Which feature best distinguishes drug-susceptible M. tuberculosis from multidrug-resistant (MDR) M. tuberculosis?

Explanation

## Drug-Susceptible vs. MDR M. tuberculosis ### Genetic Basis of Resistance **Key Point:** MDR-TB is defined by resistance to at least isoniazid (INH) and rifampicin (RIF), the two most potent first-line drugs. This resistance is conferred by specific chromosomal mutations, not by plasmids or horizontal gene transfer. ### Molecular Mechanisms of Resistance | Drug | Target Gene | Mutation Effect | Frequency in MDR-TB | |------|-------------|-----------------|---------------------| | **Rifampicin** | rpoB (RNA polymerase β-subunit) | Altered drug binding site | ~95% of RIF-resistant strains | | **Isoniazid** | katG (catalase-peroxidase) | Loss of pro-drug activation | ~50–80% of INH-resistant strains | | **Isoniazid** | inhA (enoyl-ACP reductase) | Altered NADH binding | ~20–30% of INH-resistant strains | **High-Yield:** The rpoB gene is the most common site of rifampicin resistance mutations. Isoniazid resistance typically involves loss-of-function mutations in katG (which activates the pro-drug INH) or gain-of-function mutations in inhA (target enzyme). ### Clinical Significance **Clinical Pearl:** Detection of rpoB mutations via rapid molecular tests (GeneXpert MTB/RIF, line probe assays) is now the WHO-recommended first-line diagnostic for MDR-TB, replacing culture-based drug susceptibility testing (DST). ### Why Other Options Are Incorrect - **Catalase enzyme:** Both drug-susceptible and MDR M. tuberculosis produce catalase. Loss of catalase (niacin-negative strains) is rare and unrelated to MDR status. - **Acid-fast staining:** Both susceptible and MDR strains are acid-fast positive; staining cannot differentiate resistance phenotypes. - **Niacin production:** Niacin accumulation is a biochemical marker of M. tuberculosis species identification, not a resistance marker. Both susceptible and MDR strains produce niacin. **Warning:** Do not confuse phenotypic resistance (observed lack of drug effect in culture) with genotypic resistance (presence of resistance-conferring mutations). Genotypic testing detects the mutations directly.