IDENTIFICATION OF A CLAVULANATE SUSCEPTIB LE CEPHALOSPORINASE IN A CLINICAL STRAIN OF SERRATIA-FONTICOLA

We analyzed the beta-lactamase production of a Serratia fonticola isol ated for its resistance to cefuroxime (Minimum Inhibitory Concentratio n > 256 mg/l) in December 1993 from a patient hospitalized in Meaux. T he wild strain was resistant to amoxycillin but sensitive to augmentin , that suggested the production of a beta-lactamase susceptible to cla vulanic acid. For the wild strain, beta-lactamase production was induc ible and only one enzyme with an isoelectric point of 8.12 was detecte d. beta-lactamase production was 16 mU/mg for non-induced extracts and ranged from 100 to 230 mU/mg in the presence of inducing beta-lactams (enzyme activity was measured with penicillin G as substrate). On a S zybalski gradient a constitutive strain was obtained. Its enzyme produ ction was 13 000 mU/mg. The kinetics and isoelectric points of the enz ymes produced by the two strains were identical. This beta-lactamase h ydrolyzes penicillins (amoxycillin : Vm = 60 relative to penicillin G = 100, ticarcillin : 15), first generation cephalosporins (cephalothin Vm = 930). However, this enzyme hydrolyzes efficiently oxyimino-cepha losporins : cefuroxime (Vm = 70) and cefotaxime (Vm = 120), but cepham ycins are not substrates. Clavulanic acid has a very good affinity for this beta-lactamase (K-i = 0.09 mu M) which is inactivated progressiv ely (I-50 = 0.045 mu g/ml). These properties shows some similarities w ith those of the class A beta-lactamases of P. vulgaris RO104 (pI = 8. 3), P. penneri 14HBC (pI = 6.65) and the plasmid-mediated extended-spe ctrum MEN-1 (pI = 8.4).