A 32-year-old male construction worker from rural Maharashtra presents with a 3-month history of cough, fever, and weight loss. Sputum smear microscopy is positive for acid-fast bacilli. Which is the most common Mycobacterium species responsible for tuberculosis in this patient?

Explanation

## Mycobacterial Species — TB Aetiology **Key Point:** *Mycobacterium tuberculosis* is responsible for >95% of all tuberculosis cases worldwide and in India, including pulmonary TB in immunocompetent individuals. ### Epidemiological Distribution of TB-Causing Mycobacteria | Species | Frequency | Primary Host | Clinical Setting | |---------|-----------|--------------|------------------| | **M. tuberculosis** | **>95%** | **Humans** | **Pulmonary and extrapulmonary TB in all populations** | | M. bovis | 1–3% | Cattle, unpasteurized dairy | TB in rural/pastoral populations; zoonotic | | M. avium complex (MAC) | <1% | Environmental, poultry | Immunocompromised (CD4 <50 in AIDS) | | M. marinum | <0.1% | Aquatic environments | Cutaneous TB (fish tank granuloma) | | M. africanum | <1% | Humans (West Africa) | Endemic to West African regions | **High-Yield:** In India, *M. tuberculosis* is the overwhelming cause of TB. *M. bovis* is occasionally seen in rural populations with exposure to infected cattle or unpasteurized milk, but remains <3% of cases. ### Clinical Pearl **Key Point:** The patient's presentation (sputum smear-positive pulmonary TB in a construction worker) is classic for *M. tuberculosis* infection. The organism is transmitted person-to-person via respiratory droplets and is the cause of TB in immunocompetent individuals. ### Why Other Species Are Rare - **M. bovis**: Zoonotic transmission from cattle; more common in rural dairy-farming populations but still accounts for <3% of TB in India. - **M. avium complex**: Primarily an opportunistic pathogen in severely immunocompromised hosts (CD4 <50 cells/μL in AIDS); not a cause of TB in immunocompetent individuals. - **M. marinum**: Causes cutaneous TB (fish tank granuloma) from exposure to contaminated aquatic environments; extremely rare and not a cause of pulmonary TB. **Mnemonic:** **MAC = Mostly AIDS Complication** — M. avium complex is seen in advanced AIDS, not in immunocompetent TB patients. [cite:Harrison 21e Ch 158]