1287. Double Disk Diffusion Study to Evaluate the Synergistic Effect Between Cefiderocol and Ceftazidime-Avibactam Against Cefiderocol-Non-susceptible Acinetobacter baumannii

Abstract Background Cefiderocol (CFDC), a siderophore cephalosporin, has broad coverage of Gram-negative bacteria and been approved for clinical use in USA Europe. The SIDERO-WT surveillance studies showed that CFDC shows >95% susceptibility against Acinetobacter baumannii. Against many non-susceptible isolates, most which had blaPER gene, the combination avibactam significantly decreased MIC by broth microdilution (BMD). In this study, we evaluated appropriate methodology to evaluate synergistic effect disk diffusion studies. Methods testing was conducted as recommended CLSI using isolates (MIC >4 µg/mL based on breakpoint). BMD determined iron-depleted cation-adjusted Mueller-Hinton broth, presence or absence 4 avibactam. agar, synergy stacking methods. For methods, placed agar bacterial suspension 0.5 McFarland units spread, then ceftazidime-avibactam (CZA) disks stacked top, followed adding drop (30 µL) saline disks. As an alternative method, CZA immersed 1 second instead saline, top. zone size after 24-hour incubation at 37°C. Results blaPER-positive A. baumannii >64 CZA, 0.25 methods also resistant susceptiblilty both On other hand, blaNDM-positive 64 resistance even when multiple without with correlated well produced alone disks, respectively (Figure). Conclusion between could be detected Disclosures Yoshinori Yamano, PhD, Shionogi (Employee) Miki Takemura, MS, SHIONOGI & CO., LTD. Roger Echols, MD, (Consultant) Christopher Longshaw,