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

Nineteen β-lactamase–negative ampicillin-resistant (BLNAR) and 2 β-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 MIC50 values were 4–8 times that of β-lactamase–negative ampicillin-susceptible strains. The difficulty in detection of BLNAR strains was discussed.