A-044. Prevalence and Molecular Characterization of Macrolide Resistant S. pneumoniae in Canadian Intensive Care Units: Results Obtained During 2005 and 2006 Canadian Intensive Care Unit Surveillance Study (CAN-ICU)

A. K. Wierzbowski1, K. Nichol2, M. Decorby1, G. G. Zhanel1, D. J. Hoban1,2;
1Univ. of Manitoba, Winnipeg, MB, CANADA, 2Hlth. Sci. Ctr., Winnipeg, MB, CANADA.

Background: The respiratory tract is the major source of infections in intensive care units (ICUs) and Streptococcus pneumoniae is an important pathogen in these infections. Macrolides are frequently used in combination with other agents for empiric treatment of respiratory tract infections in the ICU. Macrolide resistance in S. pneumoniae is mostly due to the presence of ribosomal erythromycin methylase (ermB), and/or to the presence of efflux pump MefA. In this study we determined the prevalence of respiratory S. pneumoniae in Canadian ICUs and studied the macrolide resistance mechanisms and genetic relatedness among these organisms. Methods: 2292 respiratory isolates were obtained from 19 medical centers across Canada between 2005 and 2006 as part of the Canadian National Intensive Care Unit (CAN-ICU) Study. MIC testing was carried out according to the CLSI approved microbroth dilution method. Macrolide resistant isolates were tested by PCR for known macrolide resistance genetic determinants, erm(B) and mef(A) by standard methods. The genetic relatedness between isolates was evaluated by PFGE of SmaI digest. Serotyping was performed by the Quellung reaction. Results: Out of a total of 2292 respiratory isolates obtained, 198 (8.6%) were identified as S. pneumoniae. 39 (19.7%) of 189 (available for testing) were non-susceptible to clarithromycin. The mefA gene was present in 17(43.6%) of the 39 isolates (clarithromycin MIC range: 0.5-8ug/ml and clindamycin MIC: <0.25ug/ml). The ermB gene was found in 12 (30.8%) isolates (clarithromycin MICs:1->16ug/ml and clindamycin >8ug/ml). Of the 39 isolates, 5 (12.8%) contained both mefA and ermB and 5 (12.8%) isolates contained neither gene. Dendrogram analysis of 39 macrolide resistant isolates identified clusters of isolates that were genetically related (≥80% homology) and others that were unrelated. Conclusion: Macrolide resistance among S. pneumoniae in Canadian ICUs is mediated by both ribosomal methylase and by efflux. The spread of macrolide resistant S. pneumoniae appears to involve both the dissemination of resistant clones and de novo acquisition of macrolide resistance determinants.