Investigation of the synergistic effects of antibiotic combinations against colistin-resistant Klebsiella pneumoniae clinical isolates

Introduction: Carbapenem-resistant Enterobacteriaceae infections are a significant public health concern worldwide. In the last decade, there has been a substantial increase in the prevalence and incidence of carbapenem-resistant Gram negative bacteria. Klebsiella pneumoniae, one of the leading antibiotic-resistant species, causes infections that are difficult to treat, have limited therapeutic options and are associated with high mortality rates. These infections also pose a burden on healthcare systems and national economies due to prolonged hospital stays, additional treatments, and infection control measures. While various mechanisms contribute to carbapenem resistance, the most significant is the production of carbapenemase genes. Detecting resistance genes is essential for both determining local resistance patterns and guiding treatment decisions. Due to the limited treatment options, clinicians have been increasingly relying on agents like colistin. Treatment has become more difficult as a result of the rise in colistin resistance over time. In this context, utilizing the synergistic effects of combination therapies has gained importance. Our study aims to investigate the presence of carbapenemase resistance genes and evaluate the synergistic efficacy of antibiotic combinations, including colistin, against multidrug-resistant Klebsiella pneumoniae clinical isolates.Materials and Methods: In total, 21 strains of multidrug-resistant Klebsiella pneumoniae that were resistant to carbapenems and colistin were included in our investigation. These strains were isolated from various clinical specimens obtained from patients aged 18 and older, admitted to Intensive Care Units and other clinics, between January 2021 and January 2024. Conventional PCR was used to investigate the presence of OXA-48, KPC, and NDM carbapenemase genes in the isolates. The synergistic effects of colistin in combination with tigecycline, ceftazidime/avibactam, meropenem, gentamicin, and trimethoprim/sulfamethoxazole were evaluated using the checkerboard broth microdilution method.Results: Of the 21 clinical isolates included in the study, 52.3% (n=11) were from male patients, and 47.6% (n=10) were from female patients. The mean age of the patients was 75.6 (±14.4) years. All of the isolates (100%) tested positive for the blaOXA-48 gene, while 4 isolates (19%) were positive for both blaOXA-48 and blaNDM, and 1 isolate (4.7%) was positive for both blaOXA-48 and blaKPC. The checkerboard method revealed the highest synergy in the combination of colistin with tigecycline, observed in 17 isolates (80.9%). Synergy was also observed in 11 isolates (52.3%) with the combination of colistin and ceftazidime/avibactam, in 8 isolates (38%) with colistin and trimethoprim/sulfamethoxazole, in 6 isolates (28.5%) with colistin and meropenem, and in 4 isolates (19%) with colistin and gentamicin. The FICI (Fractional Inhibitory Concentration Index) mean value for the colistin-tigecycline combination was significantly lower than the FICI values for the combinations of colistin with trimethoprim/sulfamethoxazole, meropenem, and gentamicin (p=0.003, p<0.001, and p<0.001, respectively).Conclusion: InTurkiye, OXA-48 remains the predominant carbapenemase enzyme, while the prevalence of NDM and KPC carbapenemases is steadily increasing. The colistin-tigecycline combination demonstrated significant in vitro synergistic activity against multidrug-resistant Klebsiella pneumoniae strains, including those resistant to both colistin and carbapenems. This combination could be considered as a treatment option for infections caused by these strains. However, for it to be recommended as a standard treatment, further in vitro and in vivo clinical studies with a broader range of strains are needed. Keywords: Klebsiella pneumoniae, Checkerboard microdilution, carbapenemase, Synergy