A patient with pulmonary TB has sputum smear positive for AFB but culture on Löwenstein-Jensen (LJ) medium is negative after 8 weeks of incubation. The most likely explanation is:

• A The organism is M. leprae which does not grow on LJ medium
• B M. tuberculosis was killed by decontamination procedure ✓
• C Smear result is a false positive due to non-viable organisms
• D Contamination suppressed mycobacterial growth

Explanation

Over-decontamination using N-acetyl-L-cysteine-sodium hydroxide (NALC-NaOH) is the most common cause of culture-negative, smear-positive specimens because the harsh alkaline conditions kill mycobacteria along with contaminants. The LJ medium's slow 3–8 week growth requirement makes this a frequent diagnostic problem. M. leprae is not detectable on smear from sputum. Contaminant suppression typically causes overgrowth on the plate rather than sterility.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.