Multi-drug resistance and molecular pattern of erythromycin and penicillin resistance genes in Streptococcus pneumoniae
The appearance and dissemination of penicillin resistant and macrolide resistant Streptococcus pneumoniae strains has caused increasing concern worldwide. The aim of this study was to survey drug resistance and genetic characteristics of macrolide and penicillin resistance in S. pneumoniae. This is a cross-sectional study, which was carried out on 70 samples suspected to be S. pneumoniae isolated from patients who were admitted in Intensive Care Unit (ICU) of southwest of Iran, in 2010 and 2011. At first, suspected colonies were identified by phenotypic and chemical tests. The isolates were confirmed as S. pneumoniae based on the presence of lytA gene by polymerase chain reaction (PCR) method. Antibiotic resistance was evaluated according to Standard Clinical and Laboratory Standards Institute (CLSI). Minimum inhibitory concentrations (MICs) of erythromycin and penicillin were determined by the E-test method. Molecular analyses of macrolide and penicillin resistance were carried out by using specific primers for detection of the resistance gene including erm(B), mef(A), pbp1a, pbp2b and pbp2x genes. The lytA gene was detected in 50 samples. There was prevalence of resistant strains to erythromycin (56%), penicillin (40%), ampicillin (56%), cefotaxime (50%), tetracycline (10%), trimethoprim-sulfamethoxazole (48%), nalidixic acid (16%), clarithromycin (48%), azithromycin (44%) and levofloxacin (4%). All strains were susceptible to chloramphenicol, amikacin, streptomycin and gentamicin. Gene analysis showed that 29 strains (58%) had mef(A) gene, and 24 strains (48%) had the erm(B) gene. Out of all the penicillin resistance and intermediate strains, 6 (20%) and 1 (3.33%) strains harbor mutations in pbp1a and pbp2x genes, respectively, but pbp2b was not identified in any sample. Resistance to penicillin, trimethoprim-sulfamethoxazole, clarithromycin and azithromycin in S. pneumoniae is a serious problem in this area and the local pattern of resistance/susceptibility must be considered for therapeutic regimens. The mef(A) gene was a predominant mechanism of macrolide resistance in this area. With regards to low frequency of pbps resistance genes, monitoring of other kinds of mechanisms is recommended.
Key words: Streptococcus pneumoniae, multi-drug resistance, erythromycin, penicillin, resistance genes.