The mechanism of action of a citrus oil blend against Enterococcus faecium and Enterococcus faecalis

The aim was to explore the mechanisms by which a blend of orange (Citrus sinensis) : bergamot (Citrus bergamia) (1 : 1 v/v) EO (essential oil) (2% v/v) and its vapour (15 mg l(-1) air) brings about its antimicrobial effect against Enterococcus faecium and Enterococcus faecalis. Cells were exposed to the blend in oil or vapour form in a sealed unit. Membrane permeability was measured using an NPN assay and intra and extracellular ATP concentrations were assessed using luminescence. Assays using 3,3-dipropylthiacarbocyanine and carboxyfluorescein diacetate succinimidyl ester measured membrane potential and intracellular pH changes. TEM images of treated cells indicate morphological differences and show the possible uptake of the EO into the cell. After cells were exposed to EO or vapour, cell permeability increased by x2 and x40 respectively. A decrease of 1.5 in intracellular pH, 20 a.u. in membrane potential and 18 pmol mg(-1) protein of intracellular ATP occurred. The EO blend affects the cell membrane and cell homeostasis resulting in inhibition of growth or cell death. Understanding the mechanisms by which EOs bring about their antibacterial effect could lead to an alternative to chemical-based bactericides for use against Enterococcus sp.