Resistance of cephoxazole-benzylpenicillin combinations to destruction by β-lactamases associated with bovine mastitis

The effectiveness of cephoxazole in preventing hydrolysis of benzylpenicillin by β-lactamases has been confirmed by inhibition studies with examples of the principal types of β-lactamase produced by organisms commonly found in the milk of mastitic cows. Even the enzyme that proved most difficult to inhibit was completely inhibited when 0·6 m m cephoxazole was added to 1 m m benzylpenicillin; in the absence of cephoxazole the benzylpenicillin was completely hydrolysed in 2·5 min by staphylococcal penicillinase under the conditions of the experiment. The benzylpenicillin-cephoxazole combination was also found to be more effective than either ampicillin-cloxacillin or benzylpenicillin-cloxacillin combinations. A 1:1 ratio by weight of benzylpenicillin: cephoxazole is therefore highly stable to the β-lactamases commonly cultured from the milk of mastitic cows.