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Background Avibactam (AVI) is a β-lactamase inhibitor with potent inhibitory activity against Class A, Class C, and some Class D serine β-lactamases. The combination of ceftazidime (CAZ) with AVI has been approved in Europe and in the United States for several indications. This study evaluated the in vitro activity of CAZ-AVI and comparators against Enterobacteriaceae (Eba) and Pseudomonas aeruginosa (Pae) isolates collected from patients with bloodstream infections as part of the ATLAS surveillance program in 2014–2017. Methods A total of 53416 Eba and 15050 Pae nonduplicate clinically significant isolates, including 5155 Eba and 845 Pae isolated from bloodstream infections, were collected by 167 hospital laboratories in 36 countries in Europe, Latin America, Asia/Pacific (excluding China), and the Middle East/Africa region. Susceptibility testing was performed by CLSI broth microdilution. CAZ-AVI was tested at a fixed concentration of 4 µg/mL AVI. Meropenem-nonsusceptible (MEM-NS) Eba and Pae isolates were screened for the presence of β-lactamase genes. Results Susceptibility data are shown in the Table. Percentages of susceptibility (% S) to the tested agents were 0.2–2.8% lower among Eba and Pae from bloodstream infections compared with isolates from combined sources in most cases. CAZ-AVI showed potent in vitro activity against all Eba bloodstream isolates and subsets of CAZ-NS and colistin-resistant (CST-R) isolates (MIC90, 0.5–2 µg/mL, 96.0–100% S). Reduced activity against MEM-NS Eba was attributable to carriage of class B metallo-β-lactamases (MBLs) because all MEM-NS MBL-negative isolates were susceptible to CAZ-AVI. CAZ-AVI also showed good in vitro activity against the majority of Pae bloodstream isolates (MIC90, 16 µg/mL, 89.5% S). Activity was reduced against CAZ-NS, MEM-NS and CST-R subsets (53.7–85.0% S), which included isolates carrying MBLs, but exceeded the activity of CAZ and MEM against these subsets by 15–65%. CST and amikacin were the only tested comparators that demonstrated comparable or greater activity against Pae bloodstream isolates. Conclusion CAZ-AVI provides a valuable therapeutic option for treating bloodstream infections caused by MBL-negative Eba and Pae isolates. Disclosures All authors: No reported disclosures.

open forum infectious diseases

693. In Vitro Activity of Ceftazidime–Avibactam and Comparator Agents Against Enterobacteriaceae and Pseudomonas aeruginosa Collected From Patients with Bloodstream Infections as Part of the ATLAS Global Surveillance Program, 2014–2017