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Different counts in MacConkey Agar

Roberto Rosati r.rosati at jack.isrim.krenet.it
Fri May 26 10:06:47 EST 2000

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


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