P-1198. Inner Zones of Inhibition in Disk Diffusion Testing of Cefiderocol Best Reflect Activity Against Acinetobacter baumannii

Abstract Background Cefiderocol (FDC) is a siderophore-conjugated cephalosporin with activity against aerobic Gram-negative bacteria, including multidrug-resistant isolates. The disk diffusion (DD) method for FDC was approved by the Clinical and Laboratory Standards Institute (CLSI), but non-reproducible appearance of colonies within the zone of growth inhibition, especially observed with Acinetobacter baumannii, can complicate endpoint reading (Fig. 1). Here we investigated if reading outer zone of growth inhibition, which is more reproducible, can be used as endpoint reading.Figure 1.Example of colonies within predominant zone of growth inhibition.A: Predominant inhibition zone (outer inhibition zone).B: Inner zone of inhibition taking colonies within zone of inhibition into account.Figure 2.Correlation of inhibition zone diameters obtained by DD and MIC of FDC determined with broth microdilution.Modal MIC (n=9 - 30) and mean diameter (n=9) were calculated. Methods 41 A. baumannii strains were used for this study. Seventeen of 41 A. baumannii strains showed colonies in disk inhibition zone. MIC measurements in iron-depleted cation-adjusted Mueller Hinton broth (Becton Dickinson BBL) and DD testing using Mueller-Hinton agar (Becton Dickinson) and FDC disk (Mast Group) were performed and interpreted according to CLSI protocols. In vivo efficacy was evaluated in a neutropenic murine thigh infection model using human-simulated regimen dosing of FDC 1, 2, 3.Figure 3.Correlation of mean inhibition zone diameters of DD test and in vivo efficacy.“S” indicates susceptible. Mean zone diameter was calculated by using 9 diameter data for each strain. Results In correlation analysis between DD zone of growth inhibition and FDC MIC, inner diameters and MIC showed no very major error (VME), while outer diameters and MIC showed 2% (1/41) VME (Fig. 2). In correlation analysis between DD zone of growth inhibition and in vivo efficacy, all strains judged susceptible by inner diameter read-out showed a reduction in bacterial burden in vivo, whereas 19% (4/41) of strains judged susceptible by outer diameter read-out showed an increase in bacterial burden in vivo (Fig. 3). Conclusion Using the inner diameter as read-out for zone of growth inhibition showed a better correlation with MIC and efficacy in vivo, and therefore use of inner diameter for judging susceptibility in A. baumannii using DD is suggested. References 1. Monogue ML, et al. Antimicrob Agents Chemother. 2017;61(11):e01022-17 2. Gill CM, et al, JAC Antimicrob Resist. 2022;4(3):dlac047 3. Gill CM, et al, J Antimicrob Chemother. 2023;78(4):983-990 Disclosures Hidenori Yamashiro, Shionogi HQ: Employee Christopher M. Longshaw, PhD, Shionogi BV: Employee Boudewijn L. DeJonge, PhD, Shionogi Inc.: Employee Yoshinori Yamano, PhD, Shionogi HQ: Employee