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  • Title: Presence of plasmid pA15 correlates with prevalence of constitutive MLS(B) resistance in group A streptococcal isolates at a university hospital in southern Taiwan.
    Author: Liu YF, Wang CH, Janapatla RP, Fu HM, Wu HM, Wu JJ.
    Journal: J Antimicrob Chemother; 2007 Jun; 59(6):1167-70. PubMed ID: 17460027.
    Abstract:
    OBJECTIVES: To investigate the role of a plasmid bearing the erm(B) gene on the prevalence of the macrolide, lincosamide and group B streptogramin (MLS(B)) phenotype of group A streptococci (GAS) and to characterize the plasmid and determine the clonal relation between the erythromycin-resistant isolates. METHODS: Two hundred and five erythromycin-resistant GAS isolates were collected from 1990 to 2006. Colony hybridization, PCR, plasmid curing and PFGE techniques were used to analyse the mechanisms behind the phenotypes. RESULTS: Among the 56 isolates with constitutive MLS(B) (cMLS(B)) resistance, 53 isolates harboured a plasmid, pA15, of 19 kb. erm(B) was on pA15 and it confered a cMLS(B) resistance phenotype. The prevalence rate of the pA15-containing isolates was 36.3% from 1993 to 1995, but the plasmid could not be detected from 2004 to 2006. To link the high-level resistance to pA15, clinical isolate A15 was selected and pA15 was cured by novobiocin. In the plasmid-cured strain SW503, the erythromycin MIC decreased from 256 to 0.032 mg/L. By electroporation, pA15 was re-introduced into the plasmid-cured erythromycin-susceptible strain, and the high-level erythromycin resistance was restored. Plasmid pA15 was also transferred to group B streptococci and group C streptococci by electroporation. In all the pA15-containing isolates, emm1 type was present and pulse type J was predominant (48 of 54 isolates). CONCLUSIONS: The plasmid pA15 mediated cMLS(B) resistance in the mid-1990s, but pA15 was not detected in the clinical isolates from 2004 onwards, which correlates with the absence of cMLS(B) resistance in this region.
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