Rosemary oil, clove oil, and a mix of thyme-orange essential oils inhibit Salmonella and Campylobacter in vitro

Abstract

The demand for foods that are free of pathogens and chemical residues has increased interest in the use of plant-based products as natural antimicrobials. Essential oils (EO) from plants are natural compounds that have been shown to have antimicrobial properties against food-borne pathogens. The objective of the current study was to determine the ability of various concentrations of 4 selected EO to inhibit Salmonella enterica (3 different serovars and a cocktail of all 3) and Campylobacter (2 strains of Campylobacter jejuni, one strain of Campylobacter coli, and a cocktail of all 3). The disc diffusion method was used to screen the oils of thyme, orange, rosemary, and clove oil. The minimum inhibitory concentration or minimum bactericidal concentration of the EO was determined using a 2-fold broth dilution method at concentrations ranging from 0.0008 to 1.000% (vol/vol). Two independent experiments were performed. Zones of inhibition (ZI) were expressed in millimeters and concentrations were expressed in percentages. All the oils demonstrated antibacterial activity against the strains tested. However, thyme oil demonstrated the strongest inhibitory activity than other oils against Salmonella (ZI of 18.5 mm). In general, Campylobacter was more susceptible to the antibacterial activity of EO, with plates containing thyme or clove oil showing no growth. Orange oil was also highly effective on Campylobacter, with a mean ZI of 17.5 mm. The least expensive treatment effective against both Salmonella and Campylobacter was a combination of 100% concentrations of thyme and orange oil combined on a 50:50 proportion. Tested on the same strains of bacteria, the thyme-orange combination (TOC) had a mean ZI of 20.5 mm for Salmonella and 21.3 mm for Campylobacter. Thyme-orange combination demonstrated a synergetic effect against Salmonella, but no such effect was noticed for Campylobacter. On average, 0.14% TOC was required to inhibit both pathogens. Hence, TOC can be considered as a potential antimicrobial for future studies on food systems.