Pathogens & Diseases

This section covers the major players in Mycobacteriology and Nocardia, focusing on the diseases they cause, how they spread, and who’s most at risk

General Principles

Opportunistic Infections: Many Mycobacteria and Nocardia are opportunistic, meaning they primarily cause disease in individuals with weakened immune systems or underlying lung conditions
Slow Growth: Mycobacteria are slow-growing organisms, which can make diagnosis challenging. Nocardia are also slow growers, but often faster than mycobacteria
Acid-Fast Stain: Mycobacteria and Nocardia are acid-fast, meaning they resist decolorization with acid-alcohol after staining with dyes like carbolfuchsin. This is a key characteristic used for initial identification
Environmental Reservoirs: Many of these organisms are found in the environment (soil, water), making complete eradication difficult

Major Pathogens and Disease States

Mycobacterium tuberculosis

Etiology: The causative agent of tuberculosis (TB). M. tuberculosis is a slow-growing, aerobic, acid-fast bacillus
Disease States
- Pulmonary TB: The most common form, affecting the lungs. Symptoms include chronic cough (often with blood-tinged sputum), fever, night sweats, weight loss, and fatigue
- Extrapulmonary TB: TB can spread to other parts of the body, including the lymph nodes (scrofula), pleura (pleurisy), bones and joints (skeletal TB), meninges (meningitis), kidneys (renal TB), and other organs
- Latent TB Infection (LTBI): The bacteria are present in the body but are inactive and cause no symptoms. Individuals with LTBI are not infectious but can develop active TB later in life
Epidemiology
- TB is a global health problem, particularly in developing countries
- Populations at higher risk include:
- People with HIV/AIDS
- People who inject drugs
- People with diabetes, silicosis, or other medical conditions that weaken the immune system
- People in close contact with individuals with active TB
- People who live or work in congregate settings (e.g., prisons, homeless shelters)
- People who have immigrated from areas with high TB rates
Transmission: Primarily through airborne droplets produced when people with active pulmonary TB cough, sneeze, speak, or sing. Prolonged close contact is usually required for transmission

Mycobacterium avium complex (MAC)

Etiology: A group of closely related mycobacteria, primarily Mycobacterium avium and Mycobacterium intracellulare
Disease States
- Pulmonary MAC: Can cause lung infections, especially in individuals with underlying lung disease (e.g., COPD, bronchiectasis) or immunocompromised individuals. Symptoms can be similar to TB but often milder
- Disseminated MAC: Common in individuals with advanced HIV/AIDS. Can affect multiple organs, causing fever, night sweats, weight loss, diarrhea, and abdominal pain
- Lymphadenitis: Primarily affects children, causing swollen lymph nodes in the neck
Epidemiology
- MAC is found in the environment (soil, water)
- Risk factors include:
- Immunocompromise (e.g., HIV/AIDS, organ transplant recipients)
- Underlying lung disease
- Advanced age
Transmission: Primarily through inhalation or ingestion of contaminated water or soil. Person-to-person transmission is rare

Mycobacterium kansasii

Etiology: A slow-growing, acid-fast bacillus
Disease States
- Pulmonary disease: Similar to TB, but often milder. Can cause cough, fever, night sweats, and weight loss
- Extrapulmonary infections: Less common, but can affect the skin, joints, and other organs
Epidemiology
- Found in tap water, particularly in the central and southwestern United States
- Risk factors include:
- Immunocompromise
- Underlying lung disease
Transmission: Likely through inhalation of contaminated water droplets. Person-to-person transmission is rare

Mycobacterium marinum

Etiology: A slow-growing, acid-fast bacillus
Disease States
- Cutaneous infections: Causes “fish tank granuloma” or “swimming pool granuloma.” Typically presents as a localized skin lesion on the hands or arms
Epidemiology
- Found in fresh and saltwater environments, particularly in aquariums and swimming pools
- Risk factors include:
- Exposure to contaminated water
- Minor skin trauma
Transmission: Through direct inoculation of the bacteria into the skin, often through cuts or abrasions

Mycobacterium abscessus

Etiology: A rapidly growing mycobacterium
Disease States
- Skin and soft tissue infections: Can cause localized abscesses, cellulitis, and wound infections, often after cosmetic procedures (e.g., tattoos, piercings) or surgery
- Pulmonary infections: Can cause chronic lung infections, particularly in individuals with cystic fibrosis or bronchiectasis
- Disseminated infections: Can occur in immunocompromised individuals
Epidemiology
- Found in water and soil
- Increasingly recognized as a cause of healthcare-associated infections
- Highly resistant to many antibiotics, making treatment challenging
Transmission: Through direct contact with contaminated water or soil, or through contaminated medical devices. Person-to-person transmission is rare

Nocardia spp.

Etiology: A group of aerobic, gram-positive, branching filamentous bacteria that are weakly acid-fast
Disease States
- Pulmonary nocardiosis: Can cause pneumonia, lung abscesses, and empyema. Symptoms include cough, fever, chest pain, and shortness of breath
- Cutaneous nocardiosis: Can cause mycetoma (a chronic, localized infection of the skin and subcutaneous tissue), cellulitis, and skin abscesses
- Disseminated nocardiosis: Can spread to other organs, including the brain, causing brain abscesses
Epidemiology
- Found in soil
- Risk factors include:
- Immunocompromise
- Chronic lung disease
- Organ transplant recipients
Transmission: Primarily through inhalation of dust or soil containing Nocardia, or through direct inoculation into the skin

Key Terms

Etiology: The cause or origin of a disease or abnormal condition
Epidemiology: The study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems
Transmission: The way a disease is spread from one person or source to another
Acid-Fast: A physical property of certain bacteria, particularly mycobacteria, that makes them resistant to decolorization by acids during staining procedures
Granuloma: A mass of immune cells that forms when the body attempts to wall off substances it perceives as foreign but cannot eliminate
Rapidly Growing Mycobacteria (RGM): A group of mycobacteria that grow more quickly than Mycobacterium tuberculosis, often within 7 days
Slowly Growing Mycobacteria (SGM): Mycobacteria that take longer than 7 days to grow in culture
Disseminated Infection: An infection that has spread from a localized area to other parts of the body
Pulmonary Infection: An infection of the lungs, such as pneumonia or bronchitis
Extrapulmonary Infection: An infection that occurs outside the lungs
Latent Infection: An infection in which the pathogen is present in the body but not causing active disease
Opportunistic Infection: An infection that occurs more frequently or is more severe in individuals with weakened immune systems
Mycetoma: A chronic, localized infection of the skin and subcutaneous tissue, often caused by bacteria or fungi
Lymphadenitis: Inflammation of the lymph nodes, often caused by infection

Specimen Sources

Stain, Morph, Growth