Lowenstein-Jensen medium, the conventional solid medium for primary isolation of Mycobacterium tuberculosis, contains coagulated egg, glycerol, asparagine and potato starch. Which constituent provides the selective action that suppresses contaminating organisms?

• A Potassium tellurite
• B Sodium deoxycholate
• C Crystal violet
• D Malachite green ✓

Explanation

Malachite green is added to Lowenstein-Jensen medium at a final concentration of approximately 0.01 percent because mycobacteria tolerate this dye far better than most contaminating bacteria and fungi, giving the medium its selective property. Egg and glycerol serve as enrichment nutrients, not inhibitors. Sodium deoxycholate and bile salts are the selective agents of MacConkey agar, while potassium tellurite is used in media for Corynebacterium species.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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