Everything about Macconkey Agar totally explained
MacConkey (also
McConkey) agar is a
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 enterococci and species of staph)),
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 can't (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 fermenting bacteria such as
Salmonella and
Shigella can't 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.
Thus, by selecting for Gram-negative bacteria and differentiating between
enteric pathogens such as
E. coli and
Salmonella, the MacConkey agar is readily implemented in the clinical diagnosis of
diarrhea.
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.
Further Information
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