Tazobactam was shown to be a potent inhibitor of group 1, 2a, 2b, and 2b' 13-lactamases. Extended kinetic studies with class A and C serine 13-lactamases showed that the PC1, TEM-2, and P99 enzymes all were reversibly inhibited prior to inactivation of the enzymes. The CcrA metallo-1-lactamase was less well inhibited, with a 50%o inhibitory concentration at least 3 orders of magnitude less favorable than those for most serine 13-lactamases. The numbers of hydrolytic turnovers of tazobactam before inactivation were 2 for PC1, 125 for TEM-2, 50 for P99, and 4,000 for the CcrA enzyme. In spectral studies, transient intermediates were formed after reaction of tazobactam with the PC1, TEM-2, and CcrA 1-lactamases, corresponding to enzymeassociated intermediates responsible for hydrolysis of tazobactam. Chromophores absorbing at 270 nm (CcrA) and 288 nm (TEM-2 and PC1) were observed for these reaction intermediates. The P99 cephalosporinase formed a stable complex with a UV maximum at 295 nm. Incubation of tazobactam with all of the enzymes resulted in accumulation of a tazobactam reaction product with a short-wavelength absorbance. This product has characteristics similar to those of the major eucaryotic metabolite of tazobactam. Possible reaction mechanisms are presented to explain the findings. In conclusion, both serine-based and metallo-13-lactamases were irreversibly inactivated by tazobactam following an initial transient inhibition phase.Tazobactam, a triazolyl-substituted penicillanic acid sulfone (3), is a ,B-lactamase inhibitor that has been successfully combined with piperacillin to protect this broad-spectrum penicillin from ,B-lactamase-mediated hydrolysis (17, 31). Although extensive microbiological studies have been described for the piperacillin-tazobactam combination (24, 29), little has been reported concerning the mechanism of inhibition of standard ,3-lactamases. In contrast to the elaborate biochemical studies describing clavulanic acid and sulbactam inactivation of the TEM-2 (7, 15, 18, 21) and PC1 ,-lactamases (13), no comparative enzymological studies have been performed with tazobactam.In this set of studies, a metallo-p-lactamase and three serine ,-lactamases from different functional groups (8,9) were examined in detail to determine the kinetics of inhibition by tazobactam. These enzymes include the P99 cephalosporinase not inhibited by clavulanic acid (Bush group 1); the PC1 penicillinase (group 2a) and the broad-spectrum TEM-2 P-lactamase (group 2b), both well inhibited by clavulanic acid; and the CcrA metallo-,-lactamase from Bacteroides fragilis (group 3), an enzyme not inhibited by the classical 1-lactamase inhibitors. The PC1 and TEM-2 ,-lactamases are members of structural class A, with serine as the active-site acylating residue (2). The CcrA enzyme is a zinc-dependent enzyme of structural class B (2, 32). P99 is a class C enzyme with a higher molecular weight, also with an active-site serine (20). Evidence is presented to show that each enzyme undergoes reversible reactions with...