Genetic and molecular characterization of beta-lactamase-negative ampicillin-resistant Haemophilus influenzae with unusually high resistance to ampicillin.

Previous studies with beta-lactamase-negative, ampicillin-resistant (BLNAR) Haemophilus influenzae from Japan, France, and North America indicate that mutations in ftsI encoding PBP3 confer ampicillin MICs of 1 to 4 micro g/ml. Several BLNAR strains with ampicillin MICs of 4 to 16 micro g/ml recently isolated from North America were studied. Pulsed-field gel electrophoresis identified 12 unique BLNAR strains; sequencing of their ftsI transpeptidase domains identified 1 group I and 11 group II mutants, as designated previously (K.