In Vitro Control of Enterococcus faecalis by Zataria multiflora Boiss, Origanum vulgare L and Mentha pulegium Essential Oils

Introduction: Essential oils are aromatic, oily liquids that are obtained from different parts of plants and known for their antimicrobial and anti-oxidative properties. These natural materials are replacing synthetic preservatives and antioxidative agents due to an increasing demand for natural food additives. Enterococcus faecalis is a commensal microorganism of the human intestinal tract, and is unusual among prokaryotic organisms because of its ability to produce extracellular superoxide. This anion damages colonic epithelial cell DNA and can lead to colorectal cancer. Therefore, a safe, effective and inexpensive method for controlling this bacterium in foods would be useful.

Purpose:  This study evaluates the antioxidant properties of the Essential Oils (EOs) from Zataria multiflora Boiss (ZMB), Origanum vulgare L (OVL) and Mentha pulegium (MP) and their influence on the growth, membrane permeability and cell surface characteristics of Enterococcus faecalis.

Methods: Two methods (disk diffusion and micro-well dilution) were tested for the antimicrobial activity of EOs. The mechanism of the antimicrobial action of EOs against E. faecalis was also studied by measurement of the release of cell constituents and observations of cells by electron microscopy. The antioxidant activities of ZMB, OVL and MP at MIC concentrations were evaluated by inhibition of free radicals using 2,2-diphenyl-1-picrylhydrazyl (DPPH) method.

Results: The minimum inhibitory concentrations (MICs) of ZMB, OVL and MP were 2, 8 and 42 mg/ml, respectively. The antioxidant activities of ZMB, OVL and MP at MIC concentrations, as evaluated by inhibition of free radicals using 2,2-diphenyl-1-picrylhydrazyl (DPPH) method, was 26.47%, 3.2% and 34.67%, for ZMB, OVL and MP, respectively. The electron micrographs obtained from the scanning microscopy showed that slight morphological changes can be seen in cells treated with EOs at MIC concentration. These data revealed an increasing release of the 260 nm absorbing material according to the oil concentration.

Significance: Our results confirmed higher antioxidant activity of ZMB than two others, suggesting it can be used to control of O₂^- (produced by E. faecalis) may in turn minimize chromosomal instability and ultimately lead to reduced risk of colorectal cancer.