Question

A 45-year-old man on antitubercular therapy is found to have a rifampicin-resistant strain of Mycobacterium tuberculosis on GeneXpert testing. Which of the following best explains the mechanism of resistance?

• A. Inactivating enzymes
• B. Activating enzymes
• C. Altered target enzymes
• D. Efflux of drug

Hint:

It is highly recommended to memorize these specific Tuberculosis Mechanism of Resistance Memory Aids for board exams:
For Rifampicin: It involves a mutation in the RNA polymerase (specifically the rpoB gene), which directly equals an Altered Target.
For Isoniazid (INH): It involves a decreased production of catalase-peroxidase (specifically via a katG gene mutation), which directly equals Decreased drug activation, as INH is a prodrug requiring this enzyme to become active.

Answer 1

The Correct Option is C

Concept:
The question specifically asks the examinee to identify the primary, underlying molecular mechanism by which strains of Mycobacterium tuberculosis (MTB) successfully develop clinical resistance to the crucial, first-line antitubercular drug, Rifampicin.
Understanding these core molecular mechanisms is fundamental to the rapidly growing field of molecular diagnostics in modern infectious disease management.
Step-by-step Explanation:

• Rifampicin remains an absolutely essential cornerstone of highly effective, modern tuberculosis treatment regimens worldwide.

• It exerts its potent bactericidal effect by binding very tightly and specifically to the beta subunit of the bacterial DNA-dependent RNA polymerase complex.

• This firm binding effectively and completely inhibits early bacterial RNA synthesis (the process of transcription), rapidly leading to bacterial cell death.

• The critical beta subunit of this RNA polymerase enzyme is genetically encoded by the specific rpoB gene located within the MTB bacterial genome.

• It is documented that over 95% of all globally isolated rifampicin-resistant MTB strains possess spontaneous, single-nucleotide point mutations or very short deletions localized entirely within a highly conserved, 81-base pair region of the rpoB gene.

• These specific genetic mutations directly alter the complex, three-dimensional structural conformation of the resulting RNA polymerase beta subunit.

• Due to this subtle but critical structural alteration, the Rifampicin drug molecule can no longer physically bind effectively to its intended target enzyme, rendering the drug entirely useless and allowing the bacteria to survive and replicate.

• Therefore, the fundamental molecular mechanism of resistance is definitively classified as an "Altered target enzyme" (Option C).

• This mechanism strictly contrasts with some other bacteria that develop broad resistance via the production of drug-inactivating enzymes (Option A, such as beta-lactamases destroying penicillins), the pathological lack of necessary activating enzymes (Option B, such as the classic KatG mutation seen in Isoniazid resistance), or the upregulation of enhanced drug efflux pumps (Option D).

• The GeneXpert MTB/RIF assay, which is explicitly mentioned and utilized in the clinical vignette, works precisely by using rapid, automated PCR technology to directly detect these exact, specific mutations within the rpoB gene, thereby simultaneously confirming the TB diagnosis and identifying rifampicin resistance within two hours.
Final Answer:
Rifampicin resistance in tuberculosis is almost exclusively caused by specific mutations in the rpoB gene, which directly results in an altered target enzyme that cannot bind the drug.