MacConkey agar was the first solid differential medium to be formulated.
It was developed in the 20th century by Alfred Theodore MacConkey.
MacConkey agar is considered the earliest selective and differential medium designed for the cultivation of coliform organisms.
The medium is specifically used for the isolation and differentiation of non-fastidious gram-negative rods.
It is particularly useful for identifying members of the family Enterobacteriaceae.
It is also applied in the study and differentiation of the genus Pseudomonas.
Composition of MacConkey Agar
Peptone (Pancreatic digest of gelatin): 17 g
Proteose peptone (meat and casein): 3 g
Lactose monohydrate: 10 g
Bile salts: 1.5 g
Sodium chloride: 5 g
Neutral red: 0.03 g
Crystal violet: 0.001 g
Agar: 13.5 g
Principle of MacConkey Agar
Pancreatic digest of gelatin and peptones (meat and casein) supply essential nutrients, vitamins, and nitrogenous compounds necessary for microbial growth.
Lactose monohydrate serves as the fermentable carbohydrate source.
Crystal violet and bile salts provide the selective property of the medium by inhibiting most Gram-positive bacteria.
Sodium chloride maintains the osmotic balance of the medium.
Lactose-fermenting strains produce red or pink colonies.
The red coloration results from acid production during lactose fermentation.
This acid leads to absorption of neutral red and a subsequent color change of the dye when the medium’s pH drops below 6.8.
Non-lactose-fermenting bacteria such as Salmonella, Proteus, Pseudomonas aeruginosa, and Shigella are unable to utilize lactose.
Instead, they metabolize peptone, leading to the production of ammonia.
This raises the pH of the medium and results in the formation of white or colorless colonies.
These colonies often appear golden to brown with dark centers, and the surrounding agar remains relatively transparent.
Modification of the medium is possible by replacing lactose with other sugars to study fermentation patterns of Gram-negative bacteria.
Example: Replacing lactose with sorbitol helps in differentiating enteropathogenic Escherichia coli serotypes such as E. coli O157:H7.
In this case, non-sorbitol fermenting E. coli produce white circular colonies on MacConkey agar.
Preparation of MacConkey Agar
Suspend 49.53 g of dehydrated MacConkey agar medium in 1000 ml of distilled water.
Heat the mixture until it reaches boiling to completely dissolve the medium.
Sterilize the medium by autoclaving at 15 lbs pressure (121°C) for 15 minutes.
Allow the sterilized medium to cool to a temperature of 45°C–50°C.
Mix thoroughly before pouring into sterile Petri plates.
Result Interpretation on MacConkey Agar
Escherichia coli: Pink to rose-red colonies, often surrounded by a zone of precipitated bile.
Enterobacter and Klebsiella: Mucoid, pink colonies.
Salmonella andShigella: Colorless colonies, sometimes with the medium appearing orange to amber.
Pseudomonas: Irregular colonies that range from colorless to pink.
Uses of MacConkey Agar
Used for the isolation of Gram-negative enteric bacteria.
Helps in the differentiation of lactose-fermenting from non-lactose-fermenting Gram-negative bacteria.
Applied in the microbiological examination of foodstuffs.
Suitable for direct plating/inoculation of water samples to perform coliform counts.
Employed in the examination of milk and dairy products.
British Pharmacopoeia recommends this medium for the subculture and identification of Escherichia coli.
Limitations of MacConkey Agar
The colonial characteristics observed on MacConkey agar provide only presumptive identification of organisms; subculture and confirmatory tests (such as biochemical tests) are required for final identification.
Certain bacterial strains may grow poorly or may even fail to grow on this medium.
Incubation under increased CO₂ conditions can reduce the growth and recovery of several Gram-negative bacilli.
Some strains of Proteus may exhibit swarming growth on this medium, which can interfere with interpretation.
References
Hardy Diagnostics. MacConkey Agar – Product Information. Available at: https://catalog.hardydiagnostics.com/cp_prod/Content/hugo/MacConkeyAgar.htm
HiMedia Laboratories. Technical Data: MacConkey Agar (M081B). Available at: http://www.himedialabs.com/TD/M081B.pdf
HiMedia Laboratories. Technical Data: MacConkey Agar Variants (M008S). Available at: http://www.himedialabs.com/TD/M008S.pdf
Teknova. Culture Media – MacConkey Agar. Available at: https://www.teknova.com/category-s/504.htm
Becton, Dickinson and Company (BD). Resource Information on MacConkey Agar. Available at: http://www.bd.com/resource.aspx?IDX=8978
Microbe Online. MacConkey Agar: Composition, Preparation, Uses, and Colony Characteristics. Available at: https://microbeonline.com/macconkey-agar-mac-composition-preparation-uses-and-colony-characteristics/
