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

Abstract

BACKGROUND: The Canadian Ward Surveillance Study (CANWARD 2007) tested isolates collected from January to December 2007 from 12 Canadian hospitals to a range of antimicrobial agents. The present paper focuses on antimicrobial resistance inStreptococcus pneumoniaein Canadian hospitals, with an emphasis on macrolide resistance. MEtHODS: Minimum inhibitory concentrations of antimicrobial agents were determined using the broth microdilution method and interpreted according to Clinical and Laboratory Standards Institute breakpoints. Macrolide-nonsusceptible strains (clarithromycin minimum inhibitory concentrations 0.5 μg/mL or greater) were analyzed by multiplex polymerase chain reaction for the presence ofmefA andermB genes. RESULTS:S pneumoniaerepresented 9.0% (706 of 7881) of all isolates collected in CANWARD 2007. Of the 706S pneumoniaeisolates collected, 33.1% (234) were from blood and 66.9% (472) were from respiratory specimens. The overall resistance (resistant and intermediate) rates forS pneumoniaeisolated 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%). Multidrug resistance (resistance to penicillin, clarithromycin and trimethoprimsulfamethoxazole) accounted for 2% (n=9) and 0.5% (n=1) of respiratory and blood isolates, respectively. Susceptibility of 95% or greater was 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 to vancomycin, tigecycline, cethromycin or telithromycin was found. mefA was present in 53.6% (52 of 97) of respiratory and 59.3% (16 of 27) of blood macrolide-nonsusceptibleS pneumoniae, while ermB was present in 38.1% (37 of 97) of respiratory and 37% (10 of 27) of blood isolates. Eight of 97 (8.2%) respiratory and one of 27 (3.7%) blood isolates contained both mefA and ermB genes. CONCLUSIONS:S pneumoniaeis a common organism isolated from clinical specimens in Canadian hospitals. Resistance was highest to penicillin and clarithromycin, while ceftriaxone and levofloxacin susceptibility were both greater than 99%. No isolates resistant to vancomycin, tigecycline, linezolid or the ketolides were found. Resistance rates were higher among respiratory tract isolates ofS pneumoniaethan among blood isolates. Macrolide efflux,mefA, was the predominant mechanism of macrolide resistance among both respiratory and blood clarithromycin-nonsusceptible isolates.