In Vitro Susceptibility of Ceftazidime-Avibactam- Resistant Klebsiella pneumoniae Isolates to the Ceftazidime-Avibactam Plus Aztreonam Combination Using the Broth Disk Elution Method.

Infections caused by multidrug-resistant Gram-negative bacteria, particularly carbapenemase-producing Enterobacterales, present significant diagnostic and therapeutic challenges, highlighting the need for practical laboratory methods to evaluate combination therapies. This study investigated 90 ceftazidime-avibactam (CZA)-resistant Klebsiella pneumoniae isolates collected from clinical specimens between January 2021 and July 2023. Species identification was performed using MALDI-TOF MS, and antimicrobial susceptibility testing was conducted with the BD Phoenix™ system according to Clinical and Laboratory Standards Institute (CLSI) criteria, with disk diffusion used for confirmation when necessary. Carbapenemase production was determined phenotypically using the modified Carbapenem Inactivation Method. For a subset of isolates with available whole-genome sequencing data, resistance genes were confirmed, although genotypic analysis was not performed for all isolates. The in vitro activity of the CZA plus aztreonam (CZA-ATM) combination was evaluated using the CLSI-recommended broth disk elution method. All isolates were resistant to CZA and aztreonam when tested individually, whereas no visible growth was observed with the combination, indicating in vitro susceptibility. These findings suggest that the broth disk elution method is a practical approach for assessing CZA-ATM activity in routine clinical laboratories. Further studies including reference method validation and evaluation of borderline or resistant isolates are needed.