Susceptibility of Haemophilus influenzae to Ampicillin as Determined by Use of a Modified, One-Minute Beta-Lactamase Test

Fifty-seven cultures of Haemophilus influenzae were tested for production of beta-lactamase by use of a modified, 1-min test. Only the 24 cultures that were ampicillin resistant, whether type b or not, yielded positive results for the enzyme; all ampicillin-susceptible isolates were uniformly negative. The beta-lactamase procedure is simple to perform and requires only commonly available reagents.