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 in Streptococcus
pneumoniae in 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 of mefA and ermB genes.
RESULTS: S pneumoniae represented 9.0% (706 of 7881) of all isolates
collected in CANWARD 2007. Of the 706 S pneumoniae isolates collected,
33.1% (234) were from blood and 66.9% (472) were from
respiratory 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%). 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-nonsusceptible S 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 pneumoniae is 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 of S pneumoniae than
among blood isolates. Macrolide efflux, mefA, was the predominant
mechanism of macrolide resistance among both respiratory and blood
clarithromycin-nonsusceptible isolates.
