When performing drug susceptibility testing (DST) for Mycobacterium tuberculosis by the proportion method on Lowenstein-Jensen medium, resistance is defined as growth on drug-containing medium that exceeds what percentage of the control?
- A 10% — resistant if > 10% of colonies grow
- B 1% — resistant if > 1% of the bacterial population grows at the critical concentration ✓
- C 50% — resistant if > 50% of colonies grow
- D 0.1% — resistant if > 0.1% of colonies grow
Explanation
The proportion method (Canetti method) defines resistance as growth of > 1% of the bacterial population on drug-containing medium compared to drug-free control medium at the critical concentration. This 1% threshold is based on the concept that a resistant mutant population constituting > 1% of total bacilli will dominate clinically and cause treatment failure. Critical concentrations are standardized (e.g., isoniazid 0.2 µg/mL, rifampicin 40 µg/mL, ethambutol 2 µg/mL, streptomycin 4 µg/mL on LJ medium). The proportion method remains the reference method but is slow (4–8 weeks).
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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