Inducible Type I βlactamases of Gram-negative bacteria and resistance to βlactam antibiotics

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Abstract
Mutants, showing either constitutive (derepressed) or non-inducible expression of chromosomally-mediated Type I β-lactamase were obtained from clinical isolates of Enterobacter cloacae, Ent. aerogenes, Citrobacter freundii, Providencia stuartii, Morganella morganii, Serratia marcescens and Pseudomonas aeruginosa. The wildtype and mutant strains were compared for susceptibility to a range of β-lactam antibiotics. Derepression of β-lactamase synthesis generally, but not always, resulted in a marked reduction in susceptibility to the agents tested, including the ‘3rd generation’ cephalosporins. In many cases, the observed resistance would preclude, or severely compromise, the therapeutic efficacy of the drugs. In this context, depressed mutants of Enterobacter spp., Citro. freundil and Ps. aeruginosa could be of primary concern although those of Ser. marcescens, Prov. stuartii and Morg. morganii often exhibited equally high resistance levels to older β-lactams. Comparison of the susceptibilities of the non-inducible mutants with that of their inducible parents suggested variation in the β-lactamase inductive potency of different compounds in different organisms. For example, cefoxitin was a powerful inducer in Ent. cloacae, Citro. freundii and one strain of Ps. aeruginosa; similarly cefazolin and cefuroxime were good β-lactamase inducers in Ser. marcescens and Morg. morganii. Aminothiazolyl-oxime cephalosporins and ureido-penicillins were generally poor inducers. From such comparisons, the contribution of inducible Type I β-lactamase to resistance phenotype could be ascertained.