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The high content of antioxidant compounds found in some essential oils is what’s responsible for their antimicrobial properties. Theses naturally occurring derivatives are beginning to be looked at as safer alternatives for fighting microorganisms. The purpose of this study was to determine the antibacterial effects of 8 commercial essential oils [oregano, cinnamon, clove, rosemary, eucalyptus, lemon, tea tree, and orange] against three bacterial species E. coli, S. aureus, and Salmonella using the disc diffusion method. Oregano oil had the highest inhibition rate for all three bacteria while orange and lemon had little to no effect on all the bacteria.



For thousands of years essential oils have been used for curing illnesses. The Ancient Greeks were the first to use and understand the medical purpose of oregano oil. In the 15th century a physician would use oregano oil to treat diarrhea, psoriasis, vomiting, jaundice, and fungal infections (Palulis, 2012). Essential oils extracted from the buds, flowers, leaves, and other parts of the plant have long been used as natural remedies and are now beginning to gain an increased interest for their antimicrobial properties. Phenolic acids like thymol, eugenol and carvacrol are secondary plant metabolites that account for the antimicrobial activity of essential oils like oregano, cinnamon, and clove [Barbosa & et al., 2009]; Terpenoids are found in other essential oils like rosemary, eucalyptus, orange, and lemon. Terpenoids are what makes the scent, flavors, or colors of spices or plants. When testing for antimicrobial activities of oregano, cinnamon, rosemary, clove, tea tree, eucalyptus, wild orange, and lemon here are very important terpenoids that make up for how the essential oils will inhibit the bacteria. When phenols or aldehydes are attached to the terpenoid they seem to have the best antimicrobial efficacy, where ketones, easters, are weaker, and hydrocarbons are the weakest inhibition according to the research Bassole & Juliani did. In the research Palulis did mention that oregano has two main compounds that account for its extreme antimicrobial properties, carvacrol and thymol. Both are natural phenols and have strong antimicrobial activity.

The non toxic, all natural, economically feasible essential oils have a broad spectrum of antimicrobial properties has led them to become increasingly studied. Our objective of this research is to test for antibacterial activity of eight essential oils, oregano, cinnamon, rosemary, clove, tea tree, eucalyptus, wild orange, and lemon on three bacteria, Escherichia coli, Staphylococcus aureus, and Salmonella by disk diffusion method.

All eight essential oils, oregano, cinnamon, rosemary, clove, tea tree, eucalyptus, orange, and lemon were donated by doTERRA oils.
Bacterial strains Escherichia coli, Staphylococcus aureus, and Salmonella were obtained from the Biology department at El Paso Community College.

Disc Diffusion method Bacteria were grown in TSB test tubes at 370C for 24hrs. The paper discs were sterilized by autoclave in preparation for the disc diffusion method. Each bacteria was placed on TSA plates by cotton swab and allowed to dry for 3-5 minutes. The paper discs were saturated in 100% concentration of each separate essential oil and then placed onto each TSA plate freshly coated with bacteria. The positive control used was 3% H2O2 solution and the negative control was DI water. The plates were incubated at 370C for 48 hours. The zone of inhibition was measured by standard ruler.
Of the eight oils tested, oregano was the most effective. It inhibited growth of all three strains of bacteria with a very large zone. Melaluca and cinnamon proved effective against all bacteria tested and had zones of inhibition in the double digits. Orange and lemon were the least effective oils. Both oils failed to inhibit any of the bacteria except for one Salmonella trial. Both E. coli and Salmonella are Gram negative but Staph. aureus is Gram positive. There seemed to be no pattern or trend of effectiveness of the oils based on Gram stain group. Salmonella proved to be the most susceptible bacteria with an average zone of inhibition of 9.2 mm. Staph. aureus was second with an average of 5.2 mm. E. coli was last with an average of 4.5 mm.

Bacteria Oregano Cinnamon Clove Rosemary Eucalyptus Lemon Tea tree Orange NC PC
E. coli 15 8 5 8 2 0 5 0 0 7
S. aureus 19 9 3 3 3 0 7 0 0 14
Salmonella 21 14 9 8 6 2 15 2 0 10

Overall the majority of the essential oils proved effective at inhibiting the growth of all three bacterial strains. Oregano proved most effective against all three strains of bacteria. Orange and lemon had little to no effect on any of the bacteria. The oils had the most antibacterial effect on the bacteria Salmonella. Future tests and experiments could help to determine the use of essential oils for the use of children fighting illness and as a aseptic removal technique versus traditional cleaning products.
We would like to acknowledge all the members of our independent project as well as Mr. Aguilar for providing us with the opportunity to perform our experiment. We would also like to thank Patty at EPCC for providing us with all our necessary lab equipment and materials. A special thanks to Julie Mac Kenzie at doTerra for providing us with all the essential oils used for the experiment.


Aguilar, M. (2012, October 16). Exercise 69 Antibiotic Sensitivity Testing Using The Kirby-Bauer Procedure. El Paso, Texas: EPCC.
Barbosa, L., Rall, V., & Fernandez, A., (2009) Essential oils against foodborne pathogens and spoilage bacteria in minced meat. Foodborne pathogens and disease, 6 (6), DOI:10.1089=fpd.2009.0282
Bassole, I. H. N., & Juliani, H. R. (2012). Essential oils in combination and their antimicrobial properties. Molecules, 17, 3989-4006.
Prabuseenivasn, S., Jayakumar, M., & Ingnacimuthu, S. (2006). In vitro antibacterial activity of some plant essential oils. BMC compplemetrary and alternative medicine , 6(39),Retrieved from http://www.biomedcentral.com/1472-6882/6/39/
Palulis, R. (2012). Oil of oregano-the strongest most effective antibiotic known to man. Retrieved from http://rpalulis.hubpages.com/hub/Oil-of-Oregano-The-Strongest-Most-Effective-Antibiotic-Known-to-Man
Warnke, P. (2009). The battle against multi-resistant strains: Renaissance of anitmicrobial essential oils as a promising force to fight hospital-acquired infections. Macillofacial surgery,37(7), 392-397. Retrieved from http://www.sciencedirect.com/science/article/pii/S1010518209000523

Abstract by Marcos Trujillo
Introduction & References by Katrina Martinez
Materials&Methods/Results- graph and table by Lina De Santiago
Results: Erica Julian
Conclusion & Acknowledgements by Starcey Care
Edited by Katrina & Lina