Fourteen plasmid-encoded extended-spectrum β-lactamases were purified from transconjugants of original clinical isolates. The Vmax, Km and Vmax/Km were each determined for ampicillin, carbenicillin, cephaloridine, cephalexin, cefuroxime, cefuroxime, cefixime, cefdinir, ceftazidime and cefotaxime as substrates with eight of these enzymes and with the narrow-spectrum β-lactamase, TEM-1. The relative rates of hydrolysis of ampicillin, cephaloridine, cephalexin, cefuroxime, cefixime, cefdinir, ceftazidime and cefotaxime were also determined for the remaining six enzymes. Cefdinir had Vmax/Km or relative rates of hydrolysis values either equal to or lower than ampicillin, cephaloridine, cephalexin and cefotaxime for all the enzymes tested. Overall, cefdinir was more stable to the 15 β-lactamases tested than either cefuroxime or cefixime; however, ceftazidime was more stable than cefdinir to hydrolysis by eight of the enzymes tested.


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