Clinical and Laboratory Evaluation of Penicillin Resistant Streptococcus pneumoniae in Relation to the Mutations of pbp1a, pbp2b and pbp2x

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A total of 210 isolates of Streptococcus pneumoniae (S. pneumoniae) were selected randomly to examine drug susceptibility which were obtained from out-and in-patients who visited Kurume University Hospital and other affiliated hospitals through May 1998 to September 1999. The prevalence of penicillin resistant S. pneumoniae in this study was 39.5% and was compatible with those of previous reports in Japan. From the clinical aspects, the resistant strains of S. pneumoniae have been shown not to be so highly virulent comparing with sensitive strains that only few severe or mortal cases had been reported. Carbapenems, glycopeptides, and fluoroquinolones were shown to be highly active against penicillin-resistant S. pneumoniae strains as evidenced by the low minimum inhibitory concentrations (MICs) though LVFX showed 4μg/ml or higher MICs against some strains. Regarding to the mechanisms of penicillin resistance, penicillin binding proteins coding gene (pbp1a, pbp2x and pbp2b) mutations that cause modifications in these proteins were also examined for all isolates. The multivariate analysis showed statistically significant correlation between higher MIC of penicillin and cephem and pbp1a mutation while no significant contribution of pbp2x and pbp2b to the resistance was demonstrated. Additionally, no significant correlation between pbp mutation and MIC of carbapenem was observed. Furthermore, there were two highly penicillin resistant S. pneumoniae strains with no pbp mutations. Thus the pbp mutations alone were not responsible for the elevation of MIC all β-lactams. Nevertheless the pbp mutations were detected in advance of actual MIC elevations by inducement experiment in vitro. It indicated that penicillin resistance might be detected earlier than conventional methods. In previous reports some other responsible genes for penicillin resistance were demonstrated. Therefore, it might be possible to presume exact values of MICs of β-lactams against resistant strains of S. pneumoniae by detecting still unknown genes other than pbps.