A β-lactamase inhibitor; inhibits Ambler class A serine penicillinases and class C cephalosporinases, including TEM-1, SHV-1, and P99 (IC50s = 97, 150, and 8.5 nM) and the class D oxacillinase OXA-2 (IC50 = 10 nM); active against Gram-positive (MIC90s = 2-128 μg/ml) and Gram-negative bacteria (MIC90s = 4-128 μg/ml) when used in combination with piperacillin; potentiates the activity of ceftolozane against Enterobacteriaceae in a neutropenic mouse model of thigh infection