A 35-year-old man with newly diagnosed pulmonary tuberculosis is found to have M. tuberculosis that is susceptible to isoniazid (INH) but resistant to rifampicin (RIF). Which structural or biochemical feature of the organism best explains this resistance pattern?

Explanation

## Rifampicin Resistance in M. tuberculosis ### Mechanism of Rifampicin Resistance **Key Point:** Rifampicin resistance in M. tuberculosis is almost always (>95% of cases) caused by mutations in the *rpoB* gene, which encodes the β-subunit of bacterial RNA polymerase. Rifampicin binds to this subunit and inhibits transcription; mutations prevent drug binding. ### Molecular Basis 1. **rpoB mutations:** Point mutations in the rifampicin-binding pocket of RNA polymerase β-subunit prevent rifampicin from binding. 2. **High-frequency resistance:** These mutations arise at a rate of ~1 in 10^6 to 10^8 bacilli, making rifampicin resistance relatively common in untreated TB. 3. **Isoniazid susceptibility:** INH resistance requires a *separate* mutation (typically in *katG* or *inhA*), which is independent of the *rpoB* mutation. This patient has RIF-R but INH-S, indicating *rpoB* mutation without *katG*/*inhA* mutations. ### Comparison: INH vs. RIF Resistance Mechanisms | Drug | Gene(s) | Mechanism | Frequency | | --- | --- | --- | --- | | **Rifampicin** | *rpoB* | RNA polymerase mutation; prevents drug binding | ~95% of RIF-R | | **Isoniazid** | *katG*, *inhA*, *fabG1* | Loss of pro-drug activation (*katG*) or target modification (*inhA*) | ~50% *katG*, ~25% *inhA* | | **Pyrazinamide** | *pncA* | Loss of pro-drug activation (pyrazinamidase) | ~90% of PZA-R | ### Why This Patient Is RIF-R but INH-S **Clinical Pearl:** Resistance to different drugs in TB arises independently. This patient has acquired a mutation in *rpoB* (causing RIF resistance) but lacks mutations in *katG* or *inhA* (which would cause INH resistance). This is a common pattern in early drug-resistant TB and highlights the importance of using combination therapy — monotherapy selects for resistance to that single drug. **High-Yield:** The presence of RIF resistance without INH resistance is typical of early MDR-TB (multidrug-resistant TB is defined as resistance to at least INH and RIF). If this patient had received RIF monotherapy, INH-resistant mutants would eventually emerge. ### Why Other Options Are Incorrect - **Efflux pumps (Option 1):** While mycobacteria possess efflux pumps, these are not the primary mechanism of rifampicin resistance. Efflux pumps contribute to fluoroquinolone and aminoglycoside resistance but not RIF resistance. - **katG deficiency (Option 2):** This causes INH resistance, not RIF resistance. The patient is INH-susceptible, so *katG* is functional. - **Increased mycolic acids (Option 3):** While mycolic acids are important for cell wall impermeability, they do not specifically confer rifampicin resistance. Rifampicin penetrates the mycobacterial cell wall and acts intracellularly on RNA polymerase.