Haemophilus influenzae from four laboratories in one Canadian city

Serotype, biotype and antimicrobial susceptibilities of 250 clinical isolates of Haemophilus influenzae from University, affiliated and community hospitals and a private laboratory were compared. For each drug, agar dilution susceptibility testing was compared to at least one other method (modified Kirby-Bauer and/or microdilution). Most isolates (86%) were non-typable, 10% were type b. Biotype II was most common (58%). The highest prevalence of serotype b (28%) was seen in the community hospital, which also had only 4% of all biotype III isolates. beta-Lactamase production ranged from 20% (private laboratory) to 5% (affiliated hospital); it was higher among type b (23%), biotype II (17%), and from non-respiratory (26%) than respiratory sites (8%). 51% of 35 beta-lactamase producers were found in the 24% of patients under age 6. Microdilution missed seven while agar dilution and disc diffusion detected all. All isolates were susceptible to cefamandole, cefuroxime, cotrimoxazole and chloramphenicol, 86%, 98%, 99% and 27% to ampicillin, cefaclor, tetracycline and erythromycin respectively. Microdilution is unreliable for detection of ampicillin resistance mediated by beta-lactamase production.