MacConkey agar is a selective and differential medium. The component responsible for selecting against Gram-positive organisms while allowing growth of Gram-negatives is:
- A Neutral red (pH indicator) at low concentration killing Gram-positive bacteria
- B Bile salts and crystal violet inhibiting Gram-positive bacteria by disrupting their cell membrane ✓
- C Lactose acting as the sole carbon source that Gram-positives cannot utilize
- D NaCl concentration creating hypertonic conditions lethal to Gram-positives
Explanation
MacConkey agar contains bile salts and crystal violet as selective agents; bile salts disrupt the lipoteichoic acid and peptidoglycan-rich cell wall of Gram-positive bacteria while Gram-negative bacteria with outer membrane are intrinsically resistant; crystal violet (a basic dye) also inhibits Gram-positive bacteria by penetrating their cell walls. Neutral red is the pH indicator differentiating lactose fermenters (pink/red colonies) from non-fermenters (colorless), not a selective agent. Lactose non-fermenting Gram-negative bacteria still grow on MacConkey. MacConkey contains standard NaCl not hypertonic concentrations.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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