Question
Classify non – tuberculous mycobacteria giving examples
Answer
Non-tuberculous mycobacteria (NTM), also called atypical mycobacteria, are mycobacterial species other than the M. tuberculosis complex and M. leprae — mostly environmental organisms (found in soil, water) causing opportunistic disease. The Runyon classification groups them by growth rate and pigment production:
Slow growers (take >7 days to form visible colonies):
- Group I — Photochromogens (produce pigment only when exposed to light): e.g., M. kansasii (causes pulmonary disease resembling TB), M. marinum (causes “fish tank/swimming pool granuloma,” a skin infection).
- Group II — Scotochromogens (produce pigment in both light and dark): e.g., M. scrofulaceum (causes cervical lymphadenitis in children).
- Group III — Non-photochromogens (non-pigmented regardless of light exposure): e.g., M. avium-intracellulare complex (MAC/MAI) (causes disseminated disease in AIDS patients, pulmonary disease in those with structural lung disease), M. ulcerans (causes Buruli ulcer).
Rapid growers (form visible colonies within 7 days):
- Group IV: e.g., M. fortuitum, M. chelonae, M. abscessus — cause post-surgical/post-injection wound infections, skin and soft-tissue infections, and catheter-related infections.
Clinical significance: NTM infections are increasingly recognized, particularly in immunocompromised patients (especially AIDS, for MAC) and in patients with pre-existing structural lung disease; they generally require different antibiotic regimens than M. tuberculosis (often resistant to standard first-line antitubercular drugs), and person-to-person transmission is not typical (acquired from environmental sources).