Antimicrobial susceptibilities and analysis of genes related to penicillin or macrolide resistance in Streptococcus pneumoniae.

One hundred and seventy-seven strains of Streptococcus pneumoniae derived from respiratory specimens between 1987 and 2001 were evaluated for their antimicrobial susceptibilities and distribution of genes related to penicillin and macrolide resistance. Resistance rates tended to be higher for the 1996-2001 isolates than for the 1987-1995 isolates for all beta-lactams tested. For benzylpenicillin the MIC(90) value of the isolates derived between 1996 and 2001 was 1.56 mg/L, while that of strains isolated between 1987 and 1990 was 0.05 mg/L. Furthermore, the number of strains susceptible to macrolides also decreased, but only two strains isolated in 1993 were resistant to levofloxacin of the 177 S. pneumoniae strains tested. When of genes relating to penicillin resistance were analysed using PCR with primers specific to susceptible alleles, although more than 50% of strains from 1987 to 1990 and 1991 to 1995 revealed no mutations in the pbp 1a, 2x and 2b genes, only 30.0% of strains derived between 1996 and 2001 showed no mutations in the pbp gene. Strains having mutations in all three pbp genes (1a, 2x and 2b) by the PCR method increased from only 2.2% in the 1987-1990 derived strains to 27.5% in the 1996-2001 strains. Furthermore, 64.1 and 60.0% of the isolates from 1987 to 1990 and 1991 to 1995, respectively, did not possess either the mefA or ermB by PCR analysis. Conversely, 75.0% of isolates from 1996 to 2001 possessed mefA and/or ermB. These genetic changes may explain the increase in the number of penicillin and macrolide resistant strains. We believe that it is important to evaluate changes in MIC as well as genetic mutations in order to select the most appropriate therapy for S. pneumoniae infections.