NOCARDIA

• Characteristics
• Aerobic, gram-positive rods
• Medium chain mycolic acid in cell wall contributes to weak acid-fastness
• Appears as branched filamentous forms similar to fungal hyphae in tissues and cultures
• Catalase positive, utilizes carbohydrates oxidatively
• Slow growing, requiring 3-5 days of incubation for culture

Epidemiology

• Ubiquitous in soil; not part of normal flora
• Causes exogenous infections, primarily in immunocompromised patients

Pathogenesis

• Causes bronchopulmonary diseases in immunocompromised individuals
• Initial colonization of upper respiratory tract via inhalation
• Primary cutaneous infections arise from trauma introducing bacteria into tissue

Clinical Disease

• Bronchopulmonary disease (e.g., bronchitis, pneumonia):
• Symptoms are similar to other pyogenic organisms
• Develops gradually, typically in the immunocompromised
• Diagnosis in patients without underlying conditions warrants comprehensive immunologic workup
• Cutaneous infections:
• Can be primary or due to secondary spread from pulmonary infections
• Mycetoma: Chronic, painless infection of the feet with subcutaneous swelling and sinus tracts
• Lymphocutaneous infections: Characterized by nodules and ulcerations along lymphatic pathways

Laboratory Diagnosis

• Microscopy is sensitive: identifies branching, partially acid-fast organisms
• Culture on selective media (e.g., BCYE) takes about a week for growth
• Genus identification via microscopy; species identification requires genomic analysis

Treatment, Prevention, and Control

• Localized infections treated with trimethoprim-sulfamethoxazole
• Severe cases may need Amikacin combined with carbapenem
• Ubiquitous in nature; prevention involves proper wound care

MYCOBACTERIUM

Characteristics

• Non-spore forming, acid-fast, slow-growing aerobic bacilli
• Complex cell wall rich in lipids and waxes (mycolic acids) contributing to environmental resistance

Important Human Pathogens

• M. tuberculosis
• M. leprae
• M. avium
• M. intracellulare

Runyon Classification System

• Classifies nontuberculous Mycobacterium based on growth rates and pigmentation:

1. Slow-growing photochromogens
2. Slow-growing scotochromogens
3. Slow-growing non-pigmented
4. Rapid-growing

M. TUBERCULOSIS

• Main cause of tuberculosis (TB), a leading global cause of death
• Transmission via aerosols during close personal contact

Pathogenesis

• Intracellular pathogen establishing lifelong infection
• Enters lungs via inhalation
• Phagocytized by macrophages, which prevents phagosome-lysosome fusion
• Induces inflammation and recruits T cells, leading to granuloma formation
• Tubercle formation can encapsulate bacteria; small tubercles may kill bacteria but large tubercles can harbor dormant bacteria
• Reactivation may occur when immunity wanes, resulting in infection spread

Epidemiology

• Only natural reservoir is humans
• Estimated 1/3 of the world is infected; 9 million new cases and 2 million deaths annually
• Populations at high risk: homeless, drug users, AIDS patients

Clinical Disease

• Primarily affects the lungs; nonspecific symptoms include malaise, weight loss, cough, night sweats
• Sputum may be scant or bloody; cavitation observed on X-rays
• Progression risk is higher in those with poor immunity or high exposure

Laboratory Diagnosis

• Identified by radiographic evidence and tuberculin skin tests
• Microscopy detects acid-fast bacteria; cultures take longer (3 weeks or more)

Treatment, Prevention, and Control

• Requires multiple-drug regimens to counteract drug resistance
• Recommended first-line drugs include isoniazid, rifampin, and pyrazinamide

M. LEPRAE

• Causal agent of leprosy; cannot be cultured in vitro
• Slow progression of disease, potentially taking 20 years to manifest symptoms

Epidemiology

• Historically significant; prevalence has decreased dramatically from over 5 million cases in 1985 to less than 300,000 in 2005
• Typically rare in the U.S. but certain states show localized endemic cases

Clinical Disease

• Two forms:
• Tuberculoid form: Milder symptoms with hypopigmented lesions
• Lepromatous form: Severe disfiguring skin lesions and widespread immune system involvement

Diagnosis

• Microscopy sensitive for lepromatous form; skin testing is necessary for tuberculoid leprosy
• Culture is not viable for diagnostics

Treatment, Prevention, and Control

• Dapsone is common treatment for tuberculoid form; clofazimine added for lepromatous form
• Requirement for prolonged therapy and management of leprosy cases important for disease control