Determination of the antimicrobial susceptibilities of Canadian isolates of Haemophilus influenzae, Streptococcus pneumoniae and Moraxella catarrhalis

The susceptibility of Canadian isolates of three respiratory tract pathogen s (Haemophilus influenzae, Moraxella catarrhalis and Streptococcus pneumoni ae) to several antimicrobial agents were tested by two different methods. b eta-Lactamase was produced by 68/211 (32.2%) of H. influenzae isolates and 64/75 (85.3%) of M. catarrhalis isolates. For S. pneumoniae, 19/156 (12.2%) isolates were resistant to penicillin (MIC greater than or equal to 0.12 m g/L) and two isolates had MICs of 1.5 mg/L. For some combinations of agents and organisms, different methods gave different values for the proportion of isolates susceptible. Regardless of methodology, for H. influenzae, the most active antimicrobials based on proportion of strains susceptible were ciprofloxacin (100%) and cefpodoxime (98.5-100%). For M. catarrhalis, the m ost active agents were azithromycin, cefaclor, cefixime, cefpodoxime, cefur oxime, ciprofloxacin, clarithromycin and loracarbef (100% each); the least active was ampicillin. Against penicillin-sensitive and -resistant pneumoco cci, the activity was not significantly different for azithromycin and clar ithromycin (93.4-100%) and ciprofloxacin (MIC90 2.0 and 1.5 mg/L, respectiv ely) but was different for cefuroxime (99.3% and 31.6%, respectively), cefa clor (MIC90 0.75 and greater than or equal to 256 mg/L, respectively), cefp odoxime (MIC90 0.047 and 1.5 mg/L, respectively) and loracarbef (MIC90 0.75 and greater than or equal to 256 mg/L, respectively). This study indicates the increasing incidence, in Canada, of beta-lactamase resistance in H. in fluenzae and M. catarrhalis and penicillin resistance in S. pneumoniae.