Due to the increase in bacterial resistance to antimicrobials (AMBs) commonly used in veterinary and human medicine, the new strategies for controlling zoonoses focus on the study of natural products with demonstrated AMB activity, such as essential oils (EOs). The aim of this study was to evaluate the in vitro effect of the combination of enrofloxacin (ENR), ceftiofur (CEF), and trimethoprim-sulfamethoxazole (SXT) with cinnamon, clove, oregano, and red thyme EOs against multiple drug-resistant strains of Salmonella enterica. The minimum inhibitory concentration (MIC) of each product was determined by microdilution and "Checkerboard" methods and their combined effect was evaluated against 15 strains of S. enterica. The results were interpreted by the calculation of fractional inhibitory concentration (FIC) and their respective indexes (FICI). Significant susceptibility of all strains to the four EOs was observed. The results showed a synergistic effect between EOs and AMBs tested, highlighting the upper percentage of total synergies of the SXT with the four EOs (FICI ≤0.5 in 60% of assays), and the most effective combination being the one of ENR and cinnamon. The MIC of cinnamon was reduced from 1250 to 312.5 μg/mL and the MIC of ENR from 2 to 0.031 μg/mL. There was no antagonism in the tested combinations (AMBs-EOs). Our results support the combined use of EOs and AMBs for the control of multiresistant strains of S. enterica with a reduction of the minimum effective dose of AMBs and their adverse effects.
Keywords: S. enterica; antimicrobials; essential oils; multiresistance; synergism.