## Most Common Mycobacterial Species in Sputum **Key Point:** *Mycobacterium tuberculosis* is responsible for >95% of all tuberculosis cases worldwide and in India, making it by far the most common mycobacterial pathogen isolated from sputum. ### Mycobacterial Species and Clinical Significance | Species | Frequency in TB | Source | Clinical Features | |---------|-----------------|--------|-------------------| | **M. tuberculosis** | >95% | Human-to-human (airborne) | Pulmonary TB, extrapulmonary TB | | M. bovis | <1–2% | Unpasteurized dairy, cattle | Primary abdominal/lymph node TB | | M. avium complex (MAC) | <1% | Environmental, immunocompromised | Disseminated disease in AIDS | | M. marinum | <0.1% | Aquatic environments | Skin/soft tissue (swimming pool granuloma) | **High-Yield:** *M. tuberculosis* is a strict human pathogen with a predilection for the respiratory tract. It is transmitted via airborne droplets and establishes infection in the lungs, making sputum the primary diagnostic specimen. ### Why M. tuberculosis Dominates 1. **Obligate human pathogen:** Does not naturally infect animals (unlike M. bovis) 2. **Respiratory tropism:** Optimally adapted to survive in alveolar macrophages 3. **Transmission efficiency:** Airborne route is highly effective for person-to-person spread 4. **Burden of disease:** India has the highest TB burden globally; ~90% of TB in India is caused by M. tuberculosis **Mnemonic:** **MABM** = *M. avium, M. bovis, M. marinum* are **rare** causes of TB; *M. tuberculosis* is the **rule**. **Clinical Pearl:** Non-tuberculous mycobacteria (NTM) like MAC and M. marinum are increasingly recognized in immunocompromised patients (HIV/AIDS) and in specific occupational exposures, but they remain uncommon in immunocompetent individuals with pulmonary TB.
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