MacConkey agar is a selective and differential medium. What is the basis of its selectivity and how does it differentiate lactose-fermenting from non-fermenting organisms?

• A Selectivity: potassium tellurite inhibits most bacteria; Differentiation: tellurite reduced to black tellurium in colonies
• B Selectivity: bile salts and crystal violet inhibit gram-positive bacteria; Differentiation: lactose fermenters produce acid → neutral red indicator turns pink/red colonies; non-fermenters remain colorless/pale ✓
• C Selectivity: NaCl 7.5% inhibits gram-negative bacteria; Differentiation: mannitol fermentation changes phenol red to yellow
• D Selectivity: thiosulfate-citrate inhibits gram-positives; Differentiation: H2S production turns center of colonies black

Explanation

MacConkey agar contains: (1) crystal violet and bile salts that inhibit gram-positive bacteria (selectivity for gram-negatives); (2) lactose as the differentiating carbohydrate; (3) neutral red pH indicator and peptone. Lactose-fermenting organisms (E. coli, Klebsiella) produce acid, lowering pH, causing neutral red to turn pink-red; bile salt precipitation gives a surrounding halo. Non-fermenters (Salmonella, Shigella, Pseudomonas) remain colorless/translucent. Potassium tellurite is in TCBS/Hoyle's medium. High NaCl with phenol red describes mannitol salt agar for Staphylococcus. Thiosulfate-citrate describes TCBS agar.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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