Mycobacterium tuberculosis is cultured from a pulmonary sample on Lowenstein-Jensen (LJ) medium. The colonies appear after 4 weeks and show a characteristic morphology. Which of the following best describes M. tuberculosis colonies on LJ medium?

• A Buff/cream-coloured, rough, dry, cauliflower-like colonies (eugonic, non-chromogenic) ✓
• B Smooth, yellow-pigmented colonies (scotochromogen)
• C Rapidly growing (within 7 days), smooth white colonies
• D Photochromogenic, smooth pale colonies turning yellow on light exposure

Explanation

M. tuberculosis on LJ medium produces characteristic rough, dry, buff-coloured, irregular 'cauliflower-like' or breadcrumb colonies after 3–6 weeks; these are eugonic (grow well) and non-chromogenic (no pigment). Scotochromogens such as M. scrofulaceum form yellow-orange pigment in dark. M. kansasii is photochromogenic (pigment only on light exposure). Rapidly growing mycobacteria (M. fortuitum complex) grow within 7 days.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.