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Antimicrobial Agents and Chemotherapy, June 2003, p. 1875-1881, Vol. 47, No. 6
0066-4804/03/$08.00+0     DOI: 10.1128/AAC.47.6.1875-1881.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Antimicrobial Resistance in Haemophilus influenzae and Moraxella catarrhalis Respiratory Tract Isolates: Results of the Canadian Respiratory Organism Susceptibility Study, 1997 to 2002

George G. Zhanel,^1,2,3 Lorraine Palatnick,³ Kimberly A. Nichol,^1,3 Don E. Low,⁴ The CROSS Study Group, and Daryl J. Hoban^1,3*

Department of Medical Microbiology, Faculty of Medicine, University of Manitoba,¹ Departments of Medicine,² Clinical Microbiology, Health Sciences Centre, Winnipeg, Manitoba,³ Mount Sinai Hospital, Toronto, Ontario, Canada⁴

Received 30 September 2002/ Returned for modification 11 February 2003/ Accepted 8 March 2003

A total of 7,566 unique patient isolates of Haemophilus influenzae and 2,314 unique patient isolates of Moraxella catarrhalis were collected between October 1997 and June 2002 from 25 medical centers in 9 of the 10 Canadian provinces. Among the 7,566 H. influenzae isolates, 22.5% produced ß-lactamase, while 92.4% of the 2,314 M. catarrhalis isolates produced ß-lactamase. The incidence of ß-lactamase-producing H. influenzae isolates decreased significantly over the 5-year study period, from 24.2% in 1997-1998 to 18.6% in 2001-2002 (P < 0.01). The incidence of ß-lactamase-producing M. catarrhalis isolates did not change over the study period. The overall rates of resistance to amoxicillin and amoxicillin-clavulanate for H. influenzae were 19.3 and 0.1%, respectively. The rank order of cephalosporin activity based on the MICs at which 90% of isolates were inhibited (MIC₉₀s) was cefotaxime > cefixime > cefuroxime > cefprozil > cefaclor. On the basis of the MICs, azithromycin was more active than clarithromycin (14-OH clarithromycin was not tested); however, on the basis of the NCCLS breakpoints, resistance rates were 2.1 and 1.6%, respectively. Rates of resistance to other agents were as follows: doxycycline, 1.5%; trimethoprim-sulfamethoxazole, 14.2%; and chloramphenicol, 0.2%. All fluoroquinolones tested, including the investigational fluoroquinolones BMS284756 (garenoxacin) and ABT-492, displayed potent activities against H. influenzae, with MIC₉₀s of ≤0.03 µg/ml. The MIC₉₀s of the investigational ketolides telithromycin and ABT-773 were 2 and 4 µg/ml, respectively, and the MIC₉₀ of the investigational glycylcycline GAR-936 (tigecycline) was 4 µg/ml. Among the M. catarrhalis isolates tested, the resistance rates derived by using the NCCLS breakpoint criteria for H. influenzae were <1% for all antibiotics tested except trimethoprim-sulfamethoxazole (1.5%). In summary, the incidence of ß-lactamase-positive H. influenzae strains in Canada is decreasing (18.6% in 2001-2002), while the incidence of ß-lactamase-positive M. catarrhalis strains has remained constant (90.0% in 2001-2002).

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* Corresponding author. Mailing address: Clinical Microbiology, Health Sciences Centre, MS673-820 Sherbrook St., Winnipeg, Manitoba R3A 1R9, Canada. Phone: (204) 787-1191. Fax: (204) 787-4699. E-mail: dhoban{at}hsc.mb.ca.

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Antimicrobial Agents and Chemotherapy, June 2003, p. 1875-1881, Vol. 47, No. 6
0066-4804/03/$08.00+0     DOI: 10.1128/AAC.47.6.1875-1881.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

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