Mycobacterium tuberculosis vs Mycobacterium leprae and avium complex
How to tell Mycobacterium tuberculosis and Mycobacterium leprae and avium complex apart when a board question makes them look alike.
Tuberculosis is airborne granulomatous lung disease; leprae is skin-nerve disease; MAC is environmental disseminated disease in very low CD4 or chronic lung disease.
Side by side
M. tuberculosis causes pulmonary or disseminated disease with chronic cough, fever, night sweats, weight loss, cavitary upper-lobe lesions, granulomas, and airborne transmission.
M. leprae causes chronic skin and peripheral nerve disease; M. avium complex causes disseminated opportunistic disease in advanced HIV or chronic lung disease.
M. tuberculosis survives inside macrophages through mycolic-acid-rich cell walls and delayed-type hypersensitivity granuloma formation.
M. leprae prefers cooler tissues and Schwann cells; M. avium complex is an environmental nontuberculous mycobacterium causing disease when cellular immunity is poor.
Acid-fast bacilli, caseating granulomas, positive IGRA or tuberculin test, cavitary apical disease, and culture or nucleic acid testing support tuberculosis.
Acid-fast organisms in skin or nerve biopsy support leprosy; disseminated MAC has blood culture positivity, anemia, fever, weight loss, diarrhea, and CD4 often below 50.
The trap
All are acid-fast mycobacteria, so organ target, transmission, and immune context separate them.
The repair move
For mycobacteria, first choose the target organ pattern: lung granulomas, skin-nerve lesions, or disseminated low-CD4 disease.
Try a free question
CoreA patient has chronic cough, fever, night sweats, weight loss, and an apical cavitary lung lesion with caseating granulomas. Which organism is most likely?
Show answer and explanation
Answer: A. Mycobacterium tuberculosis. Chronic pulmonary disease with apical cavitation and caseating granulomas supports M. tuberculosis.
Why the other choices are tempting
- B. M. leprae causes skin and peripheral nerve disease.
- C. MAC causes disseminated opportunistic disease in advanced HIV or chronic lung disease.
- D. Nocardia is weakly acid-fast and branching, often causing lung and brain abscesses.
- E. Histoplasma is a dimorphic fungus with intracellular yeast in macrophages.
Flashcards
Tap a card to flip it.
What lung pattern supports M. tuberculosis?
Chronic cough with night sweats, weight loss, upper-lobe cavitation, and granulomas supports tuberculosis.
What tissue pattern supports M. leprae?
Hypopigmented or nodular skin lesions with sensory loss and peripheral nerve involvement support leprosy.
Quick answers
What is the fastest way to tell Mycobacterium tuberculosis from Mycobacterium leprae and avium complex?
Tuberculosis is airborne granulomatous lung disease; leprae is skin-nerve disease; MAC is environmental disseminated disease in very low CD4 or chronic lung disease.
What is the key clue for Mycobacterium tuberculosis?
M. tuberculosis causes pulmonary or disseminated disease with chronic cough, fever, night sweats, weight loss, cavitary upper-lobe lesions, granulomas, and airborne transmission.
What is the key clue for Mycobacterium leprae and avium complex?
M. leprae causes chronic skin and peripheral nerve disease; M. avium complex causes disseminated opportunistic disease in advanced HIV or chronic lung disease.
What is the common trap with Mycobacterium tuberculosis vs Mycobacterium leprae and avium complex?
All are acid-fast mycobacteria, so organ target, transmission, and immune context separate them.