Cytotoxic Effects of Origanum vulgare L. and Rosmarinus officinalis L. Essential Oils Alone and Combined at Sublethal Amounts on Pseudomonas fluorescens in a Vegetable Broth

Abstract

AbstractThis study investigated the effects of the essential oils from Origanum vulgare L. (oregano [OVEO]) and Rosmarinus officinalis L. (rosemary [ROEO]) on the membrane permeability, morphology and cell viability of Pseudomonas fluorescens ATCC 11253 in a vegetable‐based broth after treatment with one essential oil (OVEO: 5 μL/mL; ROEO: 40 μL/mL) and with a combination of both essential oils (OVEO: 1.25 μL/mL + ROEO: 20 μL/mL). Release of cell material was observed immediately after the addition of the essential oils, both singly and as a mixture, in the growth media. Electron microscopy of bacterial cells exposed to OVEO, ROEO and the mixture of both revealed the following morphological changes after 2 and 3 h of exposure: alteration in the cell wall structure, rupture of the plasma membrane, shrinking of the cells, condensation of the cytoplasmic content and leakage of the intracellular material. Confocal scanning laser microscopy revealed increased cell membrane permeability, which resulted in cell death after short exposure times of 15 and 30 min. A combination of sublethal amounts of OVEO and ROEO could be applied to inhibit the growth of P. fluorescens in vegetables.Practical ApplicationsPseudomonas fluorescens is of particular concern in vegetable products, such as minimally processed vegetables, because of its ability to survive and grow at cold environments. The special risk posed by P. fluorescens on the storage and consumption of fresh vegetables has provoked studies on development of novel technologies to control this contamination. In this context, plant essential oils have received particular interest for their potential to control this bacterium in these foods. The findings of our study clearly indicate that the essential oil of Origanum vulgare and Rosmarinus officinalis, singly or in combination at sublethal amounts, caused a decrease in cell viability, loss of cellular material and drastic ultrastructural changes in P. fluorescens. These essential oils combined at subinhibitory concentrations could be rationally applied to control the survival of P. fluorescens in fresh vegetables and give the balance of microbial safety and organoleptic acceptability.