Lowenstein-Jensen medium used for primary isolation of Mycobacterium tuberculosis contains whole eggs, asparagine, glycerol and potassium dihydrogen phosphate. What is the function of malachite green in this medium?

• A Provides the main carbon source for mycobacterial metabolism
• B Inhibits contaminating bacteria and fungi during the long incubation ✓
• C Acts as a pH indicator turning yellow when acid is produced
• D Serves as a reducing agent to maintain anaerobiosis

Explanation

Malachite green is incorporated into Lowenstein-Jensen medium as a bacteriostatic agent that suppresses the growth of contaminating bacteria and fungi without materially affecting mycobacteria, whose slow growth over up to 8 weeks otherwise invites overgrowth. It also provides a pale green background against which colonies are easier to see. Glycerol, not malachite green, serves as a carbon source that stimulates the human tubercle bacillus, and LJ medium contains no carbohydrate fermentation indicator system.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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