Susceptibility of Canadian isolates of Haemophilus influenzae, Moraxella catarrhalis and Streptococcus pneumoniae to oral antimicrobial agents

We measured the susceptibility of Canadian isolates of three respiratory tract pathogens (Haemophilus influenzae, Moraxella catarrhalis and Streptocuccus pneumoniae) to several currently approved antimicrobial agents by two different methods. We also measured the susceptibility of isolates to seven fluoroquinolones. Beta-lactamase was produced by 123/566 (21.7%) of H. influnzae isolates compared with 178/200 (89%) of M. catarrhalis isolates. For S. pneumonae 83/374 (22.2%) isolates were penicillin resistant and of these 2.1% (8/3743 showed high level resistance (MIC greater than or equal to 2 mg/l). Regardless of methodology, all fluoroquinolones were highly active against H. influenzae(MIC90 less than or equal to 0.031 mg/l) and M. catarrhalis (MIC90 less than or equal to 0.064 mg/l) isolates. Susceptibility of H. influenzae to cefuroxime and amoxycillin/clavulanic acid was 99- 100% whereas 84-85.5% were susceptible to cefaclor and cefprozil. Azithromycin susceptibility ranged from 82.6 to 99.2% depending on the method. M. catarrhalis isolates were uniformly susceptible to all agents tested except amoxycillin. Cross-resistance in S. pneumoniae to all non-quinolone agents was concurrent with increasing penicillin resistance as shown by increasing MIG,, values. For the fluoroquinolones tested, the rank order of potency based on MIG,, values was as follows: gemifloxacin (0.031-0.063 mg/l), trovafloxacin (0.125 mg/l), moxifloxacin (0.125-0.25 mg/l), grepafloxacin (0.125-0.25 mg/l), gatifloxacin (0.5 mg/l), levofloxacin (1 mg/l) and ciprofloxacin (2 mg/l). Our study confirms either a high or increasing prevalence of antimicrobial resistant respiratory pathogens in Canada and also compares the new and old fluoroquinolones and their potential role as therapy for community-acquired infections. The prevalence of p-lactamase positive H. influenzae may have decreased from levels reported in previous studies. (C) 2001 Published by Elsevier Science B.V. and International Society of Chemotherapy. All rights reserved.