water dropEssential oils are plant secondary metabolites (such as polyphenols, phenylproponoids, terpenoids, ect) which are important for plants’ survival and contribute to their immune defense. (1-2) These same compounds have been extensively studied and shown to exhibit immune and wellness benefits for humans as well.

Previously, I discussed how several essential oils have demonstrated inhibition activity against various microbes. (2-9) The mechanisms behind the action of how essential oils inhibit microbes is still not fully understood. Several studies have associated their actions to the generation of irreversible damage to the membrane of bacterial cells. This leads the bacteria to “leak out its contents” and die. Other proposed mechanisms, among many, include inhibition of toxin producing enzymes, protein degeneration, and interference with cellular respiration and electron flow. (8,9)

One of the major issues in medicine today is antibiotic resistance. Several essential oils have shown action against resistant microorganisms in vitro and in some small human. (2-9) One mechanism that seems to contribute to the prowess of these “superbugs” are biofilms. Biofilms form when bacterial (or other microbial communities) all adhere to each other on a surface and form an extracellular matrix for all of them to share and reside in. Think of a biofilm as something similar to a protective shield for these tiny critters to gain superpowers. This allows these microbe inhabitants to share nutrients, transfer DNA, and make them more resistant our own immune defenses and external antibacterial agents.(10-16) Therefore, it seems logical that if these biofilms could be prevented, there would be less chance for resistant microbes to grow.

Essential Oils and Biofilms

A few studies have demonstrated that essential oils show promise in their ability to inhibit biofilm formation. (8, 17,18) Below are two such abstracts.

(1) One study tested six different chemotypes (variations in predominant constituents) of oregano and found that oregano had in vitro activity against Staphylococcus aureus strains and Psuedomonas aeruginosa. Furthermore, these essential oil species showed inhibition of new biofilm formations from food spoilage organisms:

Abstract: Essential oils from six different populations of Origanum vulgare subsp. hirtum were compared for their antibiofilm properties. The six essential oils (A to F) were characterized by a combination of gas chromatography with flame ionization detector and gas chromatography with mass spectrometer detector analyses. All oils showed weak activity against the planktonic form of a group of Staphylococcus aureus strains and against a Pseudomonas aeruginosa ATCC 15442 reference strain. The ability to inhibit biofilm formation was investigated at sub-MIC levels of 200, 100, and 50 m g/ml by staining sessile cells with safranin. Sample E showed the highest average effectiveness against all tested strains at 50 m g/ml and had inhibition percentages ranging from 30 to 52%. In the screening that used preformed biofilm from the reference strain P. aeruginosa, essential oils A through E were inactive at 200 m g/ml; F was active with a percentage of inhibition equal to 53.2%. Oregano essential oil can inhibit the formation of biofilms of various food pathogens and food spoilage organisms. (17)

2. A second in vitro study that used an active constituent in oregano, carvacrol, also showed inhibition of biofilms against microbes; however, it was not effective in breaking down pre-formed biofilms:

The formation of biofilm by bacteria confers resistance to biocides and presents problems in medical and veterinary clinical settings. Here we report the effect of carvacrol, one of the major antimicrobial components of oregano oil, on the formation of biofilms and its activity on existing biofilms. Assays were carried out in polystyrene microplates to observe (a) the effect of 0–0.8 mM carvacrol on the formation of biofilms by selected bacterial pathogens over 24 h and (b) the effect of 0–8 mM carvacrol on the stability of pre-formed biofilms. Carvacrol was able to inhibit the formation of biofilms of Chromobacterium violaceum ATCC 12472, Salmonella enterica subsp. Typhimurium DT104, and Staphylococcus aureus 0074, while it showed no effect on formation of Pseudomonas aeruginosa (field isolate) biofilms. This inhibitory effect of carvacrol was observed at sub-lethal concentrations (<0.5 mM) where no effect was seen on total bacterial numbers, indicating that carvacrol’s bactericidal effect was not causing the observed inhibition of biofilm formation. In contrast, carvacrol had (up to 8 mM) very little or no activity against existing biofilms of the bacteria described, showing that formation of the biofilm also confers protection against this compound. Since quorum sensing is an essential part of biofilm formation, the effect of carvacrol on quorum sensing of C. violaceum was also studied. Sub-MIC concentrations of carvacrol reduced expression of cviI (a gene coding for the N-acyl-L-homoserine lactone synthase), production of violacein (pigmentation) and chitinase activity (both regulated by quorum sensing) at concentrations coinciding with carvacrol’s inhibiting effect on biofilm formation. These results indicate that carvacrol’s activity in inhibition of biofilm formation may be related to the disruption of quorum sensing. (18)


Resistance and Essential Oils

BugsFew studies have been done on resistance to essential oils; although several studies suggested low probability of this due to their complex mechanisms and synergistic components. (3, 8, 19) The following abstract describes an in vitro study with oregano and cinnamon essential oil against 48 clinical isolates and 12 reference strains of Gram-negative bacilli in order to test susceptibility and resistance of them to the oils. The study found that after 50 exposures, there was low probability of resistance. (Note, there was no mention of methods or quality of the oils in the abstract):

Essential oils (EOs) are excellent antimicrobial agents sometimes used in active food packaging. This work studies the susceptibility of 48 clinical isolates and 12 reference strains of Gram-negative bacilli to oregano essential oil, cinnamon essential oil, and combinations of both. Furthermore, the tendency of the clinical isolates to develop resistance to these EOs and to different antibiotics after sequential oregano or cinnamon exposure was studied. For this purpose, antibiotic susceptibility (through disk diffusion assays and minimum inhibitory concentration [MIC] determination) and oregano and cinnamon susceptibility (through MIC and minimum bactericidal concentration [MBC] determination) were compared after 50 passages in the presence or absence of subinhibitory concentrations of oregano and cinnamon essential oils. The results showed that all strains were susceptible to both EOs and their combination independently of the antibiotic resistance profile. In addition, neither synergistic nor antagonistic effects were observed between oregano and cinnamon essential oils at the concentrations tested. After the sequential exposure to both EOs, only Serratia marcescens, Morganella morganii, and Proteus mirabilis treated with oregano changed their antibiotic resistance profile and/or increased their resistance to this EO. However, the changes in antibiotic and oregano resistance were not related. (19)

Another in vitro study used a combination of essential oils with various antibiotics in order to determine their combined actions against antibacterial resistance. The authors concluded that 4 out of the 35 essential oil-antibiotic pairs showed synergistic effects and there was no interaction between the other pairs (bold emphasis mine):

In this study we investigated the relationship between several selected commercially available essential oils and beta-lactam antibiotics on their antibacterial effect against multidrug resistant bacteria. The antibacterial activity of essential oils and antibiotics was assessed using broth microdilution. The combined effects between essential oils of cinnamon bark, lavender, marjoram, tea tree, peppermint and ampicillin, piperacillin, cefazolin, cefuroxime, carbenicillin, ceftazidime, meropenem, were evaluated by means of the checkerboard method against beta-lactamase-producing Escherichia coli. In the latter assays, fractional inhibitory concentration (FIC) values were calculated to characterize interaction between the combinations. Substantial susceptibility of the bacteria toward natural antibiotics and a considerable reduction in the minimum inhibitory concentrations (MIC) of the antibiotics were noted in some paired combinations of antibiotics and essential oils. Out of 35 antibiotic-essential oil pairs tested, four of them showed synergistic effect (FIC?0.5) and 31 pairs showed no interaction (FIC>0.5-4.0). The preliminary results obtained highlighted the occurrence of a pronounced synergistic relationship between piperacillin/cinnamon bark oil, piperacillin/lavender oil, piperacillin/peppermint oil as well as meropenem/peppermint oil against two of the three bacteria under study with a FIC index in the range 0.26-0.5. The finding highlighted the potential of peppermint, cinnamon bark and lavender essential oils being as antibiotic resistance modifying agent. Reduced usage of antibiotics could be employed as a treatment strategy to decrease the adverse effects and possibly to reverse the beta-lactam antibiotic resistance. (20)

This study is promising in that it provides preliminary evidence that essential oils should be studied more for their immune modulating properties regarding resistant superbugs. Furthermore, although the study is in vitro, it provides support that essential oils don’t seem to interact with antibiotics and may even prevent antibiotic resistance. This make sense considering their polyphenol content and the role these compounds have in supporting our microbiome.



snow angelsThe studies above suggest that incorporating the use of essential oils can decrease one’s susceptibility to resistant microbes that lead to chronic immune suppression. However, the best way to keep unwanted critters from flourishing in our bodies is by being an inhospitable host to them and preventing their overgrowth to begin with. This can be achieved through incorporating everyday activities that lead to a healthy lifestyle such as optimizing diet, minimizing “bad” stress, and participating in daily exercise and movement. Furthermore, the use of essential oils to support happy belly bugs and balance our immune system is another useful tool for keeping our bodies healthy.



Disclaimer: This information is applicable ONLY for therapeutic, Grade A essential oils. This information DOES NOT apply to essential oils that have not been AFNOR and ISO standardized. There is no quality control in the United States and oils labeled as “100% pure” need only contain 5% of the actual oil. The rest of the bottle can be filled with fillers and sometimes toxic ingredients that can irritate the skin. These studies are not specific to any essential oils company or brand. Please check the original reference for the source.

This information is for information purposes only and is not intended to diagnose, treat, or prescribe for any illness.



(1) Plant phenylpropanoids as emerging anti-inflammatory agents. Mini Rev Med Chem. 2011 Sep;11(10):823-35.

(2) Symposium on ‘Plants as animal foods: a case of catch 22?’: Antimicrobial properties of plant secondary metabolites. Proceedings of the Nutrition Society. (2004),63: 621–629.

(3) Combination of essential oils and antibiotics reduce antibiotic resistance in plasmid-conferred multidrug resistant bacteria. Phytomedicine. 2013 Jun 15;20(8-9):710-3. doi: 10.1016/j.phymed.2013.02.013. Epub 2013 Mar 26.

(4) Inhibition of methicillin-resistant Staphylococcus aureus (MRSA) by essential oils. Flavour and Fragrance Journal. 2008; 23: 444-449. DOI: 10.1002/ffj.1904

(5) Nelson, J. Selection of resistance to the essential oil of Melaleuca alternifolia in Staphylococcus aureus. J. Antimicrob Chemother. 2000; 45 (4): 549-550. doi: 10.1093/jac/45.4.549

(6) Staphylococcus aureus and wounds: a review of tea tree oil as a promising antimicrobial. Am J Infect Control. 2004 Nov;32(7):402-8.

(7) Uncontrolled, open-label, pilot study of tea tree (Melaleuca alternifolia) oil solution in the decolonisation of methicillin-resistant Staphylococcus aureus positive wounds and its influence on wound healing. Int Wound J. 2011 Aug;8(4):375-84. doi: 10.1111/j.1742-481X.2011.00801.x. Epub 2011 May 12.

(8) Essential Oils and Future Antibiotics: New Weapons against Emerging ‘Superbugs’? J Anc Dis Prev Rem. 2013;1: 105. doi:10.4172/2329-8731.1000105

(9) Djilani, A & Dicko, A. The Therapeutic Benefits of Essential Oils, Nutrition, Well- Being and Health. Dr. Jaouad Bouayed ed. 2012; 165-166.

(10) Bacterial biofilms: a common cause of persistent infections. Science. 1999 May 21;284(5418):1318-22.

(11) Microbial biofilms.Annu Rev Microbiol. 1995;49:711-45.

(12) Antibiotic resistance of bacteria in biofilms.Lancet. 2001 Jul 14;358(9276):135-8.

(13) Biofilms: Survival Mechanisms of Clinically Relevant Microorganisms. Clinical Microbiology Reviews 2002;15(2):167-193. doi:10.1128/CMR.15.2.167-193.2002.

(14) The role of bacterial biofilms in chronic infections.APMIS Suppl. 2013 May;(136):1-51. doi: 10.1111/apm.12099.

(15) Use of antimicrobial peptides against microbial biofilms: advantages and limits. Curr Med Chem. 2011;18(2):256-79.

(16) Antimicrobial Peptides. Pharmaceuticals. 2013;6(12):1543-1575. doi:10.3390/ph6121543.

(17) Origanum vulgare subsp. hirtum essential oil prevented biofilm formation and showed antibacterial activity against planktonic and sessile bacterial cells. J Food Prot.  2013 October; 76(10):1747-52.

(18) The Natural Antimicrobial Carvacrol Inhibits Quorum Sensing in Chromobacterium violaceum and Reduces Bacterial Biofilm Formation at Sub-Lethal Concentrations. PLoS ONE. 2014;9(4):e93414. doi:10.1371/journal.pone.0093414.

(19) Evaluation of bacterial resistance to essential oils and antibiotics after exposure to oregano and cinnamon essential oils. Foodborne Pathog Dis. 2012;9(8):699-705. doi: 10.1089/fpd.2011.1097. Epub 2012 Jul 24

(20) Combination of essential oils and antibiotics reduce antibiotic resistance in plasmid-conferred multidrug resistant bacteria. Phytomedicine. 2013 Jun 15;20(8-9):710-3. doi: 10.1016/j.phymed.2013.02.013. Epub 2013 Mar 26.

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