Avibactam is a diazabicyclooctane β-lactamase inhibitor possessing outstanding but incomplete efficacy against multidrug-resistant Gram-negative pathogens in combination with β-lactam antibiotics. Significant pharmaceutical investment in generating derivatives of avibactam warrants a thorough characterization of their activity. We show here through structural and kinetic analysis that select diazabicyclooctane derivatives display effective but varied inhibition of two clinically important β-lactamases (CTX-M-15 and OXA-48). Furthermore, these derivatives exhibit considerable antimicrobial activity (MIC ≤ 2 μg/mL) against clinical isolates of Pseudomonas aeruginosa, Escherichia coli, and Enterobacter spp. Imaging of cell phenotype along with structural and biochemical experiments unambiguously demonstrate that this activity, in E. coli, is a result of targeting penicillin-binding protein 2. Our results suggest that structure-activity relationship studies for the purpose of drug discovery must consider both β-lactamases and penicillin-binding proteins as targets. We believe that this approach will yield next-generation combination or monotherapies with an expanded spectrum of activity against currently untreatable Gram-negative pathogens.