Evaluation of the beta lactam disk for rapid detection of penicillinase producing bacteria: comparison with three other methods.

One thousand and four strains of gram positive and gram negative bacteria were tested for the production of beta lactamase by Beta Lactam reagent disks and the results were compared with acidometric microtiter, disk diffusion and antibiotic dilution procedures. Three hundred and sixty-three clinical isolates had a minimal inhibitory concentration (MIC) of greater than or equal to 2.0 micrograms/ml for penicillins. All the resistant strains of N. gonorrhoeae and Haemophilus tested gave positive beta lactamase reactions with both methods within 30 min. Four hundred and thirty-seven members of these two genera and all other bacteria with MIC values of less than or equal to 2.0 micrograms/ml remained negative for the beta lactamase test by both procedures in 4 h. The Beta Lactam reagent disk as well as the micro-method are simple, rapid and reliable. Cost analysis of the two procedures demonstrated the micro-method to be more economical.