MacConkey Agar : Composition, Principle and Use

MacConkey Agar is recommended for selective isolation of Escherichia coli from pharmaceutical products and is in accordance with harmonized methodology of BP. It is also recommended for selective isolation and differentiation of lactose fermenting and lactose non fermenting enteric bacteria.

Composition: 

Ingredients	Gms / Litre
Peptone (meat and casein)	3.0
Pancreatic digest of gelatin	17.0
Lactose monohydrate	10.0
Bile salts	1.50
Sodium chloride	5.0
Crystal Violet	0.001
Neutral red	0.030
Agar	13.50
pH after sterilization ( at 25°C) 7.1±0.2

Directions:

Suspend 49.53 grams of dehydrated medium in 1000 ml purified/distilled water. Heat to boiling to dissolve the medium completely. Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes i.e. validated cycle. AVOID OVERHEATING. Cool to 45-50°C. Mix well before pouring into sterile Petri plates. The surface of the medium should be dry when inoculated. 

Principle And Interpretation:

MacConkey Agar is the earliest selective and differential medium for cultivation of coliform organisms. Subsequently MacConkey Agar and Broth have been recommended for use in microbiological examination of foodstuffs and for direct plating / inoculation of water samples for coliform counts. This medium is also accepted by the Standard Methods for the Examination of Milk and Dairy Products. British pharmacopoeia has recommended this medium for the subculture and identification of Escherichia coli. 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. After enrichment of Escherichia coli in MacConkey Broth, it is then subcultured on MacConkey Agar.Gram negative bacteria usually grow well on the medium and are differentiated by their ability to ferment lactose. 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. 

Reference:1. www.himedialabs.com

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