Some molecular properties of Citrobacter diversus beta-lactamases.

Citrobacter diversus ULA-27, a clinical isolate showing a broad resistance pattern towards both penicillins and cephalosporins, produces chromosome encoded beta-lactamases. However, the strain remains susceptible to some cephamycins, imipenem, ceftazidime and tetracyclines. Crude bacterial extracts analyzed by isoelectric focusing on polyacrylamide gels, revealed the presence of two main isoforms and some "satellite" bands focusing in the pH range 5.7-7.2. The isoform showing the pIs 6.8 and 6.2 were characterized as class "A" beta-lactamases (according to Ambler's classification) based on the rate of interaction of beta-iodopenicillanate and the amino acid sequence around the active site serine. The substrate specificity of the Citrobacter diversus beta-lactamases explains the resistance phenomenon of this bacterium to penicillins and cephalosporins.