In vitro activity of cefpirome against beta-lactamase-inducible and stably derepressed Enterobacteriaceae.

Most members of the Enterobacteriaceae and Pseudomonas aeruginosa possess an inducible chromosomal class I beta-lactamase. Bacterial strains which produce high levels of beta-lactamase constitutively can be isolated from infections; these derepressed mutants are responsible for resistance to third-generation cephalosporins and ureidopenicillins. Minimum inhibitory concentrations of cefpirome, a fourth-generation cephalosporin, and other beta-lactam antibiotics were determined for a series of mutants of Citrobacter freundii, Enterobacter cloacae, Morganella morganii, Proteus vulgaris and Serratia marcescens with inducible stably derepressed or basal expression of chromosomal class I beta-lactamases. All the antibiotics tested were almost equally active against beta-lactamase-inducible organisms and their basal mutants. Imipenem and cefpirome showed better activity against derepressed mutants than third-generation cephalosporins and ureidopenicillins.