Synergistic Activity of Eugenol, Cinnamaldehyde and Carvacrol in Combination with Different Antimicrobial Molecules Against MultiDrug-Resistant Gram-Negative Clinical Isolates

Background/Objectives: WHO has identified Carbapenem-Resistant Acinetobacter baumannii (CRAb) and carbapenem-producing Enterobacterales (CPE) as the “critical priority” group of MDR organisms for which new therapeutic strategies are urgently needed. Here, we evaluated in vitro synergistic activity of eugenol, cinnamaldehyde and carvacrol in combination with β-lactams, gentamicin, or colistin against multidrug-resistant (MDR) Gram-negative bacteria (GNB). Methods: We selected seven MDR-GNB clinical isolates inclining CRAb, ESBL-producing and CPE clinical isolates displaying different antimicrobial susceptibility profiles. The genomes of clinical isolates were characterized by whole-genome sequencing and synergy testing was performed with checkerboard assay. Results: Our results demonstrated that eugenol, cinnamaldehyde and carvacrol in combination with colistin exhibited high synergistic activity against MDR-GNB clinical isolates (37.5-50%), while the effect was almost indifferent in combination with different β-lactam molecules or gentamicin (87.5-100%). Synergistic interaction of eugenol, cinnamaldehyde and carvacrol with colistin induced a statistically significant reduction (p<0.05) of the MIC values compared with the molecules tested alone. Conclusions: Our data demonstrated showed that this synergistic interaction was not affected by different antimicrobial resistance genes and/or different antimicrobial susceptibility profiles. In conclusion, our results suggest that eugenol, cinnamaldehyde and carvacrol in combination with colistin represents a potential strategy for treatment of MDR-GNB pathogens and limit their diffusion.