Susceptibility of Haemophilus influenzae to antimicrobial agents used in Canada. Canadian Study Group.

We evaluated the incidence of Haemophilus influenzae resistance to selected antimicrobials used in Canada. From 1985 to 1987, 2503 H. influenzae isolates obtained in 14 hospitals across Canada were sent to the Centre hospitalier de l'université Laval (CHUL) for identification, serotyping, biotyping and testing for beta-lactamase production. Susceptibility tests were done with the use of 12 antibiotics. Of the strains 424 (16.9%) produced beta-lactamase; the proportion varied from 12.8%, in Newfoundland, to 19.6%, in Ontario. Of the strains 18.3% were type b; 19.4% of those produced beta-lactamase. Almost 82% of the strains were not type b. The proportion of beta-lactamase-producing strains varied according to the isolation site, from 15.3% in the respiratory tract to 25.6% in the blood. The overall level of resistance was 19.3% to ampicillin, 24.2% to erythromycin, 3.8% to trimethoprim-sulfamethoxazole, 1.7% to amoxicillin-potassium clavulanate, 1.4% to cefaclor, 1.3% to tetracycline, 1.0% to rifampin, 0.7% to cefuroxime and 0.1% to cefamandole. Disc diffusion susceptibility testing revealed 64 strains (2.6%) that did not produce beta-lactamase but were resistant to ampicillin and 9 (0.4%) that produced beta-lactamase but were susceptible to ampicillin. The results of beta-lactamase production tests were identical regardless of whether the tests were done by the CHUL or by the other hospitals, but there was a marked difference in the susceptibility test results between the CHUL and the other centres. Our results suggest that the level of resistance of H. influenzae to antibiotics is increasing in Canada and that the initial choice of drug therapy may have to be modified.