In-vitro activity and beta-lactamase stability of methicillin, isoxazolyl penicillins and cephalothin against coagulase-negative staphylococci.

Forty-nine clinical strains of coagulase-negative staphylococci were investigated for susceptibility to methicillin, oxacillin, cloxacillin, dicloxacillin, flucloxacillin and cephalothin. The highest level of resistance was found to methicillin and the lowest to cephalothin. The resistance-level of the isoxazolyl penicillins showed a high degree of uniformity. However more strains were resistant to cloxacillin and oxacillin than to dicloxacillin and flucloxacillin. Only a weak correlation was found between beta-lactamase production, and resistance to the six antibiotics. Methicillin was the most stable. The inactivation of cephalothin differed from that of the other antibiotics.