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JAC Advance Access published online on August 26, 2005
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dki315
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© The Author 2005. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: journals.permissions@oupjournals.org
Received June 21, 2005
Revised August 5, 2005
Accepted August 12, 2005

Brief report

Are fluoroquinolone-susceptible isolates of Streptococcus pneumoniae really susceptible? A comparison of resistance mechanisms in Canadian isolates from 1997 and 2003

Kristen N. Schurek 1*, Heather J. Adam 1, Christine G. Siemens 2, Chris J. Hoban 2, Daryl J. Hoban 1, and George G. Zhanel 1

1 Department of Medical Microbiology, Faculty of Medicine, University of Manitoba, 5th Floor, Basic Medical Sciences Building, 730 William Avenue, Winnipeg, Manitoba, R3E 0W3, Canada; Clinical Microbiology, Health Sciences Centre, MS673-820 Sherbrook Street, Winnipeg, Manitoba, R3A 1R9, Canada
2 Department of Medical Microbiology, Faculty of Medicine, University of Manitoba, 5th Floor, Basic Medical Sciences Building, 730 William Avenue, Winnipeg, Manitoba, R3E 0W3, Canada

* To whom correspondence should be addressed.
Kristen N. Schurek, E-mail: kris_schurek{at}yahoo.com


  Abstract

Objectives: To assess the prevalence of efflux and amino acid substitutions in ParC and GyrA in Canadian clinical isolates of fluoroquinolone-susceptible Streptococcus pneumoniae with levofloxacin MICs of 1 mg/L collected before the introduction of the respiratory fluoroquinolones (1995-1997) and after 7 years of use (2003).

Methods: Quinolone resistance determining regions of parC and gyrA were sequenced for 111 clinical isolates collected from 1995 to 1997 and 665 isolates collected in 2003. Efflux was assessed using a reserpine agar dilution method.

Results: No isolates exhibited efflux. No significant increase in isolates harbouring amino acid substitutions was observed over time (0.9% in 1995-1997 to 2.1% in 2003, P = 0.32). However, the proportion of isolates with a ciprofloxacin MIC = 2 mg/L and a levofloxacin MIC = 1 mg/L versus ciprofloxacin MIC = 1 mg/L and a levofloxacin MIC = 1 mg/L increased over time (3.6% to 6.5%, P = 0.0021).

Conclusions: No increase in prevalence of first-step parC mutations was observed among all fluoroquinolone-susceptible clinical isolates of S. pneumoniae with levofloxacin MICs of 1 mg/L after the introduction of the respiratory fluoroquinolones; however, fluoroquinolones appear to be selecting for isolates with elevated ciprofloxacin MICs.

Keywords: levofloxacin; mutations; efflux.
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