The effect of essential oils on methicillin-resistant Staphylococcus aureus using a dressing model
Introduction
Despite major advances in wound management, infection still remains an important factor in wound healing. In burns, approximately 75% of deaths are due to complications with sepsis resulting from wound infection [1]. Among other adverse effects, infection delays healing, contributes to graft failure and can increase the depth of a burn. Approximately 30% of burn wounds become colonised with Staphylococcus aureus [2] and outbreaks of methicillin-resistant S. aureus (MRSA) have created major problems for burn units and intensive care units in terms of cross-infection and rehabilitation of the patient due to imposed barrier nursing [3]. Some MRSA strains, termed epidemic MRSA (EMRSA), have the ability to spread rapidly among patients and the dominant clonal EMRSA types 15 and 16 are problematic in the UK [4], [5]. A new strain, EMRSA 17, has recently been described in the UK [6]. Whilst vancomycin is one of the few remaining effective systemic antibiotics available for treatment, resistance has been reported and there is major concern that total antibiotic-resistant strains may emerge in the immediate future [7]. At present, topical anti-microbial therapy is the single most important component of wound care to prevent infection [8] and in hospitalised burn patients, Flamazine™ is by far the most frequently used topical prophylactic agent [9] but does not always penetrate into the wound [10] and cannot be used to eradicate carriage from the patient or the environment.
Alternative therapies are being sought for treatment of MRSA and one area of interest is the use of essential oils. MRSA is susceptible to tea tree oil [11] but there are concerns about its toxicity [12]. Tea tree oil has been accepted as a powerful antibacterial agent [13] and is sold commercially in a wide range of products. There are hundreds of other essential oils available for use, many with known antibacterial properties. These oils contain numerous constituents that contribute to the characteristic odour and medicinal effects. The major chemical components that account for the pleasant aromatic odours are primarily terpenenes, monoterpenes and linalool [14]. The presence and quantity of the various components varies between oils and determines the individuality of the oil [15]. Although essential oils are known for their antimicrobial properties, medical teams rarely use them. This is primarily due to lack of scientific evidence of their efficacy, toxicity issues and the availability of conventional therapy. In this study, a range of essential oils and extracts were tested for their anti-bacterial properties using an in vitro dressing model to assess the anti-microbial action of the vapours.
Section snippets
Bacterial strains
Three strains of S. aureus were used: an antibiotic-susceptible strain, S. aureus NCTC 6571 (Oxford strain); a methicillin-resistant S. aureus (non-typable) and a methicillin-resistant S. aureus (phage type15). The two strains of MRSA were isolated from the wounds of two burn patients on the Burns Unit, Withington Hospital, Manchester, UK.
Essential oils
Four essential oils were used in the study: tea tree (Melaleuca alternifolia), patchouli (Pogostemon cablin), lavender (Lavendula officinalis), geranium (
Antibacterial action of single oils: direct contact
All bacterial strains showed some susceptibility to each individual essential oil when tested in direct contact using the disc diffusion method. The size of the zone of inhibition varied depending upon the oils and the strain of S. aureus used. Tea tree oil gave the largest zones of inhibition when tested against MRSA than any of the other oils. MRSA appeared to be more susceptible to tea tree oil than the Oxford Staphylococcus. Patchouli, lavender, geranium and Citricidal™ inhibited growth of
Discussion
This study highlights the potential use of essential oil impregnated dressings for treating wounds infected with MRSA. Although the results are very promising, one must advise caution with their use on burned or open wounds. The toxicity and effect on re-epithelialisation of these compounds have not been investigated in this study: these are important issues for future research.
Several methods were tested to screen the oils for their antimicrobial effect. In previous studies, we have used
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