Determination of the effectiveness of some antimicrobial peptides along with various antibiotics against carbapenem-resistant acinetobacter baumannii clinical isolates on in vitro and in vivo galleria mellonella larva model

Determination of the Effectiveness of Some Antimicrobial Peptides along with Various Antibiotics Against Carbapenem-Resistant Acinetobacter baumannii Clinical Isolates on In Vitro and In Vivo Galleria mellonella Larva Model Antimicrobial resistance is a multifaceted crisis that poses a serious threat to global health. As antibiotics become ineffective over time, it becomes increasingly difficult to treat infections caused by resistant microorganisms. Due to the development of antimicrobial resistance, the use of traditional antibiotics in treatment is rapidly limited, leading to the development of research on alternative antimicrobial strategies. Antimicrobial peptides (AMP) are small molecular peptides with broad-spectrum antibacterial activity and immunomodulatory function that have attracted attention because they offer distinct advantages over traditional antibiotics and their clinical potential. In this study, LL-37 and Melittin AMPs were used. Over the past few decades, A. baumannii has significantly gained clinical importance, particularly as a causative agent of nosocomial infections. This is attributed to its exceptional capacity to trigger of infection and to develop resistance to nearly all commonly used antibiotics, including carbapenems. Since there are not many options left for treating infections caused by these bacteria, unconventional combination treatments are increasingly being taken into consideration. Within the scope of this study, we used 10 carbapenem-resistant A. baumannii isolates, causative agent of nosocomial infection. A. baumanii ATCC 19606 strain was used as a control to ensure standardization of experimental conditions. Meropenem, imipenem, melittin, LL-37 MIC values were determined by the liquid microdilution method in accordance with EUCAST recommendations. Synergy was investigated using checkerboard and time kill methods on A. baumannii isolates. Additionally, the effectiveness of MEL, IMP and MER, alone and in combination, were investigated on the in vivo G. mellonella larvae model. MEL MIC values ranged between 31,25-62,5 µg/mL. LL-37 MICvalues were found to be greater than 500 µg/mL except for two isolates. These isolates are ATCC 19606 and M5. MIC values were found to be 250 µg/mL. M5 isolate was the only isolate that showed synergy in all four combinations investigated using the checkerboard method. The fic index values for this isolate in the combinations of MER-MEL, MER-LL 37, IMP-MEL, IMP-LL 37 is 0,375, 0,375, 0,5 and 0,5, respectively. We studied 1x MIC and ½x MIC concentrations to investigate synergy using the time kill method. Both bactericidal activity and synergistic effect were determined at various time intervals. In addition, the most active substance was determined to be MEL, both at 1x MIC concentration and ½x MIC concentration. In the in vivo study performed on the G. mellonella larva model, survival rates were calculated and cfu determination was performed. Key Words: Acinetobacter baumannii, antimicrobial resistance, Galleria mellonella, LL-37, Melittin,