Ceftazidime-avibactam - Aztreonam overlay, the Colistin Broth Disk Elution test and the VITEK® 2 for detecting Colistin Resistance in Carbapenemase Producing Enterobacterales in Sydney Australia
Introduction: This study evaluated the performance of the Ceftazidime-avibactam-Aztreonam (CAZA/AZT) overlay, the Colistin Broth Disk Elution (CBDE), the VITEK 2, and an agar dilution method as reliable methods for screening for colistin resistance in a local collection of 30 carbapenemase-producing Enterobacterales (CPE) isolates collected between 2023 and 2025 with different sequence type (ST) lineages in Sydney, Australia. Methods: Susceptibility to aztreonam (AZT), ceftazidime-avibactam (CAZA), and CAZA/AZT overlay synergy was assessed using Etest methods. Colistin minimum inhibitory concentrations (MICs) were determined using the VITEK 2XL AST-XN24 card, CBDE as described by the Clinical Laboratory Standards Institute (CLSI, 2025), and an agar dilution method. Agreement between methods was assessed using percentage concordance, positive percentage agreement, negative percentage agreement, and overall categorical agreement. Results: A total of 14 isolates (47%) were resistant to AZT, 25 (83%) to CAZA, and 4 isolates (13%) were resistant to the CAZA/AZT overlay, with MIC values >4 mg/L. CBDE showed 93.3% concordance with VITEK 2XL and 90% concordance with agar dilution for colistin MIC categories. Using the CLSI colistin intermediate breakpoint category of ≤2 mg/L, CBDE demonstrated 100% positive percentage agreement, negative percentage agreement, and overall categorical agreement with VITEK 2XL. One Morganella morganii isolate showed high-level intrinsic resistance to colistin. Conclusion: The CBDE method showed strong categorical agreement with other established colistin MIC methods. Results from this study indicate that CBDE may provide smaller laboratories with a simple and practical option for routine screening of CPE isolates for colistin resistance in low-endemic settings where colistin MIC broth microdilution is not available or routinely performed. Continued research with a larger sample size, including more colistin-resistant isolates, is needed to further investigate the CBDE method and its routine use alongside other accepted methods currently in practice.