Unusual mode of inhibition of Citrobacter freundii beta-lactamases by ceftriaxone

Several Citrobacter freundii beta-lactamases, discernible by their substrate profiles and sensitivities to inhibition by newer cephalosporins, were inhibited by ceftriaxone in an unusual manner. Inhibition was noncompetitive at low concentrations of cephaloridine but became competitive at concentrations above 600 muM, which is close to the K(m) (570 muM) for cephaloridine.