Chromosomal beta-lactamases of Enterobacter cloacae are responsible for resistance to third-generation cephalosporins.

About 70% of all Enterobacter cloacae strains tested possessed one of two species-specific beta-lactamases. These enzymes, E. cloacae beta-lactamase A and E. cloacae beta-lactamase B, with isoelectric points of 8.8 and 7.8, respectively, had the same pH and temperature optima. Both showed similar enzyme kinetics and were inhibited by cloxacillin but not by p-chloromercuribenzoate. E. cloacae beta-lactamase B appeared to be identical with the enzyme of E. cloacae P99. By a mutation in a regulatory gene, inducible enzyme production could be converted into constitutive expression. In E. cloacae, both enzymes did not hydrolyze third-generation cephalosporins, but they were solely responsible for resistance toward these drugs. This was demonstrated by the characterization of Escherichia coli strains expressing an identical resistance pattern after transfer of the corresponding Enterobacter gene.