Iodometric detection of Haemophilus influenzae beta-lactamase: rapid presumptive test for ampicillin resistance.

Strains of Haemophilus influenzae type b sporadically isolated from clinical specimens are ampicillin resistant due to production of a beta-lactamase. This enzyme which inactivates ampicillin and penicillin G is not produced by ampicillin-susceptible strains. Various characteristics of beta-lactamase production and ampicillin resistance of three H. influenzae type b isolates were investigated. A sensitive iodometric test was employed to detect beta-lactamase; positive results were obtained in 5 min with 10(9) bacteria taken from cultures on a nutritionally adequate agar medium. This simple chemical test will enable the hospital laboratory to obtain presumptive evidence of ampicillin resistance on the same day that H. influenzae is isolated.