MacConkey Agar
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 .
Composition
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. Pancreatic digest of gelatin and peptones (meat and casein) provide the essential
nutrients, vitamins and nitrogenous factors required for growth of
microorganisms. Lactose monohydrate is the
fermentable source of carbohydrate. The selective action of this medium is
attributed to crystal violet and bile salts, which are inhibitory to most species
of gram-positive bacteria. Sodium chloride maintains
the osmotic balance in the medium. Neutral red is
a pH indicator that turns red at a pH below 6.8 and is colorless at any pH greater
than 6.8. Agar is the solidifying
agent.
Uses of MacConkey Agar
1. MacConkey agar is used for the isolation of
gram-negative enteric bacteria.
2. It is used in the differentiation of lactose
fermenting from lactose non-fermenting gram-negative bacteria.
3. It is used for the isolation of coliforms and
intestinal pathogens in water, dairy products and biological specimens.
Preparation
1. Suspend 49.53 grams of dehydrated medium in
1000 ml purified/distilled water.
2. Heat to boiling to dissolve the medium completely.
3. Sterilize by autoclaving at 15 lbs pressure
(121°C) for 15 minutes.
4. Cool to 45-50°C.
5. Mix well before pouring into sterile Petri
plates.
Prepare culture
The specimen streaking on macconkey agar.Incubation 37c 18-24h
Result Interpretation
Lactose fermenting strains grow as red or
pink and may be surrounded by a zone of acid precipitated bile.
The red colour is due to production of acid from lactose, absorption of
neutral red and a subsequent colour change of the dye when the pH of medium
falls below 6.8.
Lactose non-fermenting strains, such as Shigella and Salmonella are colourless and transparent and typically do not alter
appearance of the medium. Yersinia enterocolitica may
appear as small, non-lactose fermenting colonies after incubation at room
temperature.
References
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