Compounds used in this study. Antibiotics from each of the four classes of β-lactams were studied, including penicillins, AMP; the cephalosporins, THIN, nitrocefin (NCF), TAX, and TAZ; the monobactam, AZT; and the carbapenem, IMI. The tautomerization of the double bond within IMI upon β-lactamase hydrolysis to form the Δ2 isoform to the Δ1 isoform is depicted in the 3rd row. β-Lactamase inhibitors utilized in this work include CLAV, SUL, and TAZO.