At what % does methyl-blue become antimicrobial?

Enigl enigl at aol.com
Wed May 13 12:03:53 EST 1998

In article <01bd7af6$d75bc0e0$8b10aacc at guyf>, "Guy A. Faylor" <guyf at enter.net>

>Does anyone know at what percent methyl-blue becomes antimicrobial?

As you have suspected, many of these types of dyes/stains are antimicrobial to
some extent--but they are usually NOT broad ranging enough to be used alone.  I
co-authored a book on this _Preservative-Free and Self Preserving Cosmetics and

Antimicrobial range depends of the strain you are interested in.  Bacteria,
yeast and molds usually require _different_ concentrations to be controlled. 
The number of microorganisms you are starting with also matters. . . Because: 
Microbes do not die instantly (at some magic percent of dye)--instead, they die
by logarithmically, one log at a time, over time.  This can be measure as the
D-value--the time it takes to kill one log.  If you have two logs (10^2 = 100
CFU) then you need--not only--"dye percentage" but also contact time, twice as
high as the D-value (1+1 = 2 logs), if you have six logs to kill, you need six
times the D-value.

You can usually decrease the time to kill (D-value) by increasing the dye
concentration.  I usually formulate to get a 4 hour D-value, 10^6 kill in 24

Most of these dyes are not tested in the literature (except some are listed in
my book)-- still, I would not trust the literature and test the actual strains
needing to be controlled to get the specific D-value.  If you test three
concentrations and obtain the D-values for each concentration, simply make a
graph to tell you the "correct" concentration needed to kill the particular
bioburden associated with your application.  Call me if you need help at
916-989-8264 or e-mail at enigl at aol.com


Davin C. Enigl, Microbiology Consultant

Training classes in FDA regulations: Title 21 CFR
110 Food cGMP,
123 HACCP,
210 Pharmaceuticals,  biotechnology and dietary supplements
USP 23 Microbial limits, preservative stability testing
Cosmetic cGMPs

Co-author of _Preservative-Free and Self-Preserving Cosmetics and Drugs_,
Marcel Dekker Publisher: 1-800-228-1160

May 13, 1998
9:47 am PACIFIC
Davin C. Enigl, MEAS. Microbiology Consultant for Foods, Cosmetics,
Pharmaceuticals and Biotechnology. FDA validation protocols, water system,
autoclave, facility start-up. Author: _Preservative-Free and Self-Preserving
Cosmetics and Drugs_ Marcel Dekker.

More information about the Microbio mailing list

Send comments to us at biosci-help [At] net.bio.net