Macconkey Agar composition, uses, colony

The Macconkey agar composition culminates in a selective culture medium that isolates gram-negative and enteric bacilli and differentiates them based on lactose fermentation.

MacConkey agar composition was first developed in the 20th century by Alfred Theodore MacConkey, who was working as a bacteriologist for the Royal Commission on Sewage Disposal. MacConkey’s goal was to create a medium that would select for the growth of Gram-negative microorganisms and inhibit the growth of Gram-positive microorganisms.

He achieved the Macconkey Agar composition goal by adding bile acids to standard media, which are toxic to many organisms but not to enteric organisms that have evolved mechanisms to resist their action.

MacConkey also wanted to differentiate between types of enteric organisms and used the observation that Escherichia coli ferments lactose, whereas Salmonella does not to differentiate these organisms. Today, MacConkey agar is commonly used for the isolation of Gram-negative enteric bacteria and differentiates them based on lactose metabolism.

Macconkey agar composition

Ingredient	Quantity	Application
Peptone	17g	Peptone provides essential nutrients.
Agar	13.5g	Solidifying agent.
Lactose monohydratre	10g	Fermentable carbohydrate.
NaCl	5g	Maintain osmotic balance
Protease peptone	3g	Source of nitrogen and essential nutrients
Bile Salts	1.5g	Inhibit gram-positive bacteria and inhibit the swarming of Proteus.
Neutral red	0.03g	pH indicator turns red below pH 6.8 and colorless at pH above pH 6.8
Crystal violet	0.01g	Inhibit gram-positive bacteria and inhibit the swarming of Proteus.
Distilled water	1 liter	Added to make 1 liter

The final Adjust pH to 7.1 +- 0.2

How to prepare NaCl solution in a laboratory setting.

How to prepare Macconkey Agar

To prepare MacConkey agar, follow these steps:

1. Mix 49.53 grams of dehydrated MacConkey agar medium with 1l of distilled water. (Or preparation from macconkey agar composition)
2. Heat the mixture to boiling while stirring to ensure that the medium is completely dissolved.
3. Sterilize the mixture by autoclaving it at 15 lbs pressure (121°C) for 15 minutes.
4. Allow the mixture to cool to 50°C.
5. Mix the mixture well and pour it into sterile Petri plates.

Colony Morphology

When bacteria are grown on MacConkey agar, they form colonies that can be distinguished based on their color. Lactose fermenting bacteria produce acid when they metabolize lactose, which lowers the pH of the medium and causes the pH indicator neutral red to turn pink. As a result, lactose fermenting bacteria form pink or red colonies on MacConkey agar.

On the other hand, non-lactose fermenting bacteria do not produce acid when grown on MacConkey agar and therefore do not cause a color change in the medium. These bacteria form colorless or transparent colonies on MacConkey agar.

Uses of Macconkey Agar

Some of the specific uses of MacConkey agar include the isolation of coliforms and intestinal pathogens in water, dairy products, and biological specimens, as well as the differentiation of lactose fermenting from lactose non-fermenting gram-negative bacteria.

Lactose fermenters

Lactose fermenters are bacteria that can break down lactose, a sugar found in milk and dairy products. During the fermentation process, lactose is converted into simpler sugars such as glucose and galactose, which the bacteria can then use for energy. This process also produces lactic acid, which can affect the pH of the environment. Examples of lactose fermenters include:

• Citrobacter
• Escherichia coli
• Enterobacter
• Klebsiella

Non-Lactose fermenters

Non-lactose fermenters are bacteria that do not have the ability to break down lactose, a sugar found in milk and dairy products. These bacteria lack the necessary enzymes to convert lactose into simpler sugars such as glucose and galactose, and therefore cannot use lactose as a source of energy. Some examples of non-lactose fermenting bacteria include:

• Salmonella
• Yersinia
• Proteus
• Shigella
• Pseudomonus aureginosa

Klebsiella and Enterobacter produce mucoid colonies. This is because the organisms produce a capsule that is predominantly made from lactose.

Summary

MacConkey Agar (MAC) is a type of culture media that is both selective and differential. It is primarily used to isolate and differentiate non-fastidious gram-negative rods, focusing particularly on members of the Enterobacteriaceae family and the Pseudomonas genus . The ingredients of Macconkey Agar Composition include pancreatic digest of gelatin (peptone), meat and casein (proteose peptone), lactose monohydrate, bile salts, sodium chloride, neutral red, crystal violet, agar and distilled water . The pH of MAC is adjusted to 7.1 +/- 0.2 at 25 degrees C.