Comparison of Activity in Culture Medium

The aminoglycoside antibiotics, particularly gentamicin, are increasingly widely used and frequently in patients with impaired renal function. All these drugs are potentially ototoxic and measurement of blood levels is often desirable but may be complicated by the presence of a second drug, most commonly a penicillin or cephalosporin. Assays of streptomycin in the presence of penicillin have been done for many years by the simple expedient of adding commercial penicillinase, and Newsome (1967) suggested that this might also inactivate cephaloridine, methicillin, and cloxacillin. However, it seems that not only different brands of penicillinase, but even different batches of the same manufacturer may vary in this respect (Garrod and O'Grady, 1971) and many workers have felt that it was unwise to rely upon any and have either used an organism known to be resistant to the second drug to assay the aminoglycoside or declined to attempt the assay. Most commercial penicillinases are prepared from cultures of B. cereus or B. licheniformis which produce both beta-lactamase I which inactivates penicillin and ampicillin, and beta-lactamasellwhich inactivates the penicillinase-resistant penicillins and cephalosporins. The variability in the commercially available material is mainly due to the instability of beta-lactamase II, but the development by Sabath and Abraham (1966) of a zinc stabilized enzyme from B. cereus 569H opened up the possibility of the commercial production of a 'universal penicillinase' which could be relied upon to inactivate all the penicillins and cephalosporins in clinical use. A freeze-dried preparation containing both beta-lactamases I and II, which is made from B. cereus 569/H by the procedure of R. B. Davies and E. P. Abraham (unpublished), is now available from