MacConkey agar is for the differentiation of lactose fermenters. Lac+
colonies are purple (E. coli) or pink (Klebsiella/Enterobacter gp.) while
Lac - colonies are colorless (Proteus sp., Providencia, Serratia, and many
others). The red strains of Serratia marcescens can look reddish on this
medium, but it is a different color than the Lac + bacteria. Hope this
helps.
Judy Dilworth, M.T. (ASCP)
Microbiology
Luis Fernando Garcia Alles wrote:
> Dear all,
> hi! Just to jog my rotting memory, does anyone there remember how
> exactly the Mc Conkey test of colonies turning pink if they ferment the
> substrate sugar is supposed to work? Is the colour yellow/red to be
> obsered arounfd the cells or in the colonies? Do the cells take up the
> pigments? How?
> Any one of these qustions answered would be a great help.
> Cheers.
> Chaitanya.
