MacConkey agar was developed in the 20th century by Alfred MacConkey and it was the first solid differential media to be formulated. The MacConkey agar is a differential and selective media which is used for isolation and differentiation of non-fastidious gram-negative bacteria, particularly the Enterobacteriaceae family.
Composition of MacConkey Agar Media
|Peptone (Pancreatic digest of gelatin)|
Proteose peptone (meat and casein)
|Lactose monohydrate||10 gm|
|Bile salts||1.5 gm|
|Sodium chloride||5 gm|
|Neutral red||0.03 gm|
Add to make 1 Liter
(Final pH 7.1 +/- 0.2 at 25 degrees Celsius)
Principle of MacConkey Agar Media
MacConkey agar is a media that microbiologists use for the isolation of gram-negative bacteria and differentiation of lactose-fermentic gram-negative bacteria from the lactose non-fermenting ones. In this media, peptone provides the essential nutrients, vitamins, and nitrogenous factors that help in microbe growth. Lactose monohydrate forms the fermentable carbohydrate source. Bile salts and crystal violet help in the selective action of the media, as they inhibit gram-positive bacteria growth. NaCl or sodium chloride helps in maintaining osmotic balance whereas neutral red acts as a pH indicator (red at pH below 6.8 and colorless at higher). Last but not the least, agar acts as the solidifying agent.
Application of MacConkey Agar Plate
Isolation of gram-negative enteric bacteria.
Differentiation of lactose fermenting gram-negative bacteria from lactose non-fermenting ones.
Isolation of coliforms and intestinal pathogens present in water, dairy products, etc.
Lactose fermenting gram-negative bacteria grow as red or pink, generally surrounded by acid precipitated bile zone. This red color occurs due to acid production from lactose, and this changes pH to less than 6.8.
Lactose non-fermenting bacterial strains, such as Salmonella are colorless as they do not produce acid due to their incapability of lactose fermentation.
Limitations of MacConkey Agar Media
- Although colonial characteristics can give presumptive identification, subcultures and confirmation tests are suggested for final identification.
- Some strains may grow poorly on this medium.
- Incubation under increased CO2 can reduce the growth and recovery of many strains on MacConkey agar.
- Some Proteus strains may swarm on MacConkey agar.
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