Study on the mechanism of macrolide resistance in Streptococcus pneumoniae

Objective To investigate the mechanism of macrolide resistance and the prevalence of transposon in Streptococcus pneumoniae isolates.Methods Antibiotic susceptibility was determined by E-test or K-B disk diffusion method for 188 erythromycin-resistant S. pneumoniae isolates. Macrolide resistant phenotypes were identified using double disc test with erythromycin and clindamycin discs. The ermB, mefA, mefE, tetM and intTn genes were amplified by PCR.Results Of the 188 erythromycin-resistant isolates, 94.1% were resistant to clindamycin and 72.7% were penicillin non-susceptible S. pneumoniae (PNSSP). Amoxicillin-clavulanic acid and levofloxacin were still active against most of the isolates. The prevalence of resistant genes was: ermB, 91.5%; mefE, 38.3%; mefA 0%; transposon integrase gene (intTn) 94.5%. The most common macrolide-resistant phenotype was cMLSB (91.5%), which had ermB(+)mefE(+) and ermB(+)mefE(-) genotypes. About 8.5% was M phenotype, which had ermB(-)mefE(+) pattern. Conclusions The ermB-mediated cMLSB is the most prevalent phenotype among erythromycin-resistant S. pneumoniae isolates. Transposon may play an important role in resistance spread both horizontally and clonally.