What is a MacConkey agar plate used for?
Mia Morrison
Updated on March 25, 2026
What is a MacConkey agar plate used for?
MacConkey Agar (MAC) is a selective and differential medium designed to isolate and differentiate enterics based on their ability to ferment lactose. Bile salts and crystal violet inhibit the growth of Gram positive organisms.
What is in a MacConkey plate?
Key components of the MacConkey medium include crystal violet dye, bile salts, lactose, and neutral red (pH indicator). Crystal violet dye and bile salts halt the growth of gram-positive bacteria. This allows only gram-negative species to form colonies on MAC agar.
What type of media is MacConkey agar?
MacConkey agar is an example of a medium that is both differential and selective. The presence of bile salts, as well as crystal violet, within the media prevent gram-positive organisms from growing.
What is the purpose of pea agar?
PEA agar is a selective medium that is used for the isolation of gram- positive Staphylococcus species andStreptococcus species from clinical specimens or specimens that contain mixtures of bacterial flora (2). Typically PEA agar is used to inhibit the common contaminants such as Escherichia coli and Proteus species.
What ingredients make MacConkey agar differential?
The selective ingredients are the bile salts and the dye, crystal violet which inhibit the growth of Gram-positive bacteria. The differential ingredient is lactose. Fermentation of this sugar results in an acidic pH and causes the pH indicator, neutral red, to turn a bright pinky-red color.
What Colour is MacConkey agar?
red
Result Interpretation on MacConkey Agar 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.
What color is salmonella on MacConkey agar?
Colorless
Result Interpretation on MacConkey Agar
| Organisms | Growth results |
|---|---|
| Enterobacter, Klebsiella | Mucoid, pink colonies |
| Proteus | Colorless colonies, swarming growth |
| Salmonella, Shigella | Colorless colonies, or sometime medium color: orange to amber |
| Pseudomonas | Irregular, colorless to pink colonies |
How does sorbitol MacConkey Agar work?
In sorbitol MacConkey agar, lactose is replaced by sorbitol. Non-pathogenic strains of E. This raises the pH of the medium, allowing the pathogenic strain to be differentiated from other non-pathogenic E. coli strains through the action of the pH indicator in the medium.
How is MacConkey agar prepared?
Preparation of MacConkey Agar
- Suspend 49.53 grams of dehydrated medium in 1000 ml purified/distilled water.
- Heat to boiling to dissolve the medium completely.
- Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes.
- Cool to 45-50°C.
- Mix well before pouring into sterile Petri plates.
