Published: 1st Mar 2025, 14:59

Being a microbiologist, you are definitely aware of the terms MacConkey Agar & E. coli; if not, here in this blog you will learn about them. Don’t worry.

Escherichia coli, the well-known model organism. It is a gram-negative bacterium that is frequently found in the guts of humans and warm-blooded animals. It is found in the intestines yet is still cited as the cause of numerous ailments.

Here we will tell why.

Most strains of E. coli are harmless, but others, such as Shiga toxin-producing E. coli (STEC), can cause serious foodborne illness. It spreads to humans mostly by the ingestion of infected foods, such as raw or undercooked ground beef products, raw milk, and contaminated raw vegetables and sprouts.

Escherichia coli is a facultative anaerobic, rod-shaped, coliform bacterium, and MacConkey Agar is the ideal medium for primary culture of E. coli in the lab.

MacConkey Agar

MacConkey Agar is the selective and differential media for the culture of coliform organisms. Both MacConkey Agar and Broth are recommended for use in microbiological examinations of foods and direct plating/inoculation of water samples for coliform counts. This media is also recognized by the Standard Methods for the Examination of Milk and Dairy Products.

Gram-negative bacteria generally grow well on this medium and are distinguished by their ability to ferment lactose. After incubation at room temperature, Lactose-fermenting strains appear red or pink and may be surrounded by a zone of acid-precipitated bile. The red colour is caused by the generation of acid from lactose, the absorption of neutral red, and the consequent colour change of the dye when the pH of the medium falls below 6.8. Lactose non-fermenting bacteria, such as Shigella and Salmonella, are colourless and transparent, and they usually do not change the appearance of the medium.

Composition of MacConkey Agar

Principle of MacConkey Agar

In this, pancreatic digest of gelatin and peptones (meat and casein) supply the vital nutrients, vitamins, and nitrogenous elements required for microbial growth. Lactose monohydrate is a fermentable carbohydrate. This medium’s selective activity is attributable to crystal violet and bile salts, both of which inhibit most gram-positive bacteria species. Sodium chloride regulates the osmotic balance in the medium.

How is it a crucial tool for checking water quality?

Gram-negative enteric organisms are generally involved in causing waterborne disease. However, these bacteria do not always cause disease because they are normal inhabitants of the human gastrointestinal tract and are also found in other mammals, reptiles, and birds. Their presence is checked to confirm fecal contamination.

To check water contamination 

• Dissolve 49.53 grams of TM Media MacConkey Agar (TMH 118) in 1000 ml purified/distilled water. Boil for 1 minute with constant stirring.
• Sterilize by autoclaving at 15 psi pressure (121°C) for 15 minutes or as per validated cycle.
• Avoid overheating & cool to 45-50°C.
• Mix well before pouring into sterile Petri plates.
• After solidification, inoculate the medium with the water sample and incubate. The surface of the medium should be dry when inoculated.
• After incubation, observe the colonies.

So can you guess the colony colour of E. coli on MacConkey Agar?

E. coli is a gram-negative and lactose-fermenting bacteria; hence, it produces bright pink colonies on MacConkey agar.

TM Media’s MacConkey Agar 

At TM Media, MacConkey Agar is available in both powder and prepared forms. TM Media has more than 25 variants of powdered MacConkey agar to fulfil the varied needs of various industries. Their media formulations are as per BIS and harmonized as per USP/EP/BP/IP/JP.

To check out our range, click now.