Trapping of nonhydrolyzable cephalosporins by cephalosporinases in Enterobacter cloacae and Pseudomonas aeruginosa as a possible resistance mechanism.

Resistance to cefotaxime (CTA) and ceftriaxone (CTR) in Enterobacter cloacae and Pseudomonas aeruginosa was investigated in several strains which are susceptible or resistant to these agents. All strains produced a chromosomally mediated cephalosporinase of the Richmond type 1. beta-Lactamases in susceptible strains were inducible, whereas resistant strains produced the enzymes constitutively. CTA and CTR were very poor substrates but potent inhibitors of all enzymes. Binding to, rather than hydrolysis by, beta-lactamases was assumed to be a major reason for resistance, and combination experiments supported this assumption. Dicloxacillin, which did not inhibit the growth and which was a poor inducer but a strong inhibitor of these beta-lactamases, exerted strong synergistic activity when combined with CTA or CTR in strains which produced large amounts of beta-lactamase constitutively. Cefoxitin, on the other hand, poorly active alone, but a good inducer, strongly antagonized CTA or CTR in susceptible strains producing inducible enzymes. In marked contrast to CTA and CTR were the findings with cefsulodin. Cefsulodin was active against CTA- and CTR-resistant Pseudomonas, and its activity was hardly influenced by dicloxacillin or cefoxitin. Since cefsulodin was found to have a very low affinity for all cephalosporinases, these findings corroborate the assumption that binding of nonhydrolyzable cephalosporins, rather than hydrolysis by cephalosporinases, may play an important role in resistance to these agents and other newer cephalosporins in Enterobacteriaceae, as well as in other gram-negative bacteria.