Low concentrations of carvacrol and oil of oregano induce biofilm formation.

Important Essential Oil Warning: Over-Diluting Oil Of Oregano Can Lead To S. Aureus Biofilm Production

Oil of oregano has been called “one of the most effective plant essential oils with antimicrobial effect,” by Dr. Yifan Zhang, assistant professor of nutrition and food science in the Wayne State’s College of Liberal Arts and Sciences according to Farm and Dairy.

Meanwhile, Reuters reported that the U.S. food industry is heavily turning to essential oils and other natural antimicrobials, because of both popular demand and the fall of effectiveness of traditional antibiotics and antimicrobials. Reportedly, Perdue Farms eliminated all routine use of antibiotics by adding oregano to its chickens’ water.

A new study, published in Food Control, looked at the biofilm-forming ability of the common bacteria Staphylococcus aureus from food-contact surfaces. Researchers evaluated the effects of the essential oil known as oil of oregano and its active ingredient carvacrol against S. aureus planktonic and sessile cells. Sessile cells are cells that are permanently attached to a base (creating a biofilm) and planktonic cells are cells that are separate from the biofilm. Bacteria acts differently depending on if it is free floating or part of a biofilm. The team looked at the effects that oregano and its active ingredient carvacrol had on biofilm formation when left to sit. The researchers reportedly used S. aureus isolates that were known to be strong biofilm producers.

Eight out of ten of the isolates tested exhibited that the minimum inhibitory concentration (MIC) of oil of oregano and carvacrol was higher against sessile cells than planktonic cells. That is, once staph cells are part of biofilm, they are harder for us to control and destroy. The MICs against sessile cells were the concentrations that were able to destroy biofilm on polystyrene.

Doses that were considered below the concentration needed to inhibit the bacteria in meat broth were tested on stainless steel surfaces over time, and the effects reportedly varied depending on the concentrations and exposure times, but the sub-MIC doses decreased the sessile cell counts. Here’s the important part to many essential oil lovers. For all of the isolates that they tested, there was an increase in sessile cells after 288 hours of exposure to 0.625 μL/mL of oil of oregano.

One microliter is equal to one-thousandth of a milliliter. Meanwhile, according to conversion calculators, 0.625 microliters is about 0.0125 of a drop. There are a thousand milliliters in a liter. So, concentrations that would be considered too low might even be around 12.5 drops per liter! That might make essential oil users reconsider the concentrations of their essential oils in their spray bottles at home.

The researchers reported that the results of the experiments showed that there is a high incidence of biofilm-producing S. aureus that can be isolated from food-contact surfaces. They also report that while oil of oregano and carvacrol can certainly inhibit planktonic and sessile cells, there is a point where a dose of even oil of oregano gets to small, too diluted, it can do more harm than good when used as an antimicrobial surface cleaner.

The researchers warn that as we move forward exploring new, natural antimicrobials, we should be careful to consider the concentrations of the active ingredients, because even oil of oregano has the ability to induce biofilm production.

The study was a joint effort between researchers in the U.S. and Brazil. Researchers Jessica Bezerra dos Santos Rodrigues, Rayssa Julliane de Carvalho, Marciane Magnani and Neyrijane Targino de Souza of the Department of Food Engineering at the Center for Technology at the Federal University of Paraiba in João Pessoa, Brazil were authors on the article.

Kleber de Sousa and Oliveira, Octávio Luiz Franco of the Center for Proteomic and Biochemical Analysis at the Catholic University of Brasília in Brasília, Brazil also authored the article on the oil of oregano study.

Donald Schaffner, Extension Specialist in Food Science at Rutgers in New Jersey, and Evandro Leite de Souza of the Department of Nutrition at the Center for Health Sciences at the Federal University of Paraiba in João Pessoa, Brazil also authored the study published in Food Control.

Though the findings about oregano oil and biofilm may be disappointing to many essential oil enthusiasts, the researchers clearly feel that it’s important to know the truth about our most powerful natural alternatives to failing traditional antibiotics and antimicrobials, so that people can use them effectively as we enter the post-antibiotic era.

[Featured Image by Monicore/Pixabay]

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