A sputum isolate from a suspected TB patient grows on Lowenstein-Jensen medium after 6 weeks as rough, tough, buff-colored colonies. Biochemical workup shows strong nitrate reduction and abundant accumulation of a compound not produced significantly by other mycobacteria. Which biochemical property is characteristic of Mycobacterium tuberculosis?
- A Hydrolysis of Tween 80
- B Positive catalase activity at 68 degrees C
- C Strong positive niacin test ✓
- D Positive tellurite reduction within 3 days
Correct answer: C. Strong positive niacin test
Explanation
Mycobacterium tuberculosis is strongly niacin-positive because it lacks the enzyme that converts niacin to niacin ribonucleotide, so free niacin accumulates in the medium. It reduces nitrate, so option pairs are separated by this. The killer distractor is heat-stable catalase: M. tuberculosis catalase is thermolabile and destroyed at 68 degrees A, whereas atypical mycobacteria retain activity. Tween 80 hydrolysis is seen in M. kansasii, and rapid tellurite reduction is a feature of M. fortuitum.
Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.