Evolution and molecular characterization of macrolide-resistant Streptococcus pneumoniae in Canada between 1998 and 2008.

OBJECTIVES Studies were performed to assess resistance mechanisms, multidrug resistance (MDR), genetic relatedness, serotype distribution, heptavalent pneumococcal conjugate vaccine (PCV7) coverage and pili virulence factors among macrolide-resistant Streptococcus pneumoniae (MRSP) isolated from respiratory samples submitted to hospital laboratories across Canada from 1998 to 2008. METHODS Isolates of MRSP (n = 1518) collected by the national surveillance studies CROSS (Canadian Respiratory Organism Susceptibility Study; 1998-2006) and CANWARD (Canadian Ward Surveillance Study; 2007-08) were tested using the CLSI broth microdilution method to establish antimicrobial susceptibilities. PCR was used to detect macrolide resistance genes [mef(A) and erm(B)] and pili virulence factors (type 1 pili and type 2 pili), the Quellung reaction was used to identify serotypes and PFGE was used to determine genetic relatedness. RESULTS The prevalence of MRSP increased from 8% in 1998 to 22% in 2008 (P = 0.0001). MRSP were 51% mef(A) positive, 36% erm(B) positive, 8% dual mef(A) and erm(B) positive and 5% mef(A) and erm(B) negative. Dual mef(A)- and erm(B)-positive isolates increased in prevalence from 3% in 1998 to 19% in 2008 (P = 0.001). The prevalence of PCV7 serotypes (4, 6B, 9V, 14, 18C, 19F and 23F) decreased from 67% in 1998 to 31% in 2008 (P = 0.0072). The prevalence of serotype 19A, a non-PCV7 serotype, increased by 15% from 1998 to 2008; isolates of serotype 19A were MDR, dual mef(A) and erm(B) positive, genetically related by PFGE and associated with the presence of pili virulence factors. CONCLUSIONS From 1998 to 2008, respiratory isolates of MRSP in Canada increased significantly due primarily to the emergence and spread of serotypes 6A, 19A and other non-PCV7 serotypes.