Exploring synergistic effects of essential oils compounds with antibiotics and biocides against multidrug-resistant foodborne pathogens

Antibiotics and biocides can be more effective when combined with compounds that enhance their activity against multidrug-resistant (MDR) bacteria. Our aim was therefore to explore the synergistic effect between biocides or antibiotics with essential oil compounds against food-borne pathogenic bacteria. The study was conducted with four representative essential oil compounds (EOCs): geraniol, cinnamaldehyde, carvacrol, and eugenol, combined with antibiotics or biocides. Two strains, isolated from slaughterhouse surfaces during meat production and previously classified as MDR, were tested, Enterococcus faecalisE7.10 and Pseudomonas psychrophilaM33T02.2. Both strains exhibited resistance to various antimicrobials tested, P. psychrophilaM33T02.2 showed greater MICs for all antimicrobials except ampicillin (AMP) and chloramphenicol (CL), while E. faecalisE7.10 showed the highest resistance. Synergistic effects were observed combining subinhibitory concentrations of eugenol (EU) and cinnamaldehyde (CA) with AMP or erythromycin (E) against E. faecalisE7.10. For P. psychrophilaM.33T02.2, only EU showed synergy when combined with chloramphenicol (CL) and cetrimide (CTR), however synergy was observed combining vancomycin (VN) with cinnamaldehyde (CN), as well as for the biocides CTR and benzalkonium (BC) with geraniol (GE) or cinnamaldehyde (CN). All tested synergies, except CTR with EU for E. faecalisE7.10, furthermore showed strong inhibition of biofilm development. Similarly, all synergies were effective against P. psychrophilaM33T02.2 biofilm formation, except combinations involving BC. It is important to highlight the absence of cytotoxic effects on microglial cells U3_MG and BV2 cells (p> 0.05) at low concentrations, indicating their safety. These results allow us to propose the use of EOCs as adjuvant of antibiotics and biocides to limit the spread of MDR bacteria in the food chain and the environment, in addition to their use alone or in combination with biocides in food product preservation against foodborne pathogens.