Frequency and Mechanism of Spontaneous Resistance to Sulbactam Combined with the Novel β-Lactamase Inhibitor ETX2514 in Clinical Isolates of Acinetobacter baumannii

The novel diazabicyclooctenone ETX2514 is a potent, broad- spectrum serine beta-lactamase inhibitor that restores sulbactam activity against resistant Acinetobacter baumannii. The frequency of spontaneous resistance to sulbactam- ETX2514 in clinical isolates was found to be 7.6 x 10(-10) to < 9.0 x 10(-10) at 4 x MIC and mapped to residues near the active site of penicillin binding protein 3 (PBP3). Purified mutant PBP3 proteins demonstrated reduced affinity for sulbactam. In a sulbactam-sensitive isolate, resistance also mapped to stringent response genes associated with resistance to PBP2 inhibitors, suggesting that in addition to beta-lactamase inhibition, ETX2514 may enhance sulbactam activity in A. baumannii via inhibition of PBP2.