Re: Can E. coli on MacConkey agar appear purple?

Dear Dana,

Lactose-fermenting organisms develop a pink-purple discoloration on 
MacConkey agar.  Bile salts in this growth medium also inhibit gram-
positive bacteria from growing.  

Thus, pink-purple organisms growing on MacConkey must be gram-negative, 
lactose fermenters.  Fast fermenters include Klebsiella, E. coli, and 
Enterobacter.  Slow fermenters include Serratia and Citrobacter.

The most likely organism that you have growing is E. coli.

Bonnie