Antimicrobial-ResistantStreptococcus pneumoniaein Canadian Hospitals: Results from the 2007 CANWARD Study

BACKGROUND: The Canadian Ward Surveillance Study(CANWARD 2007) tested isolates collected from January to December2007 from 12 Canadian hospitals to a range of antimicrobial agents.The present paper focuses on antimicrobial resistance in Streptococcuspneumoniae in Canadian hospitals, with an emphasis on macrolideresistance.MEtHODS: Minimum inhibitory concentrations of antimicrobial agentswere determined using the broth microdilution method and interpretedaccording to Clinical and Laboratory Standards Institute breakpoints.Macrolide-nonsusceptible strains (clarithromycin minimum inhibitoryconcentrations 0.5 μg/mL or greater) were analyzed by multiplex polymerasechain reaction for the presence of mefA and ermB genes.RESULTS: S pneumoniae represented 9.0% (706 of 7881) of all isolatescollected in CANWARD 2007. Of the 706 S pneumoniae isolates collected,33.1% (234) were from blood and 66.9% (472) were fromrespiratory specimens. The overall resistance (resistant and intermediate)rates for S pneumoniae isolated from respiratory and blood specimens,respectively, were: penicillin (23.9%, 14.4%), clarithromycin(22.1%, 12.6%), trimethoprim-sulfamethoxazole (14.7%, 11.5%),doxycycline (7.8%, 5.1%) and clindamycin (7.1%, 3.3%). Multidrugresistance (resistance to penicillin, clarithromycin and trimethoprimsulfamethoxazole)accounted for 2% (n=9) and 0.5% (n=1) of respiratoryand blood isolates, respectively. Susceptibility of 95% or greaterwas found with amoxicillin-clavulanic acid (99.5%, 99.3%), ceftriaxone(99.5%, 100%), cefuroxime (95.0%, 96.8%), ertapenem (99.8%,100%), meropenem (96.1%, 99.5%) and levofloxacin (99.1%, 100%)for respiratory and blood specimens, respectively. No resistance tovancomycin, tigecycline, cethromycin or telithromycin was found.mefA was present in 53.6% (52 of 97) of respiratory and 59.3% (16of 27) of blood macrolide-nonsusceptible S pneumoniae, while ermBwas present in 38.1% (37 of 97) of respiratory and 37% (10 of 27) ofblood isolates. Eight of 97 (8.2%) respiratory and one of 27 (3.7%)blood isolates contained both mefA and ermB genes.CONCLUSIONS: S pneumoniae is a common organism isolated fromclinical specimens in Canadian hospitals. Resistance was highest topenicillin and clarithromycin, while ceftriaxone and levofloxacin susceptibilitywere both greater than 99%. No isolates resistant to vancomycin,tigecycline, linezolid or the ketolides were found. Resistancerates were higher among respiratory tract isolates of S pneumoniae thanamong blood isolates. Macrolide efflux, mefA, was the predominantmechanism of macrolide resistance among both respiratory and bloodclarithromycin-nonsusceptible isolates