21 - MacConkey agar

THE FIGURE ABOVE SHOWS LACTOSE FERMENTERS (TO THE LEFT) AND NON-LACTOSE FERMENTERS (TO THE RIGHT) ON MACCONKEY AGAR.

MacConkey (also McConkey) agar is a (differential) culture medium designed to grow Gram-negative bacteria and stain them for lactose fermentation.

It contains bile salts (to inhibit most Gram-positive bacteria, except Enterococcus and some species of Staphylococcus), crystal violet dye (which also inhibits certain Gram-positive bacteria), neutral red dye (which stains microbes fermenting lactose), lactose and peptone.

Alfred Theodore MacConkey developed it while working as a bacteriologist for the Royal Commission on Sewage Disposal.

Acting as a visual pH indicator, the agar distinguishes those Gram-negative bacteria that can ferment the sugar lactose (Lac+) from those that cannot (Lac-).

This medium is also known as an "indicator medium" and a "low selective medium". Absence of electrolytes serves to inhibit swarming by Proteus species.

LACTOSE FERMENTERS (LAC+) :

By utilizing the lactose available in the medium, Lac+ bacteria such as Escherichia coli, Enterobacter and Klebsiella will produce acid, which lowers the pH of the agar below 6.8 and results in the appearance of red/pink colonies.

NON-LACTOSE FERMENTERS  (LAC-) :

Non-Lactose fermenting bacteria such as Salmonella, Proteus species and Shigella cannot utilize lactose, and will use peptone instead. This forms ammonia, which raises the pH of the agar, and leads to the formation of white/colorless colonies.

SLOW LACTOSE FERMENTERS :

Some organisms ferment lactose slowly or weakly, and are sometimes put in their own category. These include Serratia and Citrobacter.

A variant, Sorbitol-MacConkey agar, (with the addition of additional selective agents) can assist in the isolation and differentiation of enteropathogenic E. coli serotypes such as E. coli O157:H7.