIC, 0.006 µg/ml) to sequentially increasing concentrations of penicillin allowed the isolation of spontaneous resistant mutants with stepwise increases in the MIC of penicillin required (0.01–0.02, 0.025–0.05, and 0.1 µg/ml), and in these laboratory isolates too, PBP groups 1 and 2 showed decreased affinity for labeled benzylpenicillin. DNA from the low-level resistant clinical or laboratory isolates could be used to transform the appropriate levels of penicillin resistance into penicillin-sensitive laboratory isolates. These findings suggest that significant fractions of natural pneumococcal populations may have acquired one or two of the low-level penicillin resistance genes.