Simple screening method for beta-lactamase-positive and -negative ampicillin-resistant Haemophilus influenzae...

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