Carvacrol Modulates Tetracycline Susceptibility and Resistance Gene Expression in Multidrug-Resistant Salmonella Typhimurium Strains Isolated from Swine

Salmonella Typhimurium is an important pathogen affecting both animal production and public health, with pigs representing a major reservoir. The increasing prevalence of antimicrobial resistance highlights the need for alternative or complementary therapeutic strategies. Carvacrol, a phenolic constituent of Origanum vulgare essential oil, has recognized antimicrobial activity and may contribute to the modulation of antimicrobial resistance mechanisms. The present study investigated the antimicrobial activity of carvacrol and tetracycline, individually and in combination, against swine-derived multidrug-resistant Salmonella Typhimurium strains. Minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) were determined by broth microdilution. Interactions between both compounds were assessed by checkerboard assays using the fractional inhibitory concentration index (FICI). In addition, the modulatory effect of carvacrol on the expression of the tetracycline resistance genes tetA and tetB was evaluated to provide preliminary insight into the molecular basis of these interactions. Relative expression of tetracycline resistance genes was analysed by quantitative real-time PCR in selected isolates exposed to tetracycline alone or combined with carvacrol. Carvacrol MIC values ranged from 0.5 to 16 mg/mL, with MIC50 and MIC90 values of 2 and 4 mg/mL, respectively. MBC and MIC values were identical in 84.9% of the isolates, indicating a strong bactericidal effect. Tetracycline MIC50 and MIC90 values were 256 and 512 µg/mL, respectively. Checkerboard assays showed synergistic interactions in 14.3% of strains, additive effects in 50.0%, indifferent responses in 28.6%, and antagonism in 7.1%. Combined exposure reduced tetA and tetB expression in most tested isolates compared with tetracycline alone, although responses were strain-dependent. These findings demonstrate that carvacrol exhibits antimicrobial activity against swine-derived multidrug-resistant Salmonella Typhimurium and may enhance tetracycline activity in a subset of isolates under the experimental conditions evaluated, while modulating resistance-associated gene expression. These findings provide a basis for future transcriptomic and proteomic studies aimed at further elucidating the molecular mechanisms underlying the interaction between carvacrol and tetracycline.