Imipenem

Antimicrobial susceptibility testing for CR-AB identification was performed using broth microdilution methods according to the Clinical Laboratory Standard Institute (CLSI) guidelines [21 ]. A. baumannii isolates were grown in cation-adjusted Mueller-Hinton broth (CAMHB, Difco, BD Diagnostic Systems, USA) before adding them into 96-well plates containing antibiotics to the final concentration (approximately 10⁵ CFU/ml). A. baumannii isolates were tested against carbapenems, including imipenem, meropenem, and doripenem (Carbosynth, UK). The microtiter plate was incubated for 16–20 h at 35±2°C. The results were evaluated based on the MIC values from the minimum concentration of drugs that showed no visible growth. Clinical and Laboratory Standards Institute (CLSI) breakpoints were used for susceptibility determination. In this study, CR-AB was defined as an isolate resistant to at least one carbapenem agent. A. baumannii ATCC 19606, obtained from the American Type Culture Collection (ATCC, Manassas, VA, USA) was used as a quality control isolate.

The antimicrobials ceftazidime (CAZ) and amoxicillin (AMX) were sourced from Sigma Aldrich (Dorset, UK). Imipenem (IMI), meropenem (MER), avibactam (AVI) and relebactam (REL) were obtained from Carbosynth (Compton, UK). Stock solutions of all compounds were prepared in sterile dH₂O, apart from AMX where DMSO was used, and stored at −20 °C, where appropriate.