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