Tea Tree Oil Treatment of MRSA: Difference between revisions

From MicrobeWiki, the student-edited microbiology resource
No edit summary
mNo edit summary
 
(48 intermediate revisions by the same user not shown)
Line 1: Line 1:
{{Uncurated}}
The prevalence of [http://www.nhs.uk/conditions/MRSA/Pages/Introduction.aspx Methicillin-resistant <i>Staphyloccocus aureus</i>] (MRSA) as a pathogen in hospitals and other community settings underscores the importance of eradicating the infection.<sup>1</sup> As MRSA is not susceptible to commonly used antibiotics, alternative antimicrobial agents are being sought for its eradication. One area of interest involves the use of tea tree oil, which studies have shown to effectively treat infections of drug-resistant bacteria, including MRSA.<sup>2</sup>
 
==Introduction==
==Introduction==
[[Image:mrsa_magn_lg.jpg|thumb|300px|Figure 1. This colorized scanning electron micrograph (SEM) depicts a grouping of methicillin resistant Staphylococcus aureus (MRSA) bacteria. [http://www.cdc.gov/mrsa/mrsa_initiative/skin_infection/mrsa_photo_9994.html. Public through the CDC]]]
<i>Staphyloccocus aureus</i>, more commonly known as staph bacteria or MRSA, is a Gram-positive coccus-shaped anaerobic bacterium pictured in Figure 1.<sup>1</sup> MRSA is a type of staph bacteria that is resistant to beta-lactam antibiotics, such as penicillin, amoxicillin, oxacillin, and methicillin. MRSA is also becoming resistant to mupirocin, the current standard antibiotic used in the treatment of MRSA infection.<sup>3</sup>. MRSA often colonizes on the skin or nostrils of healthy individuals, and is relatively harmless at these sites.<sup>1</sup> If <i>S. aureus</i> enters the body (e.g., wounds, cuts), it may cause infections. In such instances, the MRSA infection may range from mild (e.g., pimples) to life-threatening (e.g., infection of bloodstream, joints, or bones).<sup>1</sup> MRSA is spread through contact and most commonly contracted in public settings, namely hospitals.
Tea tree oil (TTO) is the essential oil derived through steam distillation from the Australian native plant <i>Melaleuca alternifolia</i>. Tea tree oil has been used for centuries as a topical antiseptic. TTO is believed to have antibacterial, antifungal, antiviral, and anti-inflammatory properties when used topically.<sup>4</sup> TTO may be found as additives in beauty and health products, including shampoo, soap, and nasal spray. Although historical anecdotes endorse TTO's medicinal properties, few clinical studies have been conducted to support such claims. Clinical studies, however few, indicate that tea tree oil can treat the skin infection caused by MRSA.<sup>4</sup> Tea tree oil's anti-microbial properties are attributed to belonging to a class of chemicals known as terpenes, specifically terpinen-4-ol.<sup>4</sup> Tea tree oil's bactericidal effects make this plant extract a plausible addition or supplement to a MRSA treatment plan.
==Tea Tree Oil Composition and Chemistry==
[[Image: Terpinen-4-ol_copy.png‎|thumb|400x220px|right| Figure 2. Chemical structure of terpinen-4-ol, one of nearly 100 compounds identified in tea tree oil. [http://commons.wikimedia.org/wiki/File:Terpinen-4-ol.svg Public through Wikimedia Commons]]]


<br> Methicillin-resistant <i>Staphyloccocus aureus</i> (MRSA) is a public health problem. The prevalence of MRSA as a pathogen in hospitals and other community settings underscores the importance of eradicating the carriers. As MRSA is not susceptible to commonly used antibiotics, alternative antimicrobial agents are being sought for its treatment. One area of interest involves the use of plant essential oils. Studies indicate the effectiveness of tea tree oil as treatment for infections of drug-resistant bacteria, including MRSA.  
Commercially available TTO is a composition of nearly 100 chemical compounds determined by gas chromatography-mass spectrometry.<sup>5</sup> TTO is primarily composed of a class of chemicals called monoterpenes. Typically, monoterpenes are volatile, aromatic hydrocarbons that are lipophilic, meaning that they can dissolve in fats or lipids.<sup>4</sup>


<i>Staphyloccocus aureus</i>, more commonly known as staph bacteria or MRSA, is a Gram-positive coccus-shaped anaerobic bacterium.<sup>1</sup> MRSA is a type of staph bacteria that is resistant to beta-lactam antibiotics, such as penicillin, amoxicillin, oxacillin, and methicillin. MRSA often colonizes on the skin or nostrils of healthy individuals, and is relatively harmless at these sites.<sup>1</sup> If <i>S. aureus</i> enters the body (e.g., wounds, cuts), it may cause infections. In such instances, the MRSA infection may range from mild (e.g., pimples)to life-threatening (e.g., infection of bloodstream, joints, or bones). MRSA is spread through contact and most commonly contracted in public settings, namely hospitals.  
Terpinen-4-ol, pictured in Figure 2, is the specific monoterpenic compound believed primarily responsible for TTO's anti-microbial activity. This hydrocarbon compound has one alcohol group attached to the fourth carbon. Like all monoterpenes, terpinen-4-ol’s mostly hydrocarbon structure gives the molecule lipophilic properties.<sup>4</sup> Researchers claim that the lipophilic nature of monoterpenes contributes to its antimicrobial activity, though the exact details of the antimicrobial mechanism are not currently known.<sup>4</sup> Other monoterpenic compounds in TTO include 1,8-cineole, terpinolene, and α-terpineol, whose medicinal properties are also under investigation.<sup>4</sup>


Tea tree oil (TTO) is the essential oil derived through steam distillation from the Australian native plant <i>Melaleuca alternifolia</i>. Tea tree oil has been used for centuries as a topical antiseptic. TTO has antibacterial, antifungal, antiviral, and anti-inflammatory properties in vitro.<sup>2</sup> Although historical anecdotes endorse TTO's medicinal properties, few clinical studies have been conducted to support such claims. Clinical studies, however few, indicate that tea tree oil can treat skin infection caused by MRSA. Tea tree oil's anti-microbial properties are attributed to its composition of a chemical class known as terpenes, specifically terpinen-4-ol.<sup>2</sup> Tea tree oil's bacteriocidal and bacteriostatic effects make this plant extract a plausible addition or supplement to a MRSA treatment plan.
TTO composition is commercially available in various chemotypes. The most common chemotype is the terpinen-4-ol chemotype where terpinen-4-ol comprises between 30 to 40% of  commercially available TTO.<sup>4</sup> A relatively high composition of terpinen-4-ol is more common in commercial production due to its demonstrated broad spectrum of biological activities. <sup>4,5</sup>
[[Image:mrsa_magn_lg.jpg|thumb|300px|This colorized scanning electron micrograph (SEM) depicts a grouping of methicillin resistant Staphylococcus aureus (MRSA) bacteria. Publicly available by the CDC.]] <br>


==Tea Tree Oil Composition and Chemistry==
== Antimicrobial Activity: Cytoplasmic Membrane Damage==  
<br> Commercially available tea tree oil (TTO) is a composition of nearly 100 chemical compounds determined by gas chromatography-mass spectrometry.<sup>1</sup> TTO is primarily composed of a class of chemicals called terpenes. Specifically, monoterpenes, sesquiterpenes, and other terpene alcohols dominate this composition. Terpenes are volatile, aromatic hydrocarbons and are typically soluble with nonpolar solvents.<sup>2</sup> While research indicates that terpene alcohols are generally effective anti-microbial agents, terpinen-4-ol is the specific terpenic compound believed primarily responsible for TTO's anti-microbial activity.<sup>2</sup>  
[[Image:Cytoplasmic-damage.png|thumb|400px| Figure 3. Membrane permeability is disrupted after tea tree oil (TTO) treatment. A physiologically normal cytoplasmic membrane (A) excludes foreign substances, like NaCl and propidium iodide. A membrane treated with TTO (B) does not properly exclude foreign material and water, which researchers believe is involved with TTO’s batericidal activity.<sup>[http://search.ebscohost.com/login.aspx?direct=true&db=aph&AN=6058422&site=ehost-live&scope=site 7],[http://aac.asm.org/content/46/6/1914.full.pdf+html 2]</sup> The exact mechanism for this phenomenon is unknown. Image created by Karen Leung.]]
The target of TTO's antimicrobial activity, and the primary reason for its laboratory and clinical effectiveness, is reported to be the cytoplasmic membrane of MRSA. The bacterial cell membrane physically separates the cell's internal environment from the external environment. It is selectively permeable to organic molecules and ions. As such, it regulates the movement of water and other substances into and out of the cell.<sup>6</sup> If the membrane permeability were compromised, then the barrier between the internal and external environment would be weakened. Consequently, foreign material could enter the cell more easily while cytoplasmic material could leave it more easily (as shown in Figure 3).<sup>7,2</sup>  


TTO composition is internationally regulated and may be commercially available in various chemotypes, including the terpinen-4-ol chemotype where terpinen-4-ol comprises between 30 to 40% of TTO's commercial composition.<sup>2</sup> International regulation of TTO's chemical properties calls for this relatively high composition of terpinen-4-ol in commercial production due to its historical medicinal properties. Other terpenic compounds in TTO include 1,8-cineole, terpinolene, and α-terpineol.<sup>3</sup> These compounds also have medicinal properties that are less reputable than those of terpinen-4-ol.   
Based on the lipophilicity of the terpinen-4-ol molecule and other TTO components, researchers have proposed that TTO can insert itself into and damage the lipid-rich biological membranes.<sup>4</sup> Studies examining membrane integrity after TTO treatment indicate that the essential oil disrupts the vital function of the biological membrane <sup>7,2</sup>


[[Image: Terpinen-4-ol_copy.png‎|thumb|200px|right| Chemical structure of the terpenic derivative, terpinen-4-ol. It is one of nearly 100 tea tree oil compounds identified through gas chromatography-mass spectrometry. Image obtained from Wikimedia Commons]]<br>
An <i>in vitro</i> (i.e. laboratory) study by Cox <i>et al.</i> found that TTO treatment disturbed the MRSA membrane permeability, which allowed propidium iodide, a foreign substance, to enter the cell and potassium ions, part of the cytoplasmic material, to leave the cell.<sup>7</sup> Another <i>in vitro</i> study demonstrated that TTO-treated MRSA had increased susceptibility to sodium chloride (NaCl), a cell toxin.<sup>2</sup> From this evidence, researchers believed that TTO altered the cell membrane.


==Topical Treatment and Effectiveness==
The loss of intracellular material and the inability to maintain osmotic regulation is consistent with a mechanism of action involving the loss of membrane integrity. Despite evidence of cytoplasmic membrane damage, the molecular mechanism of this impairment is still unknown and warrants further research.<sup>4</sup>
<br> Tea tree oil (TTO) may be included in commercial products, such as body wash, skin cream, and nasal ointment, and can used to treat MRSA. The clinical efficacy of TTO body wash and creams as a decolonization agent for MRSA has been supported, albeit with no statistical significance. Researchers have found that TTO can reduce or prevent MRSA colonization as well as the standard treatment. The following studies indicate the potential usefulness of tea tree oil in eradicating MRSA and
suggest that TTO may supplement a MRSA eradication regimen.


Caelli <i>et al.</i>'s (2000) pilot study study compared the effectiveness of TTO to that of routine care treatments, as measured by survivor MRSA concentrations on the skin and within the nose.<sup>4</sup> In the Caelli Lab's clinical study, 30 MRSA-affected adult hospital inpatients were randomly assigned to receive either the routine care (2% mupirocin nasal ointment and triclosan body wash) or tea tree oil regimen (4% TTO nasal ointment and 5% TTO body wash) for at least three days. Caelli <i>et al.</i> found that the tea tree oil regimen performed better (i.e., reduced or eradicated MRSA) in more subjects than the routine regimen, although these results were not statistically significant due to small sample size.  
==Treatment and Effectiveness==
Efforts to validate the therapeutic properties of tea tree oil (TTO) have yielded <i>in vitro</i> and clinical studies showing that TTO treatment eradicates and reduces MRSA infection, respectively. While TTO displays <i>in vitro</i> and clinical efficiency, TTO has not been demonstrated to prevent initial MRSA colonization. Several studies described below and illustrated by Figure 4, support the claims upheld by traditional healers that TTO is an effective antimicrobial agent.


A similar study conducted by Blackwood <i>et al.</i> (2013) also found inconclusive results with 445 patients belonging to two intensive care units (ICU) in Northern Ireland.<sup>5</sup> The Thompson Lab sought to determine whether the daily use of TTO body wash had a lower incidence of MRSA colonization. A randomized regimen of either Johnson's Baby Softwash (standard care) body wash or tea tree oil (5%) body wash was assigned to each patient. Patients continued treatment until they reached the study endpoints: detection of hospital-acquired MRSA, ICU discharge, or death. Thompson <i>et al.</i> reported that the TTO regimen did not significantly reduce new MRSA colonization in ICU patients, but claimed that TTO was safe and well-tolerated among patients.
====<i> In vitro </i> Effectiveness====
[[Image: Potency TTO.png‎|thumb|400px|right| Figure 4. The <i> in vitro </i> and clinical effectiveness of tea tree oil (TTO), as described by May <i>et al.</i>,Caelli <i>et al.</i> and Dryden <i>et al.</i>.<sup>[http://jac.oxfordjournals.org/content/45/5/639.full?sid=4734d06d-ea17-46eb-8288-3001f988f987 8],[http://www.sciencedirect.com/science/article/pii/S0195670100908302 9],[http://www.sciencedirect.com/science/article/pii/S0195670104000167 3]</sup> Image created by Karen Leung.]]


Even though TTO does not seem to be any stronger than the standard decolonization agent for MRSA, it can be more effective at clearing some of its symptoms. Dryden <i>et al.</i> (2004) deemed it to be sufficiently effective, safe, and well-tolerated to be considered part of a MRSA eradication regimen.<sup>6</sup> They found that 5% TTO body wash and 10% TTO applied to skin and nose three times a day for five days and tea tree was more effective than standard treatment at clearing skin lesions, variations in skin texture or color, associated with MRSA.
The efficacy of tea tree oil (TTO) against MRSA has been demonstrated in laboratory studies using cultured <i>S. aureus</i> samples. An <i>in vitro</i> study by May <i>et al.</i> showed that 99.9% of the MRSA isolate was killed by TTO within 4 hours and that all was eradicated after 6 hours of continuous exposure to 5% TTO.<sup>8</sup> These results suggest that TTO's antimicrobial properties make this a good agent to control MRSA and reduce its transmission among humans.
<br>


==Antimicrobial Activity: Cytoplasmic Membrane Damage==
====Clinical Efficiency====
<br> TTO is reported to disrupt <i>S. aureus</i> cell's membrane permeability. Researchers, Cox <i>et al</i> and Carson <i>et al</i>, examined TTO's effect on <i>S. aureus</i> physiology and found damage to the cytoplasmic membrane. A weakened barrier may allow for undesirable passage of material, such as ions or toxins, and potentially result in bacteriocide or bacteriostasis. The following studies examined the cytoplasmic membrane damage that TTO induces in MRSA.  
[[Image:Impetigo KL.jpg|thumb|400px|right| Figure 4. Impetigo, a skin infection resulting from staph infection. Dryden et al. found that tea tree oil could effectively treat a similar MRSA-related ailment.<sup>[http://www.sciencedirect.com/science/article/pii/S0195670104000167 3]</sup> [http://phil.cdc.gov/phil/home.asp Public through the CDC.]]]


Cox, S. D. <i>et al</i> (2000) found that exposure of MRSA to the minimum inhibitory concentration of TTO altered cell membrane structure.<sup>7</sup> The minimum inhibitory concentration (MIC) is the lowest concentration of antimicrobial - in this case, TTO - that will inhibit microbial growth. Cox <i>et al</i> accumulated evidence of cell membrane damage from a propidium iodide uptake assay and ion leakage measurements. As expected, cells that were not exposed to TTO displayed an intact cell membrane. Propidium iodide, a fluorescent nucleic acid staining solution, did not penetrate cells that were not exposed to TTO. Likewise, cells not exposed to TTO showed little potassium ion leakage. This showed that physiologically normal <i>S. aureus</i> cells would not uptake propidium iodide or leak potassium.
The efficiency of TTO in eradicating MRSA cultures <i>in vitro</i> led to clinical testing with human patients using products enhanced with TTO. TTO may be added to commercial products, such as body wash, skin cream, and nasal ointment due to its antimicrobial properties. The clinical efficacy of TTO body wash and creams as a decolonization agent for MRSA has been supported by Caelli <i>et al.</i> and Dryden <i>et al.</i>.<sup>9, 3</sup> Both researchers found that TTO treatment is as effective as the standard treatment regimen, indicating that TTO treatment may enhance a MRSA eradication regimen.


From the experiment, Cox <i>et al</i> found that TTO exposure resulted in propidium iodide uptake and potassium ion leakage, signs of a damaged cytoplasmic membrane.<sup>7</sup> In particular, they observed that exposing cell suspensions of MRSA to the minimal inhibitory concentration of 0.25% (v/v) tea tree oil for 30 minutes increased cell permeability to propidium iodide relative to the controls. They also demonstrated that 30 minutes of exposure to TTO caused up to 20% cellular loss of potassium ions. They proposed that TTO components diffused through the cell wall and disrupted the phospholipid regions of cell membrane structures. The ensuing passage of propidium iodide and potassium ions indicated severe cytoplasmic membrane damage. 
Caelli <i>et al.</i> clinically determined that TTO treatment was as effective as the standard MRSA medical treatment.<sup>9</sup> Caelli compared the effectiveness of TTO treatment to that of standard treatments against MRSA, as measured by post-treatment MRSA concentrations on the skin and within the nose of human patients.<sup>9</sup> Compared to the standard treatment, the tea tree oil topical regimen reduced or eradicated MRSA infection in more subjects. Despite this optimistic finding, the small sample size prevented this result from being statistically significant. Consequently, Caelli concluded that TTO treatment had a similar effect as standard medical treatment at eradicating MRSA <sup>5</sup>


Carson, C. <i>et al</i> (2002) demonstrated cytoplasmic membrane damage through increased susceptibility to NaCl, loss of cytoplasmic material, and the formation of mesosomes in MRSA treated by TTO or one of its components: terpinen-4-ol, α-terpineol, 1,8-cineole.<sup>8</sup> MRSA cells treated with TTO or one of its components were found to be more susceptible to sodium chloride (NaCl), a bacterial cell toxin. Treatment with TTO or its components reduced survivor colonies to <30% viability on NaCl-treated plates. This evidence led Carson <i>et al</i> to propose that the treatment likely damaged the <i>S. aureus</i> cell membrane and affected the bacterial cell's ability to exclude NaCl. Electron microscopy of terpinen-4-ol treated MRSA cells showed mesosome formation that was not observed in untreated cells. Mesosomes, invaginations of the cytoplasmic membrane, along with loss of cytoplasmic material, suggested that the TTO and its components compromised the <i>S. aureus</i> membrane.
In a similar study, Dryden <i>et al.</i> found that TTO cleared up MRSA-related skin infections better than the standard medical treatment.<sup>3</sup> Dryden's clinical study found that the daily application of 5% TTO body wash and 10% TTO cream was more effective than standard treatment at clearing skin lesions associated with MRSA infection.<sup>3</sup>  
Although research suggests the efficacy of TTO against MRSA, more clinical studies would have to be performed in order to comprehensively assess TTO's effects on MRSA patients. From these experiments, TTO was deemed to be sufficiently effective, safe, and well-tolerated to be considered part of a MRSA eradication regimen.<sup>3</sup>


Both Cox <i>et al</i> and Carson <i>et al</i> performed different experiments to demonstrate TTO's devastating effect on the <i>S. aureus</i> cytoplasmic membrane. While much of TTO's mechanism remains unexplored, these studies indicate that TTO targets the cytoplasmic membrane.  
====Dermal Toxicity====
<br>
Despite characterizing TTO’s antimicrobial properties, researchers have not done much work on the safety and toxicity of using TTO topically. The rationale for continued topical use of TTO primarily rests on anecdotal evidence of its safe usage at low concentrations.<sup>4</sup> No human deaths due to TTO exposure have been reported.<sup>4</sup> Though there are few substantial scientific studies regarding TTO’s safety, TTO can cause irritant or allergic reactions when applied to the skin.<sup>4</sup> More concrete evidence of TTO’s toxicity would be necessary in order to determine its optimal concentration for safe usage.


==Susceptibility to Antibiotic Resistance after TTO Exposure==
==Susceptibility to Antibiotic Resistance after TTO Exposure==
<br> There is concern that commercial TTO preparations, which may vary in its components' concentrations, provide users more opportunity to apply sub-lethal concentrations. McMahon <i>et al</i> (2007) and McMahon <i>et al</i> (2008) suggest that exposure of MRSA to sub-lethal TTO concentrations, defined as half of the minimal inhibitory concentration (MIC), makes MRSA less susceptible to later MIC TTO treatment.<sup>9, 10</sup>  
The available forms of TTO range from pure oil to retail products for personal health care, home care, and pet care.<sup>10</sup> Commercial TTO preparations, which may vary in its components' concentrations, provide users more opportunity to apply TTO at ineffective concentrations. The growing use of TTO also implies that a diversity of microbes is coming into contact with sub-lethal TTO concentrations.<sup>10</sup> There is concern that exposing MRSA to sub-lethal concentrations of TTO may lead to decreased antibiotic susceptibility (i.e. increased antibiotic resistance). <sup>11</sup>  


Two studies by the McMahon Lab suggested that exposure of MRSA to sub-lethal concentration of TTO was associated with decreased susceptibility to antibiotics.<sup>9, 10</sup> McMahon <i>et al</i> supported these claims by demonstrating that the effective MIC of TTO on MRSA increased significantly from the baseline MIC after a 72 h exposure to sub-lethal concentrations of TTO. They suggested that, even though TTO was an effective treatment for MRSA, repeated TTO use could lead to reduced susceptibility to the effects of antibiotics. <br>
====TTO-Induced Antibiotic Resistance is Unlikely====
[[Image:Thomsen-McMahon.png|thumb|400px| Figure 5. A comparison of McMahon and Thomsen's experimental parameters regarding the study of antibiotic susceptibility after TTO exposure. Both researchers performed similar experiments (i.e. same procedure) with the shown differences.<sup>[http://jac.oxfordjournals.org/content/59/1/125.full 12],[http://www.sciencedirect.com/science/article/pii/S0924857913000071 13]</sup> Although they investigated the same question, the researchers came to different conclusions. Image created by Karen Leung.]]
 
Whether TTO exposure decreases antibiotic susceptibility in MRSA remains a subject of investigation. Evidence has been presented for both sides of this debate by McMahon <i>et al</i> and Thomsen <i>et al</i>.<sup>12,13</sup> Both researchers used similar experimental design: they examined the changes in minimal inhibitory concentrations (MIC) of various antibiotics before and after MRSA was inoculated with tea tree oil for 72 hours. <sup>12,13</sup> McMahon’s key study reported that habituating MRSA isolates to a sub-lethal concentration of TTO increased MRSA’s resistance to antibiotics.<sup>12</sup> During an attempt to replicate McMahon’s findings, Thomsen instead found that habituating MRSA isolates to a sub-lethal concentration of TTO did not increase MRSA’s antibiotic resistance.<sup>13</sup> Although McMahon’s findings couldn’t be confirmed, their key study prompted a number of further studies regarding the possibility of MRSA-induced antibiotic resistance. 
 
Further investigation reports that it is unlikely that TTO induces antibiotic resistance. A study by Hammer <i>et al.</i> seconded Thomsen’s idea that exposure to sub-lethal levels of tea tree oil was not linked to development of antibiotic resistance.<sup>14</sup> Hammer also claimed that since monoterpenes target the “structure, function, and integrity of microbial membranes, it seems unlikely that true resistance will arise” from TTO usage.<sup>14</sup>
 
====Implications of TTO-Induced Antibiotic Resistance====
Given the unexplored clinical significance regarding TTO-induced antibiotic resistance, the discussion of TTO-induced antibiotic resistance remains relevant.<sup>12</sup> If found to impact antibiotic susceptibility, TTO concentrations in retail products may be increased so that there is less exposure to sub-lethal TTO concentrations.<sup>12</sup> This may be achieved through legislation or regulation of TTO formulation.


==Conclusion==
==Conclusion==
<br> Though cytoplasmic membrane damage is believed to be the major bacteriocidal effect, there are still many unexplored mechanisms of TTO effects on MRSA.  
Discovering alternative therapy for the treatment of MRSA is very important as antibiotic resistance becomes more prevalent. MRSA's increasing resistance to mupirocin, the primary antibiotic in standard MRSA eradication treatments, has been noted by Dryden.<sup>3</sup> Since TTO shows little sign of inducing antibiotic susceptibility in MRSA upon treatment, one alternative treatment is the use of tea tree oil as an antimicrobial agent.<sup>13</sup>


Overall paper length should be 3,000 words, with at least 3 figures.<br>
The application of a TTO regimen is comparable to standard MRSA treatment in eradicating MRSA.<sup>9,3</sup> The reported mechanism for TTO's bactericidal effects is damage to the cytoplasmic membrane, thus making the membrane more permeable to substances, such as water.<sup>2</sup> Impaired osmoregulation may lead to cell lysis.<sup>2</sup>


==References==
Current research focuses on TTO's mechanism of action, safety, and effect on antibiotic susceptibility. Though cytoplasmic membrane damage is believed to be the major bactericidal effect, the mechanism is still largely unexplored. Other research examines commercial tea tree oil's cytotoxic effects if applied topically to human skin.<sup>15</sup> Such investigations would determine the safety of applying TTO on a regular basis and hopefully, the optimal strength of TTO for pharmaceutical formulation.


1. National Health Service (NHS). "MRSA Infection" <i>NHS Choices</i> 23/09/2011 Published. Web. 21/03/2013 Accessed. <http://www.nhs.uk/conditions/MRSA/Pages/Introduction.aspx>


2.  [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1360273/ Carson, C. F., Hammer, K. A., Riley, T. V. Malalueca alternifolia (tea tree) oil: a review of antimicrobial and other medicinal properties.  Clin. Microbiol. Rev. 19, 50-62 (2006)]
==References==


3.[http://pubs.acs.org/doi/abs/10.1021/jf00089a027 Brophy, J. J., N. W. Davies, I. A. Southwell, I. A. Stiff, and L. R. Williams. 1989. Gas chromatographic quality control for oil of Melaleuca terpinen-4-ol type (Australian tea tree). J. Agric. Food Chem. 37:1330-1335.]
1. [http://www.nhs.uk/conditions/MRSA/Pages/Introduction.aspx MRSA Infection. National Health Service (NHS) Web site. Last revised on 23/09/2011. Accessed on 21/03/2013.]


4. [http://ac.els-cdn.com/S0195670100908302/1-s2.0-S0195670100908302-main.pdf?_tid=9bfe844a-95a9-11e2-b83e-00000aacb360&acdnat=1364256827_240afe6f133a77d6dcd595bad10a67f9 Caelli, M., Porteous, J., Carson, C. F., Heller, R., Riley, T. V. Tea tree oil as an alternative topical decolonization agent for methicillin-resistant <i>Staphylococcus aureus</i>. J. Hosp. Infect. 46, 236-237 (2000)]  
2. [http://aac.asm.org/content/46/6/1914.full.pdf+html Carson, C. F., Mee, B. J., Riley, T. V. Mechanism of action of <i>Melaleuca alternifolia</i> (tea tree) oil on <i>Staphylococcus aureus</i> determined by time-kill, lysis, leakage, and salt tolerance assays and electron microscopy. Antimicrob. Agents Chemother. 46, 1914-1920. (2002)]


5. [http://jac.oxfordjournals.org/content/early/2013/01/06/jac.dks501.long Blackwood, B., Thompson, G., McMullan, R., Stevenson, M., Riley, T. V., Alderdice, F. A., Trinder, T. J., Lavery, G. G., McAuley D. F. Tea tree oil (5%) body wash versus standard care (johnson's Baby Softwash) to prevent colonization with methicillin-resistant Staphylococcus aureus (MRSA) in critically ill adults: a randomized controlled trial. doi: 10.1093/jac/dks501 J. Abtimicrob. Chemother. (Published online 2013)]
3. [http://www.sciencedirect.com/science/article/pii/S0195670104000167 Dryden, M. S., Dailly, S., Crouch, M. A randomized, controlled trial of tea tree topical preparations versus a standard topical regimen for the clearance of MRSA colonization. J. Hosp. Infect. 56, 283-286 (2004)]


6. [http://www.sciencedirect.com/science/article/pii/S0195670104000167 Dryden, M. S., Dailly, S., Crouch, M. A randomized, controlled trial of tea tree topical preparations versus a standard topical regimen for the clearance of MRSA colonization. J. Hosp. Infect. 56, 283-286 (2004)]
4. [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1360273/ Carson, C. F., Hammer, K. A., Riley, T. V. Malalueca alternifolia (tea tree) oil: a review of antimicrobial and other medicinal properties. Clin. Microbiol. Rev. 19, 50-62 (2006)]


7. [http://search.ebscohost.com/login.aspx?direct=true&db=aph&AN=6058422&site=ehost-live&scope=site Cox, S. D., et al. The mode of antimicrobial action of the essential oil of Melaleuca alternifolia (tea tree oil). J Applied Microbiol. 88, 170-175 (2000).]
5. [http://pubs.acs.org/doi/abs/10.1021/jf00089a027 Brophy, J. J., N. W. Davies, I. A. Southwell, I. A. Stiff, and L. R. Williams. Gas chromatographic quality control for oil of Melaleuca terpinen-4-ol type (Australian tea tree). J. Agric. Food Chem. 37, 1330-1335 (1989)]


8. [http://aac.asm.org/content/46/6/1914.full.pdf+html Carson, C. F., Mee, B. J., Riley, T. V. Mechanism of action of <i>Melaleuca alternifolia</i> (tea tree) oil on <i>Staphylococcus aureus</i> determined by time-kill, lysis, leakage, and salt tolerance assays and electron microscopy. Antimicrob. Agents Chemother. 46, 1914-1920. (2002)]
6. [http://textbookofbacteriology.net/structure.html Todar, K. "Structure and function of bacterial cells" Online Textbook of Bacteriology. 2012 Published. 15/04/2013 Accessed.]


9. [http://onlinelibrary.wiley.com/store/10.1111/j.1472-765X.2008.02420.x/asset/j.1472-765X.2008.02420.x.pdf?v=1&t=heqoymgp&s=ff0f3a6f109314127dd17607e4ef4296940cd1a4 McMahon, M. A. S., Tunney, M. M., Moore, J. E., Blair, I. S., Gilpin, D. F., McDowell, D. A. Changes in antibiotic susceptibility in staphylococci habituated to sub-lethal concentrations of tea tree oil (<i>Melaleuca alternifolia</i>). Lett. Appl. Microbiol. 47, 263-268. (2008)]
7. [http://search.ebscohost.com/login.aspx?direct=true&db=aph&AN=6058422&site=ehost-live&scope=site Cox, S. D., Mann, C. M., Markham, J. L., Bell, H. C., Gustafson, J. E., Warmington, J. R., Wyllie, S. G. The mode of antimicrobial action of the essential oil of Melaleuca alternifolia (tea tree oil). J. Applied Microbiol. 88, 170-175 (2000).]


10. [http://jac.oxfordjournals.org/content/59/1/125.full McMahon, M. A. S., Blair, I. S., Moore, J. E., McDowell, D. A. Habituation to sub-lethal concentrations of tea tree oil (<i>Melaleuca alternifolia</i>) is associated with reduced susceptibility to antibiotics in human pathogens. J. Antimicrob. Chemother. 59, 125-127. (2007)]
8. [http://jac.oxfordjournals.org/content/45/5/639.full?sid=4734d06d-ea17-46eb-8288-3001f988f987 May, J., Chan, C. H., King, A., Williams, L., French, G. L. Time kill studies of tea tree oils on clinical isolates. J. Antimicrob. Chemo. 45, 639-643 (2000)]
9. [http://ac.els-cdn.com/S0195670100908302/1-s2.0-S0195670100908302-main.pdf?_tid=9bfe844a-95a9-11e2-b83e-00000aacb360&acdnat=1364256827_240afe6f133a77d6dcd595bad10a67f9 Caelli, M., Porteous, J., Carson, C. F., Heller, R., Riley, T. V. Tea tree oil as an alternative topical decolonization agent for methicillin-resistant <i>Staphylococcus aureus</i>. J. Hosp. Infect. 46, 236-237 (2000)]


11.  
10.[http://cancer.org/treatment/treatmentsandsideeffects/complementaryandalternativemedicine/herbsvitaminsandminerals/tea-tree-oil "Tea tree oil" American Cancer Society Web site. 28/11/2008 Revised. 16/04/2013 Accessed.


11. [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC153147/  Gilbert, P., McBain A. J. Potential impact of increased use of biocides in consumer products on prevalence of antibiotic resistance. Clin. Microbiol. Rev. 16, 189-208 (2003)]


12. [http://jac.oxfordjournals.org/content/59/1/125.full McMahon, M. A. S., Blair, I. S., Moore, J. E., McDowell, D. A. Habituation to sub-lethal concentrations of tea tree oil (<i>Melaleuca alternifolia</i>) is associated with reduced susceptibility to antibiotics in human pathogens. J. Antimicrob. Chemother. 59, 125-127. (2007)]


13. [http://www.sciencedirect.com/science/article/pii/S0924857913000071 Thomsen, N. A., Hammer, K. A., Riley, T. V. Belkum, A. V., Carson, C. F. Effect of habituation to tea tree (<i>Melaleuca alternifolia</i>) oil on the subsequent susceptibility of Staphylococcus spp. to antimicrobials, triclosan, tea tree oil, terpinen-4-ol and carvacrol. Int. J. Antimicrob. Agents. 41, 343-351. (2013)]


14. [http://aac.asm.org/content/56/2/909.full Hammer, K. A., Carson, C. F., Riley, T. V. Effects of Malaleuca alternifolia (Tea Tree) essential oil and the major monoterpene component terpinen-4-ol on the development of single- and multistep antibiotic resistance and antimicrobial susceptibility. Antimicrob. Agents Chemother. 56, 909-915 (2012)]


15. [http://web.ebscohost.com/ehost/detail?sid=20308a68-10a8-4e65-8646-dd0217fb7689%40sessionmgr10&vid=1&hid=28&bdata=JnNpdGU9ZWhvc3QtbGl2ZSZzY29wZT1zaXRl#db=aph&AN=31322267 Loughlin, R., Gilman, B. F., McCarron, P. A., Tunney, M. M. Comparison of the cidal activity of tea tree oil and terpinen-4-ol against clinical bacterial skin isolates and human fibroblast cells. Letters Applied Microb. 48, 428-433 (2008)]


[Sample reference] [http://ijs.sgmjournals.org/cgi/reprint/50/2/489 Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "''Palaeococcus ferrophilus'' gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". ''International Journal of Systematic and Evolutionary Microbiology''. 50, 489-500 (2000)]


Edited by Karen Leung, a student of [http://www.jsd.claremont.edu/faculty/profile.asp?FacultyID=254/ Nora Sullivan] in BIOL187S (Microbial Life) in [http://www.jsd.claremont.edu/ The Keck Science Department of the Claremont Colleges] Spring 2013.
[Edited by Karen Leung, a student of [http://www.jsd.claremont.edu/faculty/profile.asp?FacultyID=254/ Nora Sullivan] in BIOL187S (Microbial Life) in [http://www.jsd.claremont.edu/ The Keck Science Department of the Claremont Colleges] Spring 2013.


<!--Do not edit or remove this line-->[[Category:Pages edited by students of Nora Sullivan at the Claremont Colleges]]
<!--Do not edit or remove this line-->[[Category:Pages edited by students of Nora Sullivan at the Claremont Colleges]]

Latest revision as of 05:43, 14 May 2013

The prevalence of Methicillin-resistant Staphyloccocus aureus (MRSA) as a pathogen in hospitals and other community settings underscores the importance of eradicating the infection.1 As MRSA is not susceptible to commonly used antibiotics, alternative antimicrobial agents are being sought for its eradication. One area of interest involves the use of tea tree oil, which studies have shown to effectively treat infections of drug-resistant bacteria, including MRSA.2

Introduction

Figure 1. This colorized scanning electron micrograph (SEM) depicts a grouping of methicillin resistant Staphylococcus aureus (MRSA) bacteria. Public through the CDC

Staphyloccocus aureus, more commonly known as staph bacteria or MRSA, is a Gram-positive coccus-shaped anaerobic bacterium pictured in Figure 1.1 MRSA is a type of staph bacteria that is resistant to beta-lactam antibiotics, such as penicillin, amoxicillin, oxacillin, and methicillin. MRSA is also becoming resistant to mupirocin, the current standard antibiotic used in the treatment of MRSA infection.3. MRSA often colonizes on the skin or nostrils of healthy individuals, and is relatively harmless at these sites.1 If S. aureus enters the body (e.g., wounds, cuts), it may cause infections. In such instances, the MRSA infection may range from mild (e.g., pimples) to life-threatening (e.g., infection of bloodstream, joints, or bones).1 MRSA is spread through contact and most commonly contracted in public settings, namely hospitals.

Tea tree oil (TTO) is the essential oil derived through steam distillation from the Australian native plant Melaleuca alternifolia. Tea tree oil has been used for centuries as a topical antiseptic. TTO is believed to have antibacterial, antifungal, antiviral, and anti-inflammatory properties when used topically.4 TTO may be found as additives in beauty and health products, including shampoo, soap, and nasal spray. Although historical anecdotes endorse TTO's medicinal properties, few clinical studies have been conducted to support such claims. Clinical studies, however few, indicate that tea tree oil can treat the skin infection caused by MRSA.4 Tea tree oil's anti-microbial properties are attributed to belonging to a class of chemicals known as terpenes, specifically terpinen-4-ol.4 Tea tree oil's bactericidal effects make this plant extract a plausible addition or supplement to a MRSA treatment plan.

Tea Tree Oil Composition and Chemistry

Figure 2. Chemical structure of terpinen-4-ol, one of nearly 100 compounds identified in tea tree oil. Public through Wikimedia Commons

Commercially available TTO is a composition of nearly 100 chemical compounds determined by gas chromatography-mass spectrometry.5 TTO is primarily composed of a class of chemicals called monoterpenes. Typically, monoterpenes are volatile, aromatic hydrocarbons that are lipophilic, meaning that they can dissolve in fats or lipids.4

Terpinen-4-ol, pictured in Figure 2, is the specific monoterpenic compound believed primarily responsible for TTO's anti-microbial activity. This hydrocarbon compound has one alcohol group attached to the fourth carbon. Like all monoterpenes, terpinen-4-ol’s mostly hydrocarbon structure gives the molecule lipophilic properties.4 Researchers claim that the lipophilic nature of monoterpenes contributes to its antimicrobial activity, though the exact details of the antimicrobial mechanism are not currently known.4 Other monoterpenic compounds in TTO include 1,8-cineole, terpinolene, and α-terpineol, whose medicinal properties are also under investigation.4

TTO composition is commercially available in various chemotypes. The most common chemotype is the terpinen-4-ol chemotype where terpinen-4-ol comprises between 30 to 40% of commercially available TTO.4 A relatively high composition of terpinen-4-ol is more common in commercial production due to its demonstrated broad spectrum of biological activities. 4,5

Antimicrobial Activity: Cytoplasmic Membrane Damage

Figure 3. Membrane permeability is disrupted after tea tree oil (TTO) treatment. A physiologically normal cytoplasmic membrane (A) excludes foreign substances, like NaCl and propidium iodide. A membrane treated with TTO (B) does not properly exclude foreign material and water, which researchers believe is involved with TTO’s batericidal activity.7,2 The exact mechanism for this phenomenon is unknown. Image created by Karen Leung.

The target of TTO's antimicrobial activity, and the primary reason for its laboratory and clinical effectiveness, is reported to be the cytoplasmic membrane of MRSA. The bacterial cell membrane physically separates the cell's internal environment from the external environment. It is selectively permeable to organic molecules and ions. As such, it regulates the movement of water and other substances into and out of the cell.6 If the membrane permeability were compromised, then the barrier between the internal and external environment would be weakened. Consequently, foreign material could enter the cell more easily while cytoplasmic material could leave it more easily (as shown in Figure 3).7,2

Based on the lipophilicity of the terpinen-4-ol molecule and other TTO components, researchers have proposed that TTO can insert itself into and damage the lipid-rich biological membranes.4 Studies examining membrane integrity after TTO treatment indicate that the essential oil disrupts the vital function of the biological membrane 7,2

An in vitro (i.e. laboratory) study by Cox et al. found that TTO treatment disturbed the MRSA membrane permeability, which allowed propidium iodide, a foreign substance, to enter the cell and potassium ions, part of the cytoplasmic material, to leave the cell.7 Another in vitro study demonstrated that TTO-treated MRSA had increased susceptibility to sodium chloride (NaCl), a cell toxin.2 From this evidence, researchers believed that TTO altered the cell membrane.

The loss of intracellular material and the inability to maintain osmotic regulation is consistent with a mechanism of action involving the loss of membrane integrity. Despite evidence of cytoplasmic membrane damage, the molecular mechanism of this impairment is still unknown and warrants further research.4

Treatment and Effectiveness

Efforts to validate the therapeutic properties of tea tree oil (TTO) have yielded in vitro and clinical studies showing that TTO treatment eradicates and reduces MRSA infection, respectively. While TTO displays in vitro and clinical efficiency, TTO has not been demonstrated to prevent initial MRSA colonization. Several studies described below and illustrated by Figure 4, support the claims upheld by traditional healers that TTO is an effective antimicrobial agent.

In vitro Effectiveness

Figure 4. The in vitro and clinical effectiveness of tea tree oil (TTO), as described by May et al.,Caelli et al. and Dryden et al..8,9,3 Image created by Karen Leung.

The efficacy of tea tree oil (TTO) against MRSA has been demonstrated in laboratory studies using cultured S. aureus samples. An in vitro study by May et al. showed that 99.9% of the MRSA isolate was killed by TTO within 4 hours and that all was eradicated after 6 hours of continuous exposure to 5% TTO.8 These results suggest that TTO's antimicrobial properties make this a good agent to control MRSA and reduce its transmission among humans.

Clinical Efficiency

Figure 4. Impetigo, a skin infection resulting from staph infection. Dryden et al. found that tea tree oil could effectively treat a similar MRSA-related ailment.3 Public through the CDC.

The efficiency of TTO in eradicating MRSA cultures in vitro led to clinical testing with human patients using products enhanced with TTO. TTO may be added to commercial products, such as body wash, skin cream, and nasal ointment due to its antimicrobial properties. The clinical efficacy of TTO body wash and creams as a decolonization agent for MRSA has been supported by Caelli et al. and Dryden et al..9, 3 Both researchers found that TTO treatment is as effective as the standard treatment regimen, indicating that TTO treatment may enhance a MRSA eradication regimen.

Caelli et al. clinically determined that TTO treatment was as effective as the standard MRSA medical treatment.9 Caelli compared the effectiveness of TTO treatment to that of standard treatments against MRSA, as measured by post-treatment MRSA concentrations on the skin and within the nose of human patients.9 Compared to the standard treatment, the tea tree oil topical regimen reduced or eradicated MRSA infection in more subjects. Despite this optimistic finding, the small sample size prevented this result from being statistically significant. Consequently, Caelli concluded that TTO treatment had a similar effect as standard medical treatment at eradicating MRSA 5

In a similar study, Dryden et al. found that TTO cleared up MRSA-related skin infections better than the standard medical treatment.3 Dryden's clinical study found that the daily application of 5% TTO body wash and 10% TTO cream was more effective than standard treatment at clearing skin lesions associated with MRSA infection.3 Although research suggests the efficacy of TTO against MRSA, more clinical studies would have to be performed in order to comprehensively assess TTO's effects on MRSA patients. From these experiments, TTO was deemed to be sufficiently effective, safe, and well-tolerated to be considered part of a MRSA eradication regimen.3

Dermal Toxicity

Despite characterizing TTO’s antimicrobial properties, researchers have not done much work on the safety and toxicity of using TTO topically. The rationale for continued topical use of TTO primarily rests on anecdotal evidence of its safe usage at low concentrations.4 No human deaths due to TTO exposure have been reported.4 Though there are few substantial scientific studies regarding TTO’s safety, TTO can cause irritant or allergic reactions when applied to the skin.4 More concrete evidence of TTO’s toxicity would be necessary in order to determine its optimal concentration for safe usage.

Susceptibility to Antibiotic Resistance after TTO Exposure

The available forms of TTO range from pure oil to retail products for personal health care, home care, and pet care.10 Commercial TTO preparations, which may vary in its components' concentrations, provide users more opportunity to apply TTO at ineffective concentrations. The growing use of TTO also implies that a diversity of microbes is coming into contact with sub-lethal TTO concentrations.10 There is concern that exposing MRSA to sub-lethal concentrations of TTO may lead to decreased antibiotic susceptibility (i.e. increased antibiotic resistance). 11

TTO-Induced Antibiotic Resistance is Unlikely

Figure 5. A comparison of McMahon and Thomsen's experimental parameters regarding the study of antibiotic susceptibility after TTO exposure. Both researchers performed similar experiments (i.e. same procedure) with the shown differences.12,13 Although they investigated the same question, the researchers came to different conclusions. Image created by Karen Leung.

Whether TTO exposure decreases antibiotic susceptibility in MRSA remains a subject of investigation. Evidence has been presented for both sides of this debate by McMahon et al and Thomsen et al.12,13 Both researchers used similar experimental design: they examined the changes in minimal inhibitory concentrations (MIC) of various antibiotics before and after MRSA was inoculated with tea tree oil for 72 hours. 12,13 McMahon’s key study reported that habituating MRSA isolates to a sub-lethal concentration of TTO increased MRSA’s resistance to antibiotics.12 During an attempt to replicate McMahon’s findings, Thomsen instead found that habituating MRSA isolates to a sub-lethal concentration of TTO did not increase MRSA’s antibiotic resistance.13 Although McMahon’s findings couldn’t be confirmed, their key study prompted a number of further studies regarding the possibility of MRSA-induced antibiotic resistance.

Further investigation reports that it is unlikely that TTO induces antibiotic resistance. A study by Hammer et al. seconded Thomsen’s idea that exposure to sub-lethal levels of tea tree oil was not linked to development of antibiotic resistance.14 Hammer also claimed that since monoterpenes target the “structure, function, and integrity of microbial membranes, it seems unlikely that true resistance will arise” from TTO usage.14

Implications of TTO-Induced Antibiotic Resistance

Given the unexplored clinical significance regarding TTO-induced antibiotic resistance, the discussion of TTO-induced antibiotic resistance remains relevant.12 If found to impact antibiotic susceptibility, TTO concentrations in retail products may be increased so that there is less exposure to sub-lethal TTO concentrations.12 This may be achieved through legislation or regulation of TTO formulation.

Conclusion

Discovering alternative therapy for the treatment of MRSA is very important as antibiotic resistance becomes more prevalent. MRSA's increasing resistance to mupirocin, the primary antibiotic in standard MRSA eradication treatments, has been noted by Dryden.3 Since TTO shows little sign of inducing antibiotic susceptibility in MRSA upon treatment, one alternative treatment is the use of tea tree oil as an antimicrobial agent.13

The application of a TTO regimen is comparable to standard MRSA treatment in eradicating MRSA.9,3 The reported mechanism for TTO's bactericidal effects is damage to the cytoplasmic membrane, thus making the membrane more permeable to substances, such as water.2 Impaired osmoregulation may lead to cell lysis.2

Current research focuses on TTO's mechanism of action, safety, and effect on antibiotic susceptibility. Though cytoplasmic membrane damage is believed to be the major bactericidal effect, the mechanism is still largely unexplored. Other research examines commercial tea tree oil's cytotoxic effects if applied topically to human skin.15 Such investigations would determine the safety of applying TTO on a regular basis and hopefully, the optimal strength of TTO for pharmaceutical formulation.


References

1. MRSA Infection. National Health Service (NHS) Web site. Last revised on 23/09/2011. Accessed on 21/03/2013.

2. Carson, C. F., Mee, B. J., Riley, T. V. Mechanism of action of Melaleuca alternifolia (tea tree) oil on Staphylococcus aureus determined by time-kill, lysis, leakage, and salt tolerance assays and electron microscopy. Antimicrob. Agents Chemother. 46, 1914-1920. (2002)

3. Dryden, M. S., Dailly, S., Crouch, M. A randomized, controlled trial of tea tree topical preparations versus a standard topical regimen for the clearance of MRSA colonization. J. Hosp. Infect. 56, 283-286 (2004)

4. Carson, C. F., Hammer, K. A., Riley, T. V. Malalueca alternifolia (tea tree) oil: a review of antimicrobial and other medicinal properties. Clin. Microbiol. Rev. 19, 50-62 (2006)

5. Brophy, J. J., N. W. Davies, I. A. Southwell, I. A. Stiff, and L. R. Williams. Gas chromatographic quality control for oil of Melaleuca terpinen-4-ol type (Australian tea tree). J. Agric. Food Chem. 37, 1330-1335 (1989)

6. Todar, K. "Structure and function of bacterial cells" Online Textbook of Bacteriology. 2012 Published. 15/04/2013 Accessed.

7. Cox, S. D., Mann, C. M., Markham, J. L., Bell, H. C., Gustafson, J. E., Warmington, J. R., Wyllie, S. G. The mode of antimicrobial action of the essential oil of Melaleuca alternifolia (tea tree oil). J. Applied Microbiol. 88, 170-175 (2000).

8. May, J., Chan, C. H., King, A., Williams, L., French, G. L. Time kill studies of tea tree oils on clinical isolates. J. Antimicrob. Chemo. 45, 639-643 (2000)

9. Caelli, M., Porteous, J., Carson, C. F., Heller, R., Riley, T. V. Tea tree oil as an alternative topical decolonization agent for methicillin-resistant Staphylococcus aureus. J. Hosp. Infect. 46, 236-237 (2000)

10."Tea tree oil" American Cancer Society Web site. 28/11/2008 Revised. 16/04/2013 Accessed.

11. Gilbert, P., McBain A. J. Potential impact of increased use of biocides in consumer products on prevalence of antibiotic resistance. Clin. Microbiol. Rev. 16, 189-208 (2003)

12. McMahon, M. A. S., Blair, I. S., Moore, J. E., McDowell, D. A. Habituation to sub-lethal concentrations of tea tree oil (Melaleuca alternifolia) is associated with reduced susceptibility to antibiotics in human pathogens. J. Antimicrob. Chemother. 59, 125-127. (2007)

13. Thomsen, N. A., Hammer, K. A., Riley, T. V. Belkum, A. V., Carson, C. F. Effect of habituation to tea tree (Melaleuca alternifolia) oil on the subsequent susceptibility of Staphylococcus spp. to antimicrobials, triclosan, tea tree oil, terpinen-4-ol and carvacrol. Int. J. Antimicrob. Agents. 41, 343-351. (2013)

14. Hammer, K. A., Carson, C. F., Riley, T. V. Effects of Malaleuca alternifolia (Tea Tree) essential oil and the major monoterpene component terpinen-4-ol on the development of single- and multistep antibiotic resistance and antimicrobial susceptibility. Antimicrob. Agents Chemother. 56, 909-915 (2012)

15. Loughlin, R., Gilman, B. F., McCarron, P. A., Tunney, M. M. Comparison of the cidal activity of tea tree oil and terpinen-4-ol against clinical bacterial skin isolates and human fibroblast cells. Letters Applied Microb. 48, 428-433 (2008)


[Edited by Karen Leung, a student of Nora Sullivan in BIOL187S (Microbial Life) in The Keck Science Department of the Claremont Colleges Spring 2013.