Fighting resistance naturally: essential oils restore antibiotic power against foodborne pathogens

Antibiotic resistance reduces available therapeutic options, increasing the risk of morbidity and mortality, and placing an additional burden on healthcare system. Essential oils (EOs), due to their complex composition and multiple mechanisms of action targeting microbial cells, may act synergistically with antibiotics, hence enhancing or restoring antimicrobial susceptibility. This research aimed to evaluate the efficacy of selected EOs, either in combination with antibiotics or as a pre-treatment, in restoring susceptibility to commonly used antibiotics. The effect on Listeria monocytogenes and Salmonella enterica, two pathogens of major relevance in food safety and veterinary context, is proposed as a case study. In detail, eleven tetracycline-resistant S. enterica strains, isolated from the swine food supply chain, were treated with EOs derived from Thymbra capitata, and Thymus serpillum (Flora Srl, Lorenzana, Italy), in combination with tetracycline, to determine the Minimum Inhibitory Concentration (MIC) and Fractional Inhibitory Concentration Index (FICI) at 37°C for 48 h. Additionally, two food derived and two clinical isolates of L. monocytogenes were exposed to T. capitata EO (Exentiae, Italy), both as a pretreatment (1 h at 37°C at MIC/2, followed by removal) and in combination with penicillin, ampicillin, and gentamicin, at 37°C for 48 h. Growth and inhibition dynamics were monitored using the Omnilog system (Biolog Inc., USA). The combination of EOs with tetracycline reduced the antibiotic MIC to 4 μg/mL for Salmonella strains, restoring susceptibility according to CLSI and EUCAST standards. The effect was strain-dependent, with interactions ranging from additive to synergistic. In L. monocytogenes, T. capitata EO combined with antibiotics, resulted in up to a seven-fold decrease in MIC and MBC values, thus restoring susceptibility. Pre-treatment with EO also produced a bacteriostatic effect when the cells were subsequently exposed to sub-inhibitory antibiotic concentrations (MIC/2), with stronger effects observed in clinical strains. The major component of the tested EOs was carvacrol, known to disrupt cytoplasmic membranes. Other phenolic compounds likely enhance its efficacy by increasing membrane permeability and facilitating antibiotic uptake. EO-induced cellular stress may deplete cellular energy and further increase susceptibility to antimicrobial agents, including antibiotics. In conclusion, essential oils reduce the concentrations of antibiotic required to inhibit S. enterica and L. monocytogenes, highlighting their potential as adjuvants in strategies to combat antimicrobial resistance. Further studies are necessary to explore their integration into antimicrobial therapies targeting multidrug-resistance bacteria.