Objective   To detect the mutant prevention concentration (MPC) of macrolide resistant Streptococcus pneumoniae

to amplify the resistance gene of Streptococcus pneumoniae mutants selected in the mutant selection window (MSW)

and to investigate the mechanism in the resistance of Streptococcus pneumoniae to macrolides.     Methods   The Streptococcus pneumoniae strain ATCC49619 was enriched in broth

and the bacterial concentrations were adjusted to 10

10

colony forming units per milliliter. The minimal inhibitory concentr

ation (MIC)

MIC for 99 % of input cells (MIC

99

)

provisional MPC (MPCpr)

and MPC of roxithromycin and azithromycin for Streptococcus pneumoniae were determined by agar plate dilution method. The ermB and mefA of Streptococcus pneumoniae mutants selected in the MSW were obtained by PCR method and sequencing.     Results   The MPCs of roxithromycin and azithromycin for ATCC49619 were 0.80 μg/ml and 0.51 μg/ml

and the MPC/MIC

99

were 5.0 and 3.9. The ermB gene was detected in Streptococcus pneumoniae mutants selected in the MSW.     Conclusion   Adjustment of the drug dose may limit the enrichment of macrolide-resistant Streptococcus pneumoniae mutant. The mechanism in the resistance of Streptococcus pneumonia to macrolides may be associated with ermB genes carried.