Simple screening method for β-lactamase-positive and -negative ampicillin-resistant Haemophilus influenzae isolates

Abstract

A simple, inexpensive method for screening for β-lactamase-positive and β-lactamase-negative ampicillin-resistant Haemophilus influenzae isolates was developed. Disks containing 10 μg of cloxacillin yielded no zone of inhibition when placed on chocolate agar plates inoculated with β-lactamase-positive (16 strains) or ampicillin-resistant (≥1 μg/ml) β-lactamase-negative (10 strains) H. influenzae, whereas ampicillin-susceptible (≤ 0.5 μg/ml; 36 strains) H. influenzae almost always (92%) showed a zone of inhibition.