MacConkey Agar- Composition, Principle, Uses, Preparation and Colony Morphology

MacConkey agar (MAC) was the first solid differential media to be formulated which was developed at 20th century by Alfred Theodore MacConkey. 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 and the genus Pseudomonas.

Composition of MacConkey Agar

Ingredients Amount
Peptone (Pancreatic digest of gelatin)   17 gm
Proteose peptone (meat and casein)  3 gm
Lactose monohydrate   10 gm
Bile salts  1.5 gm
Sodium chloride  5 gm
Neutral red  0.03 gm
Crystal Violet  0.001 g
Agar  13.5 gm
Distilled Water Add to make 1 Liter

Final pH 7.1 +/- 0.2 at 25 degrees C.

Principle of MacConkey Agar

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 of MacConkey Agar

  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.

Result Interpretation on MacConkey Agar

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.

Colony Morphology on MacConkey Agar

Colony Morphology on MacConkey Agar

Organism

Colour

Remarks

Escherichia coli

red/pink

non-mucoid

Aerobacter aerogenes

pink

mucoid

Enterococcus species

red

minute, round

Staphylococcus species

pale pink

opaque

Pseudomonas aeruginosa

green-brown

fluorescent growth

Limitations of MacConkey Agar

  1. The colonial characteristics described give presumptive identification only of the isolated organisms. It is necessary to subculture and carry out confirmation tests for final identification.
  2. Some strains may be encountered that grow poorly or fail to grow on this medium.
  3. Incubation of MacConkey Agar plates under increased CO2 has been reported to reduce growth and recovery of a number of strains of Gram-negative bacilli.
  4. Some strains of Proteus may swarm on this medium.

References

  1. Austin Community College, 5930 Middle Fiskville Rd., Austin, Texas
  2. ASM Microbe Library: MacConkey Agar Plates Protocols
  3. Thermo Fisher Scientific Inc., Dehydrated Culture Media: MacConkey Agar
  4. Acumedia Manufacturers: MacConkey Agar
  5. HiMedia Laboratories Pvt. Ltd, Technical data: MacConkey Agar
  6. Hardy Diagnostics: MacConkey Agar
  7. Science Prof Online (SPO): MacConkey Agar
  8. Bacteriological Analytical Manual, 8th Edition, Revision A, 1998.
  9. Collin County Community College District.
  10. Microbe Online
  11. Wikipedia

12 thoughts on “MacConkey Agar- Composition, Principle, Uses, Preparation and Colony Morphology”

  1. Thank you sir for your excellent notes, it really guides me in the treatment plant where i’m working as a microbiologist in the lab. please did you have notes on other media, their composition, uses, preparation and their appearances /colour after culturing.

  2. Dear Sir,
    I salute you for the nice explanations in Microbiology you regularly upload in Facebook. I share & use your blogs for teaching my Uunder graduate students. I am sure, someday some one will recognize your writing in blogs & offer you a good post & position. Pls continue your service to Microbiology

    Thank You Sir,

    Dr. Manjunath Ramanna
    Assistant Professor of Agricultural Microbiology,
    University of Horticultural Sciences, Bagalkot, Karnataka, India

  3. If MAC is for non-fastidious Gram-negative bacilli, why would Enterococcus and Staphylococcus give a reaction on this medium? Shouldn’t they show no growth? It may be that Enterococcus and Staphylococcus ferment lactose, but wouldn’t the crystal violet and bile salts inhibit their growth? Just curious.

  4. I am samiksha working as a microbiologist in ferm we are using macconkeys agar for water pathogen testing some times we found some submerged precipitation like something but not grow when subculture, in macconkeys agar plate after 60 to 72 hrs of incubation at 30 to 35 degrees incubation. Please suggest .

  5. Asante sana (thank you much) for your post. Am a foods, nutrition & dietetics student currently on practicum in a dairy factory. Initially couldn’t understand this MacConkey that i encountered in the microbiology lab but now i can understand it’s uses. For this am thankful.

  6. This is remarkably helpful. Thank you so much. Am a student of food and technology, working on the microbiology of pap was like been in the dark cos I was just following manual instructions without having a full understanding of my project work but now I know better. Thank you very much

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