Q.64 Which one of the following statements is TRUE about MacConkey (MAC) agar medium?
(A) MAC agar medium is a selective and differential medium for Gram-positive bacteria.
(B) MAC agar medium is a selective and differential medium for Gram-negative bacteria.
(C) MAC agar medium is an enriched medium for Gram-positive bacteria.
(D) MAC agar medium is a synthetic medium for Gram-positive and Gram-negative bacteria.
MacConkey agar medium serves as a cornerstone in microbiology labs for isolating enteric bacteria. The correct answer to Q.64 is option (B).
Correct Answer
MAC agar medium is a selective and differential medium for Gram-negative bacteria. Crystal violet dye and bile salts inhibit Gram-positive bacteria, while lactose and neutral red allow differentiation of lactose fermenters (pink colonies) from non-fermenters (colorless).
Option Analysis
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(A) Incorrect: MAC agar targets Gram-negative bacteria, not Gram-positive; bile salts and crystal violet suppress Gram-positive growth.
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(B) Correct: Selective via inhibitors for Gram-positives; differential by lactose fermentation showing pink/red for fermenters like E. coli.
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(C) Incorrect: Not enriched; lacks blood or serum, focuses on selection/differentiation rather than boosting fastidious Gram-positive growth.
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(D) Incorrect: Not purely synthetic (uses peptones, bile salts); supports Gram-negatives selectively, not both Gram types equally.
Medium Composition
Key ingredients include peptones for nutrients, lactose as fermentable sugar, bile salts/crystal violet for Gram-positive inhibition, neutral red pH indicator, sodium chloride for osmosis, and agar for solidification. Final pH around 7.1 ensures optimal enteric bacteria growth.
Principle and Uses
Bile salts disrupt Gram-positive membranes; lactose fermenters lower pH, turning neutral red pink. Used for isolating Enterobacteriaceae/Pseudomonas from clinical samples, differentiating pathogens in stool cultures.
