The other day I was given a sample of some lovely (if a little strange) oregano and rose soap. I was rather surprised to be told by the shop assistant that oregano could kill MRSA. It seems I missed this story towards the end of last year about a University of West England project that had received funding to further investigate the potential. The antimicrobial properties are apparently due to a compound called carvacrol. This looks like an interesting line of research, but what about my soap? How much cavracrol do you need to be effective, and how much of it is there in my soap?
I only have access to the abstracts on pubmed, but this study indicates a concentration of 200mg/l had an antimicrobial effect on E. Coli and this study found an antimicrobial effect with carvacrol levels of approximately 1.0%. Wikipedia tell me that carvacrol has a density of 0.9772 g/cm3, so that would make a 1% concentration in the order of 9800 mg/l, or almost 50 times more than the first study. I'm not sure of the details of the studies and how they would affect the concentrations of carvacrol required, but clearly they make a big difference. In lieu of further information, I'll go with the 1% figure for now.
The soap list oregano oil (origanum vulgare) as one of its ingredients, of which carvacrol makes up between 44 and 85% according to this paper. So, it would take roughly 2% of the soap being oregano oil to reach the 1% concentration. You can't tell from the list of ingredients just how much oregano oil is used, but 2% sounds reasonable to me*. I haven't taken into account dilution of the soap in water, but I have taken the conservative figures in the calculations, so I think that it shows that the soap is plausibly acting as an antimicrobial.
*This is definitely a weak spot in my analysis, I have no soap making experience on which to base this statement.
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I agree with the antimicrobial effects of Oregano, however my notes/texts suggest the concentration of components will vary widely depending on such variables as source and distillation. One text lists carvacrol at 4.3% and thymol at 85%, another lists the pair at 55%.
I have reservations about the wisdom of it being used in soap, though. It's a dermal irritant and can cause sensitisation (ie eventually trigger dermatitis (The chemistry of aromatherapeutic oils, Bowles). At a concentration of 2% I'd imagine this effect would be noticeable, so the concentration may be much less than that.
My cynicism suspects that manufacturers would add only enough oil to get the desired smell and this may be possible at fractions of a percent.
To kill E.Coli, my texts suggest Eucalyptus Citriodora is optimal (diluted 1:10 with an oil high in monoterpenes such as Lemon); I'd save a blend containing oregano oil in a known quantity for the occasions when MRSA is a risk.
I didn't know that about oregano being an irritant. I believe you are right, they probably only put enough in for the smell. I certainly haven't found it irritating when I've used it.
Also, thanks for reposting my link :)
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