Objective: To investigate the characterization of ermB gene expression and dissemination in macrolide-resistant Streptococcus pneumoniae (Sp) in Shanghai.
Methods: Eighty-six erythromycin-resistant isolates of Sp were isolated from 3 hospitals in Shanghai. E-test or K-B disk diffusion test were used to determine the susceptibility to 12 antibiotics according to the National Committee for Clinical Laboratory Standards. Macrolide resistant genes ermB and mefE, and transposable elements Tn1545 and Tn917 were amplified by PCR. The isolates were divided into Tn1545 and Tn917 groups according to the transposable elements thereof. Double disc test with erythromycin and clindamycin discs divided the isolates into 2 macrolide resistant phenotypes: cMLS(B) (inducible) phenotype and M phenotype (resistant to erythromycin and sensitive to clindamycin). BOX-PCR was used to analyze the homology of the S. pneumoniae.
Results: (1) Of the 86 erythromycin-resistant isolates, the positive rates of ermB, mefE, Tn1545, and Tn917 were 94%, 46%, 87% and 7% respectively. Tn1545 and Tn917 were not detected in 5 ermB-mdfE + strains. (2) Most strains in the Tn1545 and Tn917 groups were highly resistant to erythromycin with a MIC50 of 256 microg/ml. The Tn917 group had a lower MIC to beta-lactam antibiotics and lower resistance to tetracycline, levofloxacin, and compound sulfonamide in comparison with the Tn1545 group. (3) The most common macrolide resistance phenotype of the Tn1545 group was cMLS(B) phenotype. Three strains in the Tn917 group had a 194 bp deletion in the promoter region of ermB and an insertion of TAAA motif in the N end of leader peptide, resulting in the change of the ermB gene from inducible to constitutive expression. (4) BOX-PCR showed that Tn1545 and Tn917 might spread horizontally.
Conclusion: In Shanghai ermB-mediated cMLS(B) is the most prevalent phenotype in macrolide-resistant Streptococcus pneumoniae isolates. Primarily, the ermB gene was carried and spread horizontally by Tn1545.