Objective: In this study, we aimed to determine phenotypic characteristics and genetic determinants of macrolide resistance in clinical isolates of Streptococcus pneumoniae.

Materials and Methods: In 50 erythromycin resistant S. pneumoniae isolates, 14-, 15- membered macrolides and lincosamide susceptibilities were determined by both disk diffusion and broth microdilution methods. Erythromycin - clindamycin double disk method was applied for the detection of macrolide resistance phenotypes. Genetic determinants of macrolide resistance, erm(B), erm(TR) and mef(A)/(E) were investigated by PCR.

Results: The percentages of the isolates presenting cMLSB, M and iMLSB phenotypes were 70%, 28%, and 2% respectively. According to the PCR results, 20 isolates (40%) had erm(B) alone and 15 isolates (30%) had mef(E)/(A) alone. In the remaining 15 (%30) isolates, erm(B) and mef(A)/(E) genes were found concomitant. All erm(B) positive isolates (70%) presented high level macrolide and lincosamide resistance. In contrast, macrolide MICs of the isolates carrying mef(A)/(E) gene alone (30%), were relatively low while their clindamycin MICs remained in the susceptible ranges.

Conclusion: Demonstration of mef(A)/(E) gene and M phenotype in the 60% and 28% of the isolates, respectively in this study suggests that active macrolide efflux together with the ribosomal target mutation has significant role in the macrolide resistance of our pneumococcal isolates.