MacConkey Agar and Coliform Detection

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16 Questions

What is the primary purpose of using MacConkey agar?

To identify lactose fermenting and non-lactose fermenting Gram-negative bacilli

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


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

Crystal violet

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

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

Gram-negative, lactose fermenting

Why is MacConkey agar considered a selective medium?

For its inhibition of Gram-positive bacterial growth

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

Lactose fermentation

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

Due to the absence of phenol red

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

Inhibit the growth of Gram-positive bacteria

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

Suggests fecal contamination

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


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

Drop in pH and phenol red indicator

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

Precipitation of bile salts from drop in pH

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

Detecting contaminated products with enteric bacilli

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

Their presence suggests fecal contamination

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

By distinguishing between lactose fermenting and non-lactose fermenting bacteria

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.

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.

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