Plate count agar/broth for heterotrophic plate counts

enigl at aol.com enigl at aol.com
Fri Jan 24 22:40:13 EST 1997

In article <5c8j04$n4v at news.orst.edu>, bakerr at ucs.orst.edu (Rocky Baker)

>Why does m-TGY broth (tryptone, glucose, yeast extract) for membrane 
>filtration have twice the concentration of components as the equivalent 
>Plate Count Agar.  Since you can lay a membrane on either an agar surface

>or a pad wetted with broth, it doesn't make sense to me that the broth 
>would be twice as concentrated.

Good point.  Dextrose Tryptone Agar and m-Dextrose Tryptone Broth also do
this.  The liquid medium has twice the nutrients as the agar medium.  

The theory I was told (in the 1970s) was:  The (dairy) product wets the
filter and tends not to dry the filter enough to prevent spreaders.  That
still does not make logical sense to me.  But, to me it is equally
unlikely the nutrients need to be drawn through the membrane by
concentration gradients or by a lack of good contact between the felt pad
vs. the agar substrata.

The theory goes:  With liquid medium at twice the concentration, the wet
membrane will "dry" the top due to lower water activity than agar medium. 

But the agar medium absorbs the excess dairy product moisture through the

I've heard other theories.  I've never looked up the original 1960
_Standard Methods for the Examination of Dairy Products_, but the theory
might be discussed here.

Davin C. Enigl, (Sole Proprietorship) MEAS

Microbiology Consulting,  Hazard Analysis and
Critical Control Points (HACCP), CGMP, and Validations
for the Food, Cosmetic, Nutritional  Supplement, and Pharmaceutical

enigl at aol.com

January 23, 1997
6:26 pm

More information about the Microbio mailing list

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