MacConkey Agar and Coliform Detection

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Questions and Answers

What is the primary purpose of using MacConkey agar?

To promote the growth of coliforms

To isolate enteric Gram-positive bacilli

To identify lactose fermenting and non-lactose fermenting Gram-negative bacilli (correct)

To detect non-lactose fermenting bacteria

What color do colonies appear as if they can ferment lactose on MacConkey agar?

Colorless

Pink/red (correct)

Beige

Blue

Which component of MacConkey agar is responsible for inhibiting the growth of Gram-positive bacteria?

Phenol red

Coliform bacteria

Bile salts

Crystal violet (correct)

Why do coliform bacteria sometimes appear surrounded by a black coloration on MacConkey agar?

Due to the precipitation of bile salts from the drop in pH Signup and view all the answers

Which type of bacteria are common in digestive tracts and are targeted with MacConkey agar?

Gram-negative, lactose fermenting Signup and view all the answers

Why is MacConkey agar considered a selective medium?

For its inhibition of Gram-positive bacterial growth Signup and view all the answers

What does pink colored growth on MacConkey agar indicate about the bacteria?

Lactose fermentation Signup and view all the answers

Why do colonies that do not ferment lactose on MacConkey agar remain colorless?

Due to the absence of phenol red Signup and view all the answers

What is the role of crystal violet and bile salts in MacConkey agar?

Inhibit the growth of Gram-positive bacteria Signup and view all the answers

Why is the presence of coliforms in a water supply concerning?

Suggests fecal contamination Signup and view all the answers

What color do colonies appear as if they cannot ferment lactose on MacConkey agar?

Colorless Signup and view all the answers

What causes the pink/red color in colonies that ferment lactose on MacConkey agar?

Drop in pH and phenol red indicator Signup and view all the answers

Why might coliform bacteria appear surrounded by a black coloration on MacConkey agar?

Precipitation of bile salts from drop in pH Signup and view all the answers

What is the main purpose of using MacConkey agar in industries like food and pharmaceuticals?

Detecting contaminated products with enteric bacilli Signup and view all the answers

Why are coliforms considered significant in terms of public health concern?

Their presence suggests fecal contamination Signup and view all the answers

In what way does MacConkey agar serve as a differential medium?

By distinguishing between lactose fermenting and non-lactose fermenting bacteria Signup and view all the answers

Study Notes

MacConkey Agar

MacConkey agar is used for isolating enteric Gram-negative bacilli, such as Escherichia coli and Salmonella typhimurium.
It is used in industries like food, dairy, cosmetic, and pharmaceutical to detect contaminated products.
MacConkey agar is also used to detect coliforms, which are Gram-negative, lactose-fermenting bacilli common in human and animal digestive tracts.

Selective and Differential Properties

MacConkey agar is both selective and differential.
Its selective properties come from crystal violet and bile salts, which inhibit Gram-positive growth.
It differentiates between lactose-fermenting and non-lactose-fermenting Gram-negatives.

Lactose Fermentation

Pink or red coloration on MacConkey agar indicates lactose fermentation due to a pH drop and phenol red pH indicator change.
Coliform bacteria may also have a black coloration around the colony due to bile salt precipitation from pH drop.
Colorless or beige growth indicates no lactose fermentation.

Public Health Implications

Presence of coliforms in a water supply can suggest fecal contamination, concerning public health officials.

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Description

Learn about the usage of MacConkey agar in isolating Gram-negative bacilli like Escherichia coli and Salmonella typhimurium in various industries. Also, discover how this medium is beneficial in detecting coliforms, which are common in the digestive tracts of humans and animals, posing concern in water supplies.