Comparative stabilities of penicillins and cephalosporins to staphylococcal β-lactamase and activities against Staphylococcus aureus

The relative stabilities of five penicillinase-stable penicillins and five cephalosporins to staphylococcal β-lactamase have been determined by (1) measurement of rates of hydrolysis by an isolated enzyme preparation, (2) comparison of antibacterial activities against non-β-lactamase and β-lactamase-producing strains of Staph. aureus and (3) demonstration of rates of inactivation by actively growing cultures of β-lactamase positive staphylococci. Overall, the penicillinase-stable penicillins showed greater stability to staphylococcal β-lactamase than the cephalosporins and in descending order the relative stabilities of the compounds were methicillin > cloxacillin = dicloxacillin = flucloxacillin > oxacillin = cephalothin = cephradine = cephalexin > cephazolin = cephaloridine.