Neutralization of beta-lactam antibiotics in an environmental monitoring medium

The neutralizing ability of Difco Bacto Penase, BBL Penicillinase, and Genzyme beta-lactamase for penicillin G, oxacillin, cephalothin, cefazolin, ceftazidime, ceftriaxone, cefotaxime, imipenem, and meropenem was determined in an agar medium. Trypticase soy agar plates (20 mL) containing 200 MU/L Difco Penase, 200 MU/L BBL Penicillinase, or 2 vials/L of Genzyme beta-lactamase were dosed with 20, 50, and 100 micrograms of each antibiotic and then inoculated (50-100 CFU/plate) with a susceptible microorganism. Percentage recoveries of the organism were calculated after incubation. Genzyme beta-lactamase effectively neutralized 100 micrograms of all the antibiotics except ceftazidime (32% recovery with 20 micrograms). The two penicillinases effectively neutralized 100 micrograms of penicillin G, oxacillin, cephalothin, and cefazolin; were slightly effective against ceftriaxone and cefotaxime (20-57% recovery with 20 micrograms); and totally ineffective against ceftazidime, imipenem, and meropenem. This investigation resulted in a useful procedure for qualifying and/or selecting a beta-lactamase for use in environmental monitoring medium to neutralize a particular beta-lactam.