SUMMARY: Sensitivity to ampicillin and cephalothin, the formation and induction of penicillinase and cephalosporinase, and the role of cephalosporinase in resistance to cephalothin were investigated in nine strains of Gramnegative bacteria. Naturally occurring strains resistant to cephalothin formed cephalosporinase, and ampicillin-resistant strains formed penicillinase. Cephalothin-resistant strains were all resistant to ampicillin, but the ampicillin-resistant strain of was sensitive to cephalothin and did not form cephalosporinase. Penicillinase and cephalosporinase activity were induced by benzylpenicillin in five strains of Gram-negative bacteria and cephalosporinase was also induced by cephalothin and 6-amino-penicillanic acid in four of the inducible strains examined. Methicillin and quinacillin were variable in effect as inducers and cloxacillin partially inhibited cephalosporinase activity of two strains. Intrinsic resistance to cephalothin was high in four of five naturally occurring resistant strains.


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