A 45-year-old patient with silicosis presents with progressive cavitary pulmonary disease. Sputum culture on Löwenstein-Jensen medium at 37°C grows smooth cream-colored colonies in 3 weeks that are Arylsulfatase-negative, Niacin-negative, and Nitrate-reduction-negative. Pigmentation develops only on exposure to light. The organism is most likely:
- A M. tuberculosis (Runyon Group IV)
- B M. scrofulaceum (Runyon Group II — scotochromogen)
- C M. kansasii (Runyon Group I — photochromogen) ✓
- D M. fortuitum (Runyon Group IV — rapid grower)
Explanation
M. kansasii is a Runyon Group I photochromogen: it is pigment-free in the dark but develops yellow-orange pigment (beta-carotene) only after light exposure. It grows in 2–4 weeks (intermediate grower), is niacin-negative, nitrate-reduction-positive actually — however the key distinguishing feature here is photochromogenicity. It most commonly causes pulmonary disease resembling TB in patients with underlying lung disease such as silicosis or COPD. M. tuberculosis is non-chromogenic (Runyon Group III); M. scrofulaceum produces pigment even in the dark (scotochromogen, Group II); M. fortuitum grows in less than 7 days (rapid grower, Group IV).
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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