Inhibitory parameters of essential oils to reduce a foodborne pathogen

Abstract

Technological application of essential oils, as natural sanitizing agents, to reduce food pathogens in the post-harvest processing of foods requires the establishment of the optimal conditions. The present work evaluated the parameters of antimicrobial activity of the essential oils of eucalyptus (Eucalyptus globules), tea tree (Melaleuca alternifolia), rosemary (Rosmarinus officinalis), mint (Mentha piperita), rosa moschata (Rosa moschata), clove (Syzygium aromaticum), lemon (Citrus limonum), oregano (Origanum vulgare), pine (Pinus silvestrys) and sweet basil (Ocimum basilicum) on survival and growth of different strains of E. coli O157:H7. The strains of E. coli exhibited similar susceptibilities to the action of the essential oils assayed. The essential oil with the lowest MIC and MBC (Minimum Inhibitory and Bactericidal Concentration, respectively) was clove (0.25 ml/100 ml and 0.3 ml/100 ml, respectively) and the results demonstrated that clove exerted a significant bactericidal and bacteriostatic action.

Introduction

Currently, there is a strong debate about the safety aspects of chemical preservatives since they are considered responsible for many carcinogenic and teratogenic attributes as well as residual toxicity. For these reasons, consumers tend to be suspicious of chemical additives and thus the demand for natural and socially more acceptable preservatives has been intensified (Skandamis, Koutsoumanis, Fasseas, & Nychas, 2001). The exploration of naturally occurring antimicrobials for food preservation receives increasing attention due to consumer awareness of natural food products and a growing concern of microbial resistance towards conventional preservatives (Schuenzel & Harrison, 2002). Also, there is a growing interest in organically produced foods that the general public associated with healthier food. Another problem is the use of animal waste as organic fertilizer, whether in organic or non-organic agriculture, that gives rise to concerns about the possible contamination of agricultural produce with pathogens (especially E. coli O157:H7) and the possible contamination of ground and surface water.

Many spices and herbs exert antimicrobial activity due to their essential oil fractions. Nychas (1995) reported antimicrobial activity of essential oils from oregano, thyme, sage, rosemary, clove, coriander, garlic and onion against both bacteria and molds. The means by which microorganisms are inhibited by essential oils seems to involve different modes of action. Phenolic components present in essential oils have been know to possess antimicrobial activity and some are classified as Generally Recognized as Safe (GRAS) substances and therefore could be used to prevent post-harvest growth of native and contaminant bacteria. Phenolic components of oils sensitize the phospholipid bilayer of the cell membrane, causing an increase of permeability and leakage of vital intracellular constitutes or impairment of bacterial enzyme systems (Singh, Singh, Bhunia, & Stroshine, 2002).

Escherichia coli O157:H7 was first recognized as a pathogen in 1982 and is considered the main cause of hemorrhagic colitis and of hemolytic uraemic syndrome. E. coli infections are disseminated by foods, water and person to person contact (Singh et al., 2002). Skandamis et al. (2001) reported that oregano essential oils and EDTA inhibited the growth of E. coli by inducing morphological changes at close to minimum bactericidal concentration (MBC). Elgayyar, Draughon, Golden, and Mount (2001) showed antimicrobial activity of some essential oils from plants against selected pathogenic and saprophytic microorganisms. They found that coriander and basil were highly inhibitory to E. coli O157:H7 (MIC approximately 0.25–0.50 ml/100 ml).

Technological application of essential oils, as natural sanitizing agents, to reduce food pathogens in the post-harvest processing of foods, in particular those produced by organic methods, requires the establishment of the optimal conditions, such as sensitivity of a pathogen, optimal concentrations and the time of contact between the oils and the pathogen. The aim of the present work was to evaluate antimicrobial activity parameters (minimum inhibitory concentration, minimum bactericidal concentration and optimal time of contact) of some essential oils as eucalyptus (Eucalyptus globules), tea tree (Melaleuca alternifolia), rosemary (Rosmarinus officinalis), mint (Mentha piperita), rosa moschata (Rosa moschata), clove (Syzygium aromaticum), lemon (Citrus limonum), oregano (Origanum vulgare), pine (Pinus silvestrys) and sweet basil (Ocimum basilicum) on survival and growth of different strains of E. coli O157:H7.