Abstract
Background/Objectives: Acinetobacter baumannii is a globally emerging pathogen with widespread antimicrobial resistance driven by multiple mechanisms, such as altered expression of efflux pumps like AdeABC, placing it as a priority for research. Driven by the lack of new treatments, alternative approaches are being explored to combat its infections, among which efficacy-enhancing adjuvants can be found. This study presents and characterizes MV6, a synthetic cyclic peptide that boosts aminoglycoside efficacy. Methods: MV6's activity was assessed through antimicrobial susceptibility testing in combination with different antibiotic classes against A. baumannii strains characterized by PCR and RT-qPCR. PAβN served as a reference efflux pump inhibitor. Synergy was evaluated using checkerboard assays, and spontaneous mutants were generated with netilmicin with/without MV6 (100 mg/L). Whole-genome sequencing and variant calling analysis were then performed. Results: MV6 presented low antimicrobial activity in A. baumannii with MICs higher than 2048 mg/L. MV6 showed a better boosting effect for aminoglycosides, especially netilmicin, exceeding that of PAβN. Checkerboard assays confirmed a strong synergy between netilmicin and MV6, and a significant correlation was found between netilmicin MIC and adeB overexpression, which was mitigated by the presence of MV6. MV6 reduced, by 16-fold, the mutant prevention concentration of netilmicin. Mutations in a TetR-family regulator and ABC-binding proteins were found in both groups, suggesting a direct or indirect implication of these proteins in the resistance acquisition process. Conclusions: MV6 lacks intrinsic antimicrobial activity, minimizing selective pressure, yet enhances netilmicin's effectiveness except for strain 210, which lacks the AdeABC efflux pump. Resistant mutants indicate specific aminoglycoside resistance mechanisms involving efflux pump mutations, suggesting synergistic interactions. Further research, including transcriptomic analysis, is essential to elucidate MV6's role in enhancing netilmicin efficacy and its resistance mechanisms.