Tazobactam is a β-lactamase inhibitor with greater activity against Ambler class A serine penicillinases and class C cephalosporinases, including TEM-1, SHV-1, and P99 (ICs = 97, 150, and 8.5 nM) than class B metallo-β-lactamases and the class D oxacillinase OXA-1 (ICs = 400,000 and 1,400 nM, respectively). However, it also inhibits the class D oxacillinase OXA-2 (IC = 10 nM). Tazobactam, in combination with piperacillin, is active against Gram-positive (MICs = 2-128 µg/ml) and Gram-negative bacteria (MICs = 4-128 µg/ml). In a neutropenic mouse model of thigh infection, tazobactam potentiates the activity of ceftolozane against Enterobacteriaceae. Formulations containing tazobactam, alone and in combination with piperacillin, have been used in the treatment of β-lacatamase-producing bacteria.