Sublethal concentrations of essential oils and hydrolates can restore antimicrobial sensitivity in resistant Listeria monocytogenes strains

Antimicrobial resistance is a significant health concern, and foodborne Listeria monocytogenes strains have been reported to develop resistance to antibiotics currently used to treat listeriosis, thus future outbreaks might be more challenging to contain. Therefore, researchers are exploring innovative antimicrobial compounds for food preservation and medical applications. We previously demonstrated that sublethal concentrations of essential oils (EOs) and hydrolates (HYs) affect cell growth dynamics by determining a stress that initiates microbial response. Therefore, we exposed L. monocytogenes ATCC 7644 to sublethal concentrations of EOs and HYs at 37°C for 1 hour and then evaluated modifications in cell growth performance in the presence of antibiotics and antimicrobials. The contact of 1.25 μL/mL Origanum vulgare L. extended the lag phase and reduced the growth capabilities of the cells in the presence of lithium chloride and nalidixic acid that normally allow L. monocytogenes growth. Similar results were obtained by exposing the cells to 400 μL/mL Corydothymus capitatus hydrolate, obtaining a lag phase extension of about 14 and 22 hours respectively for lithium chloride and nalidixic acid. Interesting results were obtained for the strategic antibiotic vancomycin: treated cells were completely inhibited, while controls could grow. Results were similar for lincomycin and troleandomycin. Regarding sodium lactate, increasing C. capitatus hydrolate concentrations extended the lag phase (1.75 hours and 17.05 hours respectively for control and cells exposed to 400 μL/mL HY), and progressively reduced growth rate and maximum growth values, up to 500 μL/mL HY that caused a complete inhibition. The evaluation of the EOs effect on the sensitivity of L. monocytogenes isolates to gentamicin, ampicillin and other antibiotics used to treat listeriosis are ongoing. Our results demonstrate that sublethal concentrations of essential oils and hydrolates can restore the antibiotic and antimicrobial susceptibility of Listeria monocytogenes and can represent a new weapon to counteract antimicrobial resistance.