I am not aware of any measures of biocidal activity applied to essential oils.

Antimicrobial activity of essential oil is usually expressed by MIC minimal inhibitory concentracion of oil in μg/ml. μg/ml are not the same as μl/ml, μg are approx. 1/1000 μl, depends of density.

An essential oil is non aqueous and has to be dispersed in oil or water before it becomes effective against micro-organisms (for health and safety reasons) Few Essential Oils are active against the whole spectrum of micro-organisms anyway and so a specific answer is required for any given essential oil against any given micro-organism.

If you have a specific concentration of a specific Essential Oil in mind against a specific micro-organism then we could begin to look at an answer for you.

Essential oils are liquids and from my previous research experience i would suggest to use PPM (Parts per million) as the measurement unit. This can be later converted into mg/L or ug/ml.

For this a weighed amount of essential oil is used rather than volume. Eg. 1 mg of essential oil in 1 liter would give you 1 PPM or 1 mg/L or 1 ug/ml.

Caution: The density of each essential oils have to be determined prior to using.

How determine density of EOs without know his elements?

You need not know the elements of EOs to know the density.

The density of 1 ml of pure water = 1.0 gm similarly weigh your essential oils.

Take exactly 1 ml of your essential oil and note down the weight. That weight is the density of your EOs. Easy.

Caution: Repeat the weighing at least 3-5 times to confirm the density or take the mean of 3-5 replicates. Hope this is helpful.

Who mix EOs with broth in order to develop antimicrobial assay?

Sorry How

This is quite tricky a question. But, there are methods to mix EOs in broth. I had used a simple method by making EOs stock of 10,000 PPM in ethanol and mixing know volume of the stock in broth to get a desired concentration.

This is done by adding 1 g of your essential oil in 100 ml of ethanol = 10,000 PPM

Now that you have a stock of 10,000 PPM you can make different concentration as required. Here ethanol would act as a emulsfier. Below is a table given, that i used for my research:

Add 10uL (10 microliter) of stock to 100 ml broth to get 1PPM or 1mg/L or 1ug/ml concentration, similarly

20uL - 100 ml broth - 2 PPM

50uL - 100 ml broth - 5 PPM

100uL - 100 ml broth - 10 PPM

So, mix the stock to broth accordingly to get the range of concentrations you require.

But make sure to have a control with only ethanol. You may even use acetone, although acetone is a strong anti-microbial; at the concentration what we use is rendered useless.

Please keep the stock at -20 degree C. And mix the EOs stock after sterilization of your broth. Hope this helped. Good luck.

Thank you very much

Antimicrobial activity of essential oils is measured by MIC by ug/ml (ppm). the most solvent used in essential oil dilutions is Dimethyl sulfoxide (DMSO)

This paper is very useful in making dilutions from essential oils and antibiotic powder according to the antibiotic potency

Thank u Mohamed. Great paper.

you are welcome Roman

the concentration for whole essential oil is ug/ml as IC 50

An important issue when reporting MIC values for any substance is the need to make your results comparable with those that were previously published or those that will come from future work. That means that when one wants to compare MIC values of different compounds (molecules with different molecular masses) these should be expressed as mol per unit of volume and not as mass/volume (although one can always convert these to mol/volume by oneself). Essential oils are more or less complex mixtures of organic compounds of different polarity (from highly non-polar up to semipolar compounds) that are immiscible with water (hence with nutrient both as well). Since essential oils probably exert their antimicrobial effect through the action of several compounds present in different concentrations it is very difficult to make such precise comparison as for pure compounds. It is then customary to report you results as mass of the oil provoking a certain antimicrobial activity in a certain volume of both. Perhaps it is easier to measure the volume of a liquid than its mass, but this practice (giving MIC concentrations in volume of the oil per volume of the broth) introduces another unknown (density) and makes the results more vague. The density of an essential oil sample can vary significantly with its composition although it might come from the same plant source and have the same amount of active compounds (the amount of other (diluting) oil constituents may change).

Due to the mentioned immiscibility of essential oils and broths, as well as the inactivity of most oils in concentrations of true water solutions of essential oils (very low values), one has to make an emulsion of an oil to ascertain a certain degree of homogeneity and contact between the oil and microorganisms. This can be done by using either a solution of the oil in organic solvents that are miscible with water (e.g. ethanol, DMSO, etc), but making sure that the solvent is not the one truly active (negative controls with the used solvent), or by making a water emulsion with a nonionic surfactant/emulsifier (such as Tween 80, again include a negative control).

The lower the polarity of an oil sample is you will be less likely to dissolve your sample in DMSO and will need to use Tween.

the concentration is IC50 unit ug /ml in case of croude extract , But if u have the pure compounds shoud be calculate as mm/l

For testing the antimicrobial activity of an essential oil first one has to choose the kind of the microorganism, which is to be tested. It is better to use low quantities of the crude oil than dilutions in organic solvents. Especialy alcohols can be metabolize from several microorganism, which disrupt the growth and adulterate the result.

My own testing of some commercial oils and herb extracts are descibed in the following paper.

Data Examination of fungicidal and antimicrobial properties of ph...

Thanks Mr Schreiber, I will read the paper.

There are many publications where essential oil concentrations are given as percentages based on volumes of oil and diluent (vol/vol) or mass of oil and diluent (w/vol). Another unit used frequently is parts per million (ppm), although it is used less than percentages. The concentration of sIngle compounds can be reported a variety of ways but milligrams per millilitre (mg/ml) is widely used. Molarity (mol/litre ) is also often used for single components. I agree with Niko Radulović that being able to compare results with other published values is important.

I have an essential oil with a MIC of 0.5% (v/v).How do I convert the percentage in microgramme/ml?

cordialy

You will find by attachment  why i want to convert MIC % of essential oil to microgram/ml.