Investigation of the efficacy of ceftazidime-avibactam in combination with different antimicrobials against carbapenem-resistant Klebsiella pneumoniae isolates

Klebsiella pneumoniae is the second most commonly isolated Gram-negative pathogen after Escherichia coli in bloodstream infections. The organism, which exhibits high antibiotic resistance as a result of alterations in its genetic structure, can produce extended-spectrum beta-lactamases (ESBL). Although carbapenems have been the treatment of choice for infections caused by ESBL-producing strains, the increasing resistance has led to the investigation of new antibiotics and combination therapies. This study aimed to evaluate the in vitro activity of ceftazidime/avibactam in combination with fosfomycin, meropenem, and colistin against carbapenem-resistant K. pneumoniae isolates recovered from blood cultures, using the checkerboard and time-kill assays. Between May 1 and October 31, 2024, a total of 100 carbapenem-resistant K. pneumoniae isolates identified by the VITEK 2 system were included in the study. These isolates were obtained from blood cultures submitted to the Medical Microbiology Laboratory of Selçuk University Faculty of Medicine Hospital. Carbapenemase genes were detected using the Bio-Speedy® Carbapenem Resistance qPCR Kit (Bioksen, Turkey) on the BMS MIC qPCR Cycler (Bio Molecular Systems, Australia). Minimum inhibitory concentrations (MICs) for ceftazidime/avibactam, meropenem, and colistin were determined by broth microdilution, while fosfomycin MICs were determined by agar dilution. Selected isolates were further tested for antibiotic combination efficacy using checkerboard and time-kill assays. The isolates included in the study were predominantly obtained from patients in intensive care units, accounting for 74% of all cases. Among the patients, 52% were male and 48% female, with no statistically significant difference between the patients in terms of gender characteristics and age distribution. OXA-48, NDM, and KPC genes were detected in 87%, 77%, and 69% of the isolates, respectively, with KPC+NDM+OXA-48 being the most common gene combination (31%). Resistance rates to ceftazidime/avibactam (CZA), meropenem (MEM), and colistin were 72%, 97%, and 84%, respectively. For fosfomycin (FOS), 67 isolates had MIC values >32 μg/mL. The checkerboard assay revealed the highest synergy/partial synergy with the FOS+CZA combination, while other combinations predominantly showed indifference interactions In the time-kill assay, the FOS+CZA combination demonstrated synergy in one isolate, antagonism in another, and indiferance in the remaining isolates. For the MEM+CZA combination, synergy was observed in one isolate and an indiferance in the other. The COL+CZA combination showed only an indiferance. The concordance between the two methods was determined to be 42,86%. In our study, the highest synergy rate determined by the checkerboard method was observed with the FOS+CZA combination, followed by the MEM+CZA combination. Synergistic activity was also observed for both FOS+CZA and MEM+CZA combinations in the time-kill assay. The absence of standardized interpretation criteria for the checkerboard method and lack of consensus on implementation and evaluation for the time-kill assay contribute to variability among studies. These discrepancies highlight the need for further research and standardization of both testing methods.