American Society for Microbiology © 2016 1
MacConkey Agar Plates Protocols
Created: Friday, 30 September 2005
MacConkey
agar was the first solid differential media to be formulated. It
was developed at the turn of the 20th century by Alfred Theodore
MacConkey, M.D, then Assistant Bacteriologist to the Royal Commission
on Sewage Disposal, in the Thompson-Yates Laboratories of Liverpool
University, England. The goal was to formulate a medium that would
select for the growth of gram-negative microorganisms and inhibit the
growth of gram-positive microorganisms. Dr. MacConkey first developed
a bile salt medium containing glycocholate, lactose and litmus, to be
incubated at 22°C (MacConkey, 1900). This formula was soon altered by
the replacement of glycocholate with taurocholate and the incubation
temperature was raised to 42°C (MacConkey, 1901). MacConkey later
changed the recipe again by substituting neutral red for litmus
(MacConkey, 1905), following the suggestion that neutral red be used as
an indicator in bile salt lactose medium (Grunbaum and Hume, 1902).
The final media formulation was designed to support growth
of Shigella and is the one that is most commonly used today.
Purpose
MacConkey agar is used for the isolation of gram-negative enteric
bacteria and the differentiation of lactose fermenting from lactose non-
fermenting gram-negative bacteria. It has also become common to use
the media to differentiate bacteria by their abilities to ferment sugars
other than lactose. In these cases lactose is replaced in the medium by
another sugar. These modified media are used to differentiate gram-
negative bacteria or to distinguish between phenotypes with mutations
that confer varying abilities to ferment particular sugars.
Theory
MacConkey agar is a selective and differential media used for the
isolation and differentiation of non-fastidious gram-negative rods,
particularly members of the family Enterobacteriaceae and the
genusPseudomonas. The inclusion of crystal violet and bile salts in the
media prevent the growth of gram-positive bacteria and fastidious gram-
negative bacteria, such as Neisseria and Pasteurella. The tolerance of
gram-negative enteric bacteria to bile is partly a result of the relatively
bile-resistant outer membrane, which hides the bile-
sensitive cytoplasmic
membrane (Nikaido, 1996). Other species specific bile-resistance