MacConkey agar - Composition, Principle, Uses, and Preparation

What is MacConkey Broth?

MacConkey Broth is a modification of the medium developed by scientist MacConkey. MacConkey Broth is used for the cultivation of gram-negative Bacteria.

Storage of MacConkey’s Broth

MacConkey’s Broth medium should be kept upright at 2-30 °c.
Adequate storage definitely increases the life and quality of the product.

Freezing and overheating will cause the scale to deteriorate of the medium.
MacConkey’s Broth should not be used beyond the expiration date.
Expiration dates apply to tubes and packages that can be opened, adequately stored.

Composition of MacConkey Broth

Peptone – 20 gm
Sodium taurocholate – 5 gm
Lactose – 10 gm

Sodium chloride (NaCl) – 5 gm
pH – 7.2
Distilled water – 1 liter

Neutral red (1% sol.) – 4 ml

Ingredients	Amount
Peptone	20 gm
Sodium taurocholate	5 gm
Lactose	10 gm
Sodium chloride (NaCl)	5 gm
Neutral red (1%)	4 ml
Distilled Water	1 Liter (As our need)

Preparation of MacConkey broth

The dye should be added only after adjusting the pH.
Distribute 10 ml in different test tubes.

Put Durham’s tubes in an inverted position in these tubes.
Sterilize by autoclaving at 121 °c for 15 minutes.

What is MacConkey agar?

MacConkey agar (MAC) is a solid differential media.

This agar is developed in the 20th century by Alfred Theodore MacConkey.
It is a solidifying agent which solidifies the liquid medium.

Composition of MacConkey agar

Peptone – 20 gm

Sodium taurocholate – 5 gm
Lactose – 10 gm
Sodium chloride (NaCl) – 5 gm

Distilled water – 1 liter
pH – 7.2
Neutral red (1% sol.) – 4 ml

Agar – 2-3%

Ingredients	Amount
Peptone	20 gm
Sodium taurocholate	5 gm
Lactose	10 gm
Sodium chloride (NaCl)	5 gm
Neutral red (1%)	4 ml
Agar (2-3%)	13.5 gm
Distilled Water	1 Liter (As our need)

Preparation of MacConkey agar

Sterilize by autoclaving at 121 °c for 15 minutes.
The method of plates preparation is the same as that of Nutrient agar. This medium is used for the cultivation of enteric bacteria.

Sodium taurocholate ( bile salt) inhibits non-enteric bacteria. The lactose fermenters ( E.coli, Klebsiella spp.) utilize lactose in the medium and produce acid and gas.
Acid production changes colour from neutral red to pink and gas production is detected in inverted Durham’s tubes in MacConkey’s broth tubes.
In MacConkey’s agar plates color of colonies of lactose fermenting organisms are pink and are opaque as at acidic pH, neutral red precipitates in the colony.

Lactose non-fermenters (Salmonella spp. and Vibrio spp.)show no pink coloration around the colony, in fact, the medium changes the colour to yellow and the colonies remain translucent.

Uses of MacConkey agar

MacConkey agar is used in selective and differential media for the isolation and differentiation of non-fastidious gram-negative rod-shaped bacteria, particularly members of the family Enterobacteriaceae and the genus Pseudomonas.
It is also used for the differentiation of lactose fermenting from lactose non-fermenting gram-negative bacteria.

MacConkey agar is used for the isolation as well as differentiation of gram-negative bacteria.

Limitations of MacConkey Agar

Some strains may be encountered that grow poorly or fail to grow on this medium.
Some strains may swarm on MacConkey Agar medium.

Result and Observation of MacConkey Agar

“Lactose fermenting strains, grow red or pink and may be surrounded by a zone of acid precipitated bile. The red colour is due to the production of acid from lactose, absorption of neutral red, and a subsequent colour change of the dye when the pH of the medium falls below 6.8.”

“Lactose non-fermenting strains, such as Shigella and Salmonella are colourless and transparent and typically do not alter the appearance of the medium. Yersinia enterocolitica may appear as small, non-lactose fermenting colonies after incubation at room temperature.”

Reference and Sources

http://www.cabri.org/guidelines/micro-organisms/M203Ap1.html

https://www.carlroth.com/com/en/media-for-salmonella-shigella/macconkey-agar/p/x922.1