A 28-year-old woman comes to the physician because of a two-month history of fatigue and low-grade fevers. Over the past 4 weeks, she has had increasing shortness of breath, a productive cough, and a 5.4-kg (11.9-lb) weight loss. Three months ago, the patient returned from a two-month trip to China. The patient appears thin. Her temperature is 37.9°C (100.2°F), pulse is 75/min, and blood pressure is 125/70 mm Hg. Examination shows lymphadenopathy of the anterior and posterior cervical chain. Rales are heard at the left lower lobe of the lung on auscultation. Laboratory studies show a leukocyte count of 11,300/mm3 and an erythrocyte sedimentation rate of 90 mm/h. An x-ray of the chest shows a patchy infiltrate in the left lower lobe and ipsilateral hilar enlargement. Microscopic examination of the sputum reveals acid-fast bacilli; polymerase chain reaction is positive. Sputum cultures are pending. After placing the patient in an airborne infection isolation room, which of the following is the most appropriate next step in management?

Administer isoniazid, rifampin, pyrazinamide, and ethambutol for 2 months, followed by isoniazid and rifampin for 4 months

Administer only isoniazid for 9 months

Await culture results before initiating treatment

Perform interferon-γ release assay

A 55-year-old woman comes to the physician because of fevers for 2 weeks. She works as a nurse and recently returned from a charity work trip to India, where she worked in a medically-underserved rural community. A tuberculin skin test 3 months ago prior to her trip showed an induration of 3 mm. Physical examination is unremarkable. An x-ray of the chest shows right-sided hilar lymphadenopathy. A sputum culture shows acid-fast bacilli. Which of the following immunologic processes most likely occurred first?

Replication of bacteria within alveolar macrophages

Production of interferon-gamma by T-helper cells

Migration of T-helper cells to the lungs

Formation of a nodular tubercle in the lung

Transportation of bacterial peptides to regional lymph nodes

A 45-year-old man presents with well-circumscribed, hypopigmented patches on his face and arms. The lesions are asymmetric and show decreased sensation to light touch. Microscopic examination of a skin biopsy stained with Fite stain shows acid-fast bacilli within macrophages in dermal nerve bundles. Which of the following is the most appropriate treatment?

Multidrug therapy with dapsone, rifampin, and clofazimine

A 68-year-old man comes to the physician because of a 1-month history of fatigue, low-grade fevers, and cough productive of blood-tinged sputum. He has type 2 diabetes mellitus and chronic kidney disease and underwent kidney transplantation 8 months ago. His temperature is 38.9°C (102.1°F) and pulse is 98/min. Examination shows rhonchi in the right lower lung field. An x-ray of the chest shows a right-sided lobar consolidation. A photomicrograph of specialized acid-fast stained tissue from a blood culture is shown. Which of the following is the strongest predisposing factor for this patient's condition?

Sharing of unsterile IV needles

Exposure to contaminated air-conditioning unit

Mycobacteria overview for USMLE Step 1: High-Yield Microbiology Lessons

Mycobacteria - Waxy & Wonderful

Acid-fast bacilli due to high mycolic acid content in their cell wall, creating a waxy, impermeable layer.

Resists decolorization by acid-alcohol after staining with carbolfuchsin (Ziehl-Neelsen stain).
Obligate aerobes, hence their predilection for the lungs.
Slow growers on specialized media (e.g., Löwenstein-Jensen medium). 📌 Mnemonic: "My Ass is Fast" (Mycolic Acid, Acid-Fast).

Mycobacterium tuberculosis: Acid-fast bacilli

⭐ As obligate aerobes, M. tuberculosis thrives in high O₂ tension, classically causing cavitary lesions in the lung apices.

M. tuberculosis - Captain of Consumption

Organism: Acid-fast bacillus (stains red with Ziehl-Neelsen), obligate aerobe.
Virulence: Cord factor (serpentine growth, inhibits WBCs), sulfatides (prevent phagolysosome fusion).
Culture: Slow growth on Löwenstein-Jensen agar.
Transmission: Inhaled respiratory droplets.

Ghon Complex in Primary Tuberculosis

Pathogenesis Flow:

⭐ Cord Factor (trehalose-6,6'-dimycolate) is a critical virulence factor that correlates with pathogenicity. It causes the bacilli to grow in serpentine cords and is toxic to mammalian cells.

M. leprae - The Great Divider

Acid-fast bacillus causing Hansen's disease (leprosy); an obligate intracellular parasite.
Thrives in cool body areas: skin, superficial nerves, eyes, testes.

Clinical spectrum of leprosy: tuberculoid vs lepromatous

Tuberculoid (Paucibacillary):
- Strong Th1 cell-mediated immunity contains bacteria.
- (+) Lepromin skin test; low bacillary load.
- Well-demarcated, hypopigmented, anesthetic skin plaques.
Lepromatous (Multibacillary):
- Deficient Th1, strong (ineffective) Th2 response.
- (-) Lepromin skin test; high bacillary load.
- Diffuse skin nodules (leonine facies), symmetric glove-and-stocking neuropathy.

⭐ M. leprae cannot be grown on artificial media. It is cultured in the footpads of armadillos, which have a lower body temperature.

📌 Tx: Dapsone + Rifampin (Tuberculoid); add Clofazimine for Lepromatous.

Atypical Mycobacteria - The Uninvited Guests

Widespread in the environment (soil, water); not transmissible person-to-person.
Classified by Runyon groups based on growth rate & pigment production.
- Group I (Photochromogens): Pigment w/ light. M. kansasii (TB-like disease), M. marinum (fish tank granuloma).
- Group II (Scotochromogens): Pigment in dark. M. scrofulaceum (cervical lymphadenitis in kids).
- Group III (Nonchromogens): No pigment. M. avium-complex (MAC).
- Group IV (Rapid Growers): M. fortuitum, M. abscessus (iatrogenic skin/soft tissue infections).

Mycobacterium marinum infection on finger, before and after

⭐ Mycobacterium avium complex (MAC) causes disseminated disease in AIDS patients with CD4+ counts < 50 cells/mm³. Prophylaxis with azithromycin is crucial.

High‑Yield Points - ⚡ Biggest Takeaways

Mycobacteria are acid-fast bacilli due to high mycolic acid content; they do not Gram stain.

M. tuberculosis causes TB, characterized by caseating granulomas with central necrosis.

M. leprae causes leprosy: tuberculoid form shows a strong CMI response, while lepromatous has a weak response.

M. avium complex (MAC) causes disseminated disease in AIDS patients with CD4 counts <50.

Diagnosis relies on acid-fast stain and culture on Löwenstein-Jensen medium.

Treatment requires prolonged, multi-drug therapy to combat resistance.

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