# DBT JRF 2022 — Question 31

Exam: DBT JRF 2022 · Section: B · Topic: Microbiology › Selective & Differential Culture Media · Difficulty: Medium

## Question

Given below are two statements: Statement I: MacConkey agar is a media used for recovery of Gram positive bacteria from mixed bacterial culture. Statement II: The bile salts and crystal violet in MacConkey agar inhibit growth of some bacteria. In the light of the above statements, choose the correct answer from the options given below:

## Options

- **A.** Both Statement I and II are true and Statement II is the correct explanation of Statement I
- **B.** Both Statement I and II are true but Statement II is not the correct explanation of Statement I
- **C.** Statement I is true but Statement II is false
- **D.** Statement I is false but Statement II is true

## Answer

Option D

## Explanation

MacConkey agar selectivity: MacConkey agar is a SELECTIVE and DIFFERENTIAL medium: • Selective agents: Bile salts and crystal violet — these inhibit the growth of GRAM-POSITIVE bacteria (disrupting their less protective cell membranes). • Selective for: GRAM-NEGATIVE bacteria (enteric bacteria such as E. coli, Klebsiella, Salmonella, Shigella, etc.). • Differential: Lactose fermenters produce acid → pH indicator (neutral red) turns colonies PINK/RED. Non-fermenters remain colourless. Evaluation of statements: • Statement I: 'MacConkey agar is used for recovery of Gram-POSITIVE bacteria' — FALSE (it selects for Gram-NEGATIVE). • Statement II: 'Bile salts and crystal violet inhibit growth of some bacteria' — TRUE (they inhibit Gram-positive bacteria). Answer: Statement I is false but Statement II is true (Option 4).

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