Resistance mechanism of tb bacillus for att full details for md exams 15marks
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Mycobacterium tuberculosis resistance mechanisms anti-tuberculosis drugs ATT
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Resistance Mechanisms of Mycobacterium tuberculosis to Anti-Tuberculosis Drugs (ATT)
15-Mark Answer | MD Examination Standard
INTRODUCTION
Mycobacterium tuberculosis (MTB) is an obligate aerobic, non-spore-forming, acid-fast bacillus (AFB) with a uniquely waxy, lipid-rich cell wall. Resistance to ATT is one of the most serious challenges in global tuberculosis (TB) control. Unlike many other bacteria, MTB does not acquire resistance via plasmid transfer or transposons — resistance arises exclusively through spontaneous chromosomal mutations, which are then selected under inadequate drug pressure.
CLASSIFICATION OF DRUG RESISTANCE IN TB
| Type | Definition |
|---|---|
| Primary (Initial) Resistance | Resistance in a patient never previously treated — infected by a resistant strain |
| Acquired (Secondary) Resistance | Resistance emerging during treatment due to inadequate therapy |
| MDR-TB | Resistant to at least Isoniazid + Rifampicin (the two most potent first-line drugs) |
| Pre-XDR-TB | MDR/RR-TB + resistance to any fluoroquinolone |
| XDR-TB | MDR/RR-TB + resistance to fluoroquinolone + at least one of bedaquiline/linezolid |
| TDR-TB | Totally drug-resistant — resistant to all tested ATT agents |
I. INTRINSIC (NATURAL) RESISTANCE MECHANISMS
These are innate properties of MTB that confer baseline resistance independent of mutations.
1. Lipid-Rich, Hydrophobic Cell Wall
- The mycobacterial cell wall contains mycolic acids, arabinogalactan, and peptidoglycan, forming an extraordinarily thick, waxy, hydrophobic barrier.
- This wall drastically reduces permeability to hydrophilic drugs and limits intracellular drug accumulation.
- Porins in mycobacteria (e.g., MspA-like channels) are fewer and less efficient than in gram-negative bacteria, restricting drug entry.
2. Drug Efflux Pumps
MTB encodes a large number of efflux pump systems that actively expel drugs:
| Efflux Pump Family | Examples | Drugs Expelled |
|---|---|---|
| MFS (Major Facilitator Superfamily) | Rv1258c (Tap), EfpA | Isoniazid, tetracyclines |
| ABC Transporters | DrrA, DrrB, DrrC | Multiple drugs |
| RND Family | MmpL5 | Bedaquiline, clofazimine |
| SMR Family | Small multidrug resistance | Various |
- Efflux pump activity is a key mechanism in low-level resistance and can serve as a stepping stone to high-level mutational resistance.
3. Drug-Inactivating Enzymes (Intrinsic)
- β-lactamases (BlaC): MTB produces a constitutive class A β-lactamase (BlaC) and a β-lactamase inhibitor-resistant enzyme, explaining natural resistance to most β-lactam antibiotics.
- Amidases: Enzymatic modification of certain antibiotic classes.
II. ACQUIRED RESISTANCE MECHANISMS (Drug-Specific)
All acquired resistance in MTB is due to spontaneous point mutations or deletions in specific chromosomal genes. Since MTB replicates in large numbers (particularly in cavitary disease), mutants arise naturally and are selected when drug pressure is inadequate.
The Principle of Mutation Frequencies
| Drug | Spontaneous Mutation Frequency |
|---|---|
| Isoniazid | ~1 in 10⁶ bacilli |
| Rifampicin | ~1 in 10⁸ bacilli |
| Ethambutol | ~1 in 10⁵ bacilli |
| Streptomycin | ~1 in 10⁶ bacilli |
In a pulmonary cavity with ~10⁸–10⁹ bacilli, pre-existing resistant mutants are virtually certain — this is why monotherapy is absolutely contraindicated.
1. ISONIAZID (INH) Resistance
Target: Pro-drug requiring activation. INH is activated by the catalase-peroxidase enzyme (KatG) to form an isonicotinoyl radical, which then inhibits InhA (enoyl-ACP reductase) — a key enzyme in mycolic acid biosynthesis.
Resistance Mechanisms:
| Gene Mutated | Protein Affected | Mechanism | Frequency |
|---|---|---|---|
| katG (codon 315 most common — Ser315Thr) | KatG catalase-peroxidase | Loss of INH activation; pro-drug not converted to active form | ~50–80% of INH-resistant strains |
| inhA (promoter region –15C→T most common) | InhA (enoyl-ACP reductase) | Drug target overexpression; INH cannot adequately inhibit the overproduced target | ~15–20% |
| inhA (structural mutations) | InhA | Altered drug-binding site | ~5–10% |
| ahpC | Alkyl hydroperoxide reductase | Compensatory upregulation after KatG loss | ~10–15% |
| kasA | β-ketoacyl ACP synthase | Alternate target in mycolic acid pathway | ~5% |
| ndh | NADH dehydrogenase | Altered NADH/NAD+ ratio, affects InhA inhibition | Rare |
Clinical Note: katG mutations confer high-level resistance (MIC >5 μg/mL); inhA promoter mutations confer low-level resistance and are also associated with ethionamide cross-resistance.
2. RIFAMPICIN (RIF) Resistance
Target: Rifampicin binds the β-subunit of DNA-dependent RNA polymerase (encoded by rpoB), blocking RNA synthesis.
Resistance Mechanism:
| Gene | Mutation Hotspot | Effect |
|---|---|---|
| rpoB | 81-bp RRDR (Rifampicin Resistance Determining Region), codons 516, 526, 531 | Altered RNA polymerase β-subunit; rifampicin cannot bind effectively |
- Codon 531 (Ser531Leu) — most common (~40–50%)
- Codon 526 mutations — second most common
- Codon 516 mutations — less common, lower-level resistance
Key Point: RIF resistance is a surrogate marker for MDR-TB because >90% of RIF-resistant strains are also INH-resistant. WHO now uses "RR-TB" (Rifampicin Resistant TB) synonymously with MDR-TB for treatment purposes.
Rapid molecular tests like Xpert MTB/RIF specifically detect rpoB RRDR mutations.
3. PYRAZINAMIDE (PZA) Resistance
Target: PZA is a pro-drug converted by pyrazinamidase (PZase), encoded by pncA, to pyrazinoic acid (POA). POA disrupts membrane energy (proton motive force) and inhibits fatty acid synthase I (FAS-I) and trans-translation.
Resistance Mechanism:
| Gene | Mutation | Effect |
|---|---|---|
| pncA (>90% of cases) | Multiple diverse point mutations, insertions, deletions throughout gene | Loss of pyrazinamidase activity; PZA not converted to active POA |
| rpsA | Ribosomal protein S1 mutations | Altered POA binding target |
| panD | Aspartate decarboxylase | Alternate resistance mechanism |
- Mutations in pncA are highly diverse (>400 different mutations reported), making molecular prediction of PZA resistance challenging.
- Phenotypic DST for PZA is technically difficult (requires acidic pH 5.5, BACTEC MGIT).
4. ETHAMBUTOL (EMB) Resistance
Target: Inhibits arabinosyl transferases (EmbA, EmbB, EmbC), encoded by the emb operon, which are essential for arabinogalactan synthesis (cell wall component).
Resistance Mechanism:
| Gene | Mutation Hotspot | Effect |
|---|---|---|
| embB (most common) | Codon 306 (Met306Val/Ile/Leu) | Altered drug target; EMB binding reduced |
| embA, embC | Various mutations | Reduced EMB efficacy |
| embB promoter | Overexpression of embCAB operon | Target overproduction |
5. STREPTOMYCIN (SM) Resistance
Target: Inhibits 30S ribosomal subunit, causing misreading of mRNA and inhibition of translation initiation.
Resistance Mechanisms:
| Gene | Protein | Mechanism |
|---|---|---|
| rpsL (codon 43/88) | Ribosomal protein S12 | Mutation reduces streptomycin affinity for 30S subunit |
| rrs (16S rRNA) | 16S rRNA | Nucleotide mutations at positions 530 loop and 912 region alter ribosomal binding site |
| gidB | 7-methylguanosine methyltransferase | Low-level resistance; methylation of 16S rRNA |
6. FLUOROQUINOLONES (FQs) Resistance
Target: Inhibit DNA gyrase (topoisomerase II) and topoisomerase IV, preventing DNA replication.
Resistance Mechanisms:
| Gene | Protein | Hotspot | Effect |
|---|---|---|---|
| gyrA | DNA gyrase A subunit | QRDR codons 88–94 (Asp94Gly most common) | Altered gyrase; FQ cannot bind/inhibit |
| gyrB | DNA gyrase B subunit | Codons 447–500 | Reduced FQ binding affinity |
| Efflux pumps | MmpL5, Rv1258c | — | Active efflux of FQs |
FQ resistance is critical because fluoroquinolones (levofloxacin, moxifloxacin) are backbone drugs in MDR-TB treatment.
7. SECOND-LINE INJECTABLE AGENTS: Amikacin / Kanamycin / Capreomycin
Target: 30S ribosomal subunit (aminoglycosides); also affects protein synthesis.
| Gene | Protein | Effect |
|---|---|---|
| rrs (A1401G most common) | 16S rRNA | Cross-resistance to amikacin + kanamycin + capreomycin |
| eis promoter (C→T at –10) | Enhanced intracellular survival protein (aminoglycoside acetyltransferase) | Low-level kanamycin resistance via drug acetylation |
| tlyA | rRNA methyltransferase | Capreomycin resistance |
8. BEDAQUILINE Resistance
Target: Inhibits mycobacterial ATP synthase (F₁F₀-ATPase), subunit c, depleting cellular energy.
| Gene | Protein | Mechanism |
|---|---|---|
| atpE | ATP synthase subunit c | Direct target mutation |
| mmpL5 / mmpS5 | MmpL5 efflux pump | Increased drug efflux |
| Rv0678 | Transcriptional repressor of MmpL5 | Loss-of-function → efflux pump overexpression; also causes cross-resistance to clofazimine |
9. LINEZOLID Resistance
Target: Inhibits 50S ribosomal subunit (23S rRNA peptidyl transferase center), blocking protein synthesis.
| Gene | Mechanism |
|---|---|
| rrl (23S rRNA) | Point mutations at positions 2061, 2447, 2576 |
| rplC (ribosomal protein L3) | Altered 50S binding site |
| Efflux (Rv1979c) | Active drug efflux |
III. MECHANISMS PROMOTING RESISTANCE DEVELOPMENT
1. Heteroresistance
Mixed populations of susceptible and resistant organisms exist simultaneously — under drug pressure, resistant population dominates.
2. Intracellular Location
MTB resides within macrophage phagosomes — intracellular environment may limit drug penetration and create micro-environments (acidic pH, anaerobic) that impair drug activity.
3. Biofilm and Persister Cells
- MTB can form biofilms and persister cells (metabolically dormant, drug-tolerant) — these are not resistant by genetic mutation but tolerate drugs by reducing their metabolic activity, avoiding drug targets.
- Persisters require prolonged treatment (6–24 months) and are responsible for relapse if therapy is shortened.
4. Inadequate Drug Levels
- Malabsorption, drug interactions, poor adherence, and subtherapeutic doses create selective pressure that enriches resistant mutants.
IV. SUMMARY TABLE: Key Mutations at a Glance
| Drug | Key Gene | Key Mutation | Mechanism |
|---|---|---|---|
| Isoniazid | katG | Ser315Thr | ↓ Pro-drug activation |
| Isoniazid | inhA promoter | –15C→T | ↑ Target overexpression |
| Rifampicin | rpoB | Ser531Leu | ↓ Drug binding to RNA pol |
| Pyrazinamide | pncA | Multiple | ↓ Pro-drug activation |
| Ethambutol | embB | Met306Val | ↓ Drug-target affinity |
| Streptomycin | rpsL, rrs | Lys43Arg; A512C | ↓ 30S binding |
| Fluoroquinolones | gyrA | Asp94Gly | ↓ DNA gyrase inhibition |
| Amikacin/Kan | rrs | A1401G | ↓ 30S binding |
| Bedaquiline | atpE, Rv0678 | Various | ↓ Target binding; ↑ efflux |
| Linezolid | rrl, rplC | Various | ↓ 50S binding |
V. CLINICAL IMPLICATIONS & PREVENTION OF RESISTANCE
- Combination chemotherapy is mandatory — simultaneous resistance mutations to 4 drugs is mathematically improbable (~10⁻³⁰).
- DOTS (Directly Observed Treatment, Short-course) ensures adherence.
- Rapid molecular diagnostics (Xpert MTB/RIF, line probe assays — LPA) detect resistance early.
- Drug susceptibility testing (DST) — phenotypic and genotypic — guides regimen selection.
- Adequate drug dosing and management of malabsorption are critical.
- Treatment of latent TB reduces the reservoir for resistance emergence.
IMPORTANT EXAM POINTS (Quick Recall)
- MTB resistance = chromosomal mutation only (no plasmids, no transposons)
- Largest reservoir of resistance mutations = large cavitary lesions (10⁸–10⁹ bacilli)
- katG Ser315Thr = most common INH resistance mutation (~60–70%)
- rpoB Ser531Leu = most common RIF resistance mutation (~40%)
- RIF resistance = practical surrogate for MDR-TB
- rrs A1401G = cross-resistance to amikacin + kanamycin + capreomycin
- Rv0678 mutations = cross-resistance between bedaquiline + clofazimine
- inhA promoter mutations = cross-resistance between INH + ethionamide
- Persister cells ≠ resistant mutants — drug tolerance without genetic mutation
- XDR-TB (new 2021 WHO definition): MDR/RR-TB + FQ + bedaquiline/linezolid resistance
Sources: Treatment of Drug-Susceptible Tuberculosis (WHO Guidelines); Treatment of Drug-Resistant Tuberculosis (WHO 2022); Harrison's Principles of Internal Medicine; Ananthanarayan & Paniker's Microbiology; Rang & Dale's Pharmacology.
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