A 35-year-old woman with a 3-week history of productive cough and night sweats is suspected of having pulmonary tuberculosis. A sputum sample is collected for microbiological investigation. Which of the following is the most appropriate investigation of choice for primary isolation and diagnosis of *Mycobacterium tuberculosis*?

Question

Accepted Answer

A. Löwenstein-Jensen (LJ) solid medium incubated aerobically at 37°C for 2–4 weeks. ## Investigation of Choice for *Mycobacterium tuberculosis* Isolation

### Why Löwenstein-Jensen Medium is the Gold Standard

**Key Point:** Löwenstein-Jensen (LJ) medium is the traditional, most widely used solid medium for primary isolation of *Mycobacterium tuberculosis* in resource-limited settings and remains the reference standard globally.

**High-Yield:** Critical features of LJ medium:
1. **Egg-based medium** — provides lipids and nutrients essential for slow-growing mycobacteria
2. **Glycerol enrichment** — promotes growth of *M. tuberculosis*
3. **Malachite green** — inhibits contaminant bacteria while allowing mycobacterial growth
4. **Aerobic incubation at 37°C** — *M. tuberculosis* is strictly aerobic
5. **Prolonged incubation (2–4 weeks or longer)** — accounts for slow generation time (~15–20 hours)

### Comparison of Mycobacterial Culture Media

| Medium | Type | Incubation | Time to Growth | Advantages | Disadvantages |
|--------|------|-----------|-----------------|-----------|----------------|
| **Löwenstein-Jensen** | Solid, egg-based | Aerobic, 37°C | 2–8 weeks | Gold standard; cost-effective; inhibits contaminants | Slow; requires skilled reading; media preparation complex |
| Middlebrook 7H10 | Solid, agar-based | 5–10% CO₂, 37°C | 2–6 weeks | Faster than LJ; transparent (easier to read) | More expensive; requires CO₂ |
| Ogawa | Solid, egg-based | Aerobic, 37°C | 2–8 weeks | Similar to LJ; slightly faster | Rarely used now; similar limitations to LJ |
| Blood agar | Solid, general-purpose | Aerobic, 37°C | 24–48 hours | Fast | *M. tuberculosis* will NOT grow; designed for non-fastidious bacteria |

**Clinical Pearl:** In India and most developing countries, LJ medium remains the primary choice for TB diagnosis due to cost-effectiveness, reliability, and availability. Middlebrook media are reserved for drug-susceptibility testing and research settings.

**Mnemonic:** **LJ = Long-term, Lipid-rich, Justified** — the standard for TB culture.

Answer

B. Middlebrook 7H10 agar incubated in 5–10% CO₂ at 37°C for 2–4 weeks

Answer

C. Ogawa medium incubated aerobically at 37°C for 2–4 weeks

Answer

D. Blood agar plate incubated aerobically at 37°C for 48 hours

Explanation

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