Antibacterial activity of essential oils: potential applications in food

Abstract

Due to its antibacterial activity, oregano oil has lately become interesting as a potential 'natural' food preservative. Oregano oil was found to be a fast acting and effective inhibitor of a strain of Escherichia coli O157:H7, the causative agent of a serious gastro-enteritis, and was lethal to this strain when used in high enough concentrations. Starting with 106 bacteria per millilitre broth, 156 ppm was lethal within 5 min; 625ppm was lethal within 1 min. Treated bacterial cells observed under an electron microscope were lysed and apparently empty of cell contents. The chemical composition of two oregano oils was determined. Carvacrol and thymol, both phenolic compounds, appeared to be responsible for the antibacterial activity. The other two major components p-cymene and gamma-terpinene were found to be inactive against bacteria. The food stabiliser agar improved the antibacterial action of oregano oil in broth and the action of carvacrol was improved by both agar and stabiliser carrageenan. This enhancement is presumed to be due to the delayed separation of the essential oil or carvacrol droplets from suspension. The emulsifier lecithin markedly reduced the efficacy of oregano oil, possibly because it enclosed the carvacrol in micelles so that it came less into contact with bacterial cells. Changes in protein synthesis in E. coli O157:H7 cells during overnight incubation with a sub-lethal concentration of carvacrol were demonstrated. Heat shock protein 60 (HSP60) was induced in significant amounts and flagellin synthesis was almost completely inhibited, resulting in bacteria without flagella. Flagella are essential to bacterial motility and there is some evidence to indicate that they mediate adherence of pathogenic E. coli to epithelial cells in the gut and urinary tract. Further research is planned to investigate whether carvacrol can render E. coli O157:H7 aflagellate in vivo and whether this inhibits adhesion and colonisation of gut epithelial cells. Carvacrol in the form of a vapour was effective in significantly reducing viable numbers of Salmonella enteritidis on raw chicken at temperatures ranging from 4C to 37 C. This is promising for practical applications in the food sector since many foods are stored and transported at low temperatures. Possible uses could be the inclusion of a carvacrol carrier within the packaging for fresh products to reduce growth of spoilage bacteria. Alternatively, small carcases or pieces of meat could be decontaminated by vapour before further processing. In conclusion, there are several possible applications for oregano essential oil, and the constituent carvacrol in particular, in improving food safety. The addition of essential oil or carvacrol to food products could inhibit growth and reduce numbers of spoilage bacteria and pathogens. The use of carvacrol vapour within food packaging to extend the shelf life and improve safety or as a novel decontaminant for fresh meat is another possibility. A further potential application is in improving the intestinal health of food animals, when used as a feed additive.