Background: With respect to clinical burden of pneumococcal infections and the role of erythromycin in the treatment of these infections, this study was performed to characterize phenotypic and genotypic patterns of erythromycin resistance and distribution of tetracycline resistance genes among clinical isolates of Streptococcus pneumoniae.
Methods: Clinical isolates of S. pneumoniae were collected from 15 hospital and clinical laboratories in Tehran for 1 year during 2011. Identification was confirmed by biochemical and molecular tests. Erythromycin- and tetracycline-resistant isolates were selected by standard disc diffusion method. Minimal inhibitory concentration was determined by E-test method for erythromycin. Erythromycin resistance phenotype was determined by double-disc test. Finally, 3 erythromycin resistance genes were detected by a multiplex polymerase chain reaction (PCR) for erm(A), erm(B), and mef(A/E), then 4 tetracycline resistance genes were detected by multiplex PCR with the specific primers for tet(M), tet(O), tet(K), and tet(L).
Results: A total of 88 pneumococcal isolates were collected from clinical samples. Fifty-two percent of the isolates were found to be resistant to erythromycin, and 62%, to tetracycline by disc diffusion method. E-test method showed that the minimal inhibitory concentration ranges of the isolates were from 2 up to greater than 256 μg/mL. Erythromycin-clindamycin double-disc test exhibited that 79% of resistant isolates were constitutive MLS (cMLS) phenotype, and the remaining isolates were M phenotype. erm(B)and mef(A/E) genes were detected in all cMLS and M phenotypes, respectively, by multiplex PCR. Six isolates were double positive for mef(A/E)and erm(B)genes. Eighty-five percent of erythromycin-resistant isolates were resistant to tetracycline as well, and only tet(M) genes were detected in these isolates.
Conclusions: To the best of our knowledge as a first report about the phenotypes and genotypes of erythromycin resistance in clinical pneumococci isolates in Iran, the high prevalence of erythromycin resistance with predominant cMLS phenotype together with coresistance to tetracycline can be alarming and need more attentions in Iran. It seems necessary to design a national multicenter study to characterize macrolide-resistant pneumococci in various geographical areas of Iran and determine if either these resistant isolates belong to the dominant serotypes and clones or not.