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Different counts in MacConkey Agar

david.lawton1 david.lawton1 at ntlworld.com
Fri May 26 13:34:15 EST 2000

""Roberto Rosati"" <r.rosati at jack.isrim.krenet.it> wrote in message
news:001d01bfc723$abbdaae0$b0828fc2 at isrim.krenet.it...
> Has anybody of you experienced variability in colony counting between
> different brands of MacConkey Agar?
> In this lab we're working with E.Coli (ATCC 25922 and wild type) and we
> routinely use MacConkey Agar inclusion plating as a counting method for
> these microorganisms. (Inclusion plating is not the best method with
> MacConkey, I know...)
> I found that there is a marked difference in the number of colonies
> obtained from two different brands of MacC.A.: the wild-type strain gave
> a 2-times difference, and strain ATCC 25922 showed a tenfold difference!
> One "macroscopic" difference between the two brands, the lack of crystal
> violet in one of the two, seems not to be responsible for this effect
> (supplementing the lacking one with the appropriate quantity of crystal
> violet gives only a slight decrease in counts).
> I'd appreciate any comments and suggestions.
> Thanks in advance
> Roberto Rosati
> r.rosati at jack.isrim.krenet.it
Some types of MacConkey vary greatly in their composition.
Have you got the formulae for each brand?


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