A sputum sample from a suspected TB patient is processed using the Petroff's method (4% NaOH decontamination) before culture. What is the primary purpose of this decontamination step?

• A To concentrate mycobacteria by lyzing non-acid-fast organisms
• B To neutralize the antibiotics present in the sputum before culture
• C To convert dormant mycobacteria into the replicating form
• D To selectively kill non-mycobacterial contaminants while sparing acid-fast bacilli, which are resistant due to their thick lipid-rich cell wall ✓

Explanation

Respiratory specimens contain diverse normal flora that would overgrow Mycobacterium tuberculosis on culture media. Petroff's 4% NaOH (or N-acetyl-L-cysteine–NaOH) decontamination kills non-mycobacteria while sparing mycobacteria because their waxy, mycolic acid–rich cell wall provides resistance to alkali. The sample is then neutralized and centrifuged; the sediment is used for smear and culture on Lowenstein-Jensen medium or liquid BACTEC systems.

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

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