Canadian beta-lactamase-negative Haemophilus influenzae isolates showing decreased susceptibility toward ampicillin have significant penicillin binding protein 3 mutations.

Nineteen beta-lactamase-negative ampicillin-resistant (BLNAR) and 2 beta-lactamase-positive amoxicillin-clavulanic acid-resistant Canadian Haemophilus influenzae strains were characterized. All 21 isolates were found to have the N526K mutations in their ftsI genes, and their ampicillin MIC(50) values were 4-8 times that of beta-lactamase-negative ampicillin-susceptible strains. The difficulty in detection of BLNAR strains was discussed.