Potentiation of the Antimicrobial Effect of Oxytetracycline Combined with Cinnamon, Clove, Oregano, and Red Thyme Essential Oils against MDR Salmonella enterica Strains

Salmonella spp. of both human and animal origin have a high resistance percentage to tetracyclines. Essential oils, including cinnamon, clove, oregano, and red thyme, have demonstrated bactericidal activity against this bacterium. However, in many cases, the minimum inhibitory concentration (MIC) exceeds the cytotoxicity limits. The aim of this study was to evaluate the in vitro effectiveness of combining oxytetracycline with essential oils against multidrug-resistant Salmonella enterica strains. The results indicated a positive interaction (synergy and additivity) between oxytetracycline and the four oils that were tested. This led to a reduction in the MIC of both the oils and the antibiotic. The reduction was between 2 and 4 times the initial value for the oils and between 2 and 1024 times for the antibiotic. The best results were achieved with the combination of oxytetracycline and cinnamon, which decreased the effective concentration of this antibiotic to below the sensitivity threshold. Although differences in response were observed depending on the bacterial strain, there was no antagonistic effect in any case. The study suggests that combining oxytetracycline with cinnamon oil may be an effective alternative for controlling tetracycline-resistant strains of Salmonella, although further studies would be advisable. Tetracyclines have a high resistance percentage in Salmonella spp. of both human and animal origin. Essential oils, such as cinnamon (Cinnamomum zeylanicum), clove (Eugenia caryophyllata), oregano (Origanum vulgare), and red thyme (Thymus zygis), have shown bactericidal activity against this bacterium. However, in many cases, the minimum inhibitory concentration (MIC) exceeds the cytotoxicity limits. The objective of this study was to assess the in vitro efficacy of combining oxytetracycline with essential these oils against field multidrug-resistant (MDR) Salmonella enterica strains. The MIC of each product was determined using the broth microdilution method. The interaction was evaluated using the checkerboard method, by means of the fractional inhibitory concentration index (FICindex) determination. The results showed a positive interaction (synergy and additivity) between oxytetracycline and the four oils tested, resulting in a reduction in both products' MICs by 2 to 4 times their initial value, in the case of oils, and by 2 to 1024 times in the case of the antibiotic. The combination of oxytetracycline and cinnamon achieved the best results (FICindex 0.5), with a decrease in the antibiotic effective concentration to below the sensitivity threshold (MIC of the combined oxytetracycline 0.5 mu g/mL). There was no antagonistic effect in any case, although differences in response were observed depending on the bacterial strain. The results of this study suggest that combining oxytetracycline with cinnamon oil could be an effective alternative for controlling tetracycline-resistant strains of Salmonella. However, its individual use should be further evaluated through in vitro susceptibility tests.