F3 Prevalence and Mechanisms of Erythromycin (Ery) resistance in Group A Streptococcus (GAS) and Group B Streptococcus (GBS): Implications for Reporting Susceptibility Results

DELGATY KL*, DESJARDINS M, RAMOTAR K, SEETERAM C, B. TOYE  Division of Microbiology, The Ottawa Hospital, Ottawa, ON, Canada

Objective: Increased Ery resistance in GBS and GAS has been reported. Cross-resistance to Clindamycin (Clinda) may be present depending on the mechanism of resistance. We determined the prevalence of macrolide resistant determinants in Ottawa to guide laboratory reporting of Ery and Clinda susceptibilities.

Methods:  Ery and Clinda susceptibilities were determined for 338 GBS and 593 GAS clinical isolates by disk diffusion (DD) and broth microdilution (BD) according to NCCLS guidelines (2003). Inducible and constitutive resistance to Clinda was determined by double disk induction (DDI). MLSB resistance (ermTR/B), and M phenotype (mefA) were confirmed by PCR.

Results:  For GBS isolates, 53 (16%) were resistant to Ery compared to 28 (8%)for Clinda.  Of these 53 isolates, erm methylase was identified in 46 (82%), 21 with inducible resistance (MLSi) and 25 with constitutive resistance (MLSc) to Clinda. The remaining 7 isolates were resistant by mefA efflux. For GAS, 47 (8%) and 6 (1%) isolates were resistant to Ery and Clinda, respectively. Ery resistance was due to mefA in 33/47 (70%) compared to 14/47 (30%) with erm mediated resistance (9 with MLSi, 5 with MLSc). 

Conclusions: In our isolates, Ery resistance in GAS is mostly due to mefA whereas in GBS, it is predominantly due to erm. These differences need to be taken into consideration when deciding whether to report Clinda susceptibility results based on in-vitro results. DDI testing would be an approach that could be used to address this issue especially for GAS.