MacConkey agar is a selective and differential medium. The selective components that inhibit Gram-positive organisms are:
- A High concentration of NaCl and selective peptone composition
- B Bile salts and crystal violet, which disrupt the lipoteichoic acid-mediated membrane stability of Gram-positive organisms ✓
- C Tellurite and potassium selenite
- D Colistin (polymyxin E) and nalidixic acid
Correct answer: B. Bile salts and crystal violet, which disrupt the lipoteichoic acid-mediated membrane stability of Gram-positive organisms
Explanation
MacConkey agar contains bile salts and crystal violet as selective agents. Crystal violet (gentian violet) inhibits Gram-positive organisms by penetrating their membrane at the concentrations used; bile salts inhibit many non-enteric organisms. The Gram-negative outer membrane effectively excludes these agents. Differentiation is achieved by lactose fermentation: fermenters produce acid, turn the neutral red indicator pink/red (e.g., E. coli, Klebsiella); non-fermenters form pale/colorless colonies (e.g., Shigella, Salmonella, Pseudomonas).
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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