Introduction
- Nocardia is a group of aerobic, Gram-positive, branching filamentous bacteria that can cause infections in humans and animals.
- These organisms are found naturally in soil, water, decaying vegetation, and dust.
- Nocardia mainly causes disease in people with weakened immune systems, but healthy individuals may also become infected after inhaling contaminated dust or after traumatic skin injuries.
- The most common disease caused by Nocardia is pulmonary nocardiosis, although it can spread to the brain, skin, and other organs.
Classification
| Characteristic | Description |
|---|---|
| Kingdom | Bacteria |
| Phylum | Actinomycetota |
| Class | Actinomycetes |
| Order | Corynebacteriales |
| Family | Nocardiaceae |
| Genus | Nocardia |
Important Species of Nocardia
| Species | Disease |
|---|---|
| Nocardia asteroides complex | Pulmonary and disseminated infections |
| Nocardia brasiliensis | Cutaneous nocardiosis and mycetoma |
| Nocardia farcinica | Severe disseminated infection |
| Nocardia otitidiscaviarum | Skin and lung infections |
| Nocardia nova | Opportunistic infections |
Morphology
Nocardia has several unique microscopic characteristics.
- Gram-positive bacteria
- Thin branching filamentous rods
- Aerobic organism
- Non-motile
- Non-spore forming
- Weakly acid-fast due to mycolic acids in the cell wall
- Forms branching hyphae-like structures
Colony Characteristics
- Dry, chalky colonies
- White, cream, yellow, orange or pink pigmentation
- Rough surface
- Earthy odor
- Slow-growing (2–7 days or longer)
Habitat
Nocardia is widely distributed in nature.
Common sources include:
- Soil
- Dust
- Decaying plants
- Fresh water
- Compost
- Organic matter
Humans acquire infection from the environment rather than from infected individuals.
Epidemiology
- Worldwide distribution
- More common in tropical and subtropical climates
- Rare but serious opportunistic infection
- Men are affected more frequently than women
- Usually affects adults
- Incidence has increased because of more immunocompromised patients
Risk Factors
People at highest risk include:
- HIV/AIDS
- Organ transplant recipients
- Long-term corticosteroid therapy
- Cancer patients
- Diabetes mellitus
- Chronic lung disease
- Tuberculosis
- Chronic kidney disease
- Alcoholism
- Immunosuppressive medications
Healthy people may develop localized skin infections after trauma.
Mode of Transmission
Nocardia is not usually transmitted from person to person.
Routes of infection
- Inhalation of contaminated dust (most common)
- Traumatic implantation through skin injury
- Contaminated wounds
- Rarely through surgical procedures
Virulence Factors
Several factors help Nocardia survive inside the human body.
| Virulence Factor | Function |
|---|---|
| Mycolic acids | Resist destruction by immune cells |
| Catalase | Protects against oxidative killing |
| Superoxide dismutase | Neutralizes reactive oxygen species |
| Cord factor | Enhances survival inside macrophages |
| Biofilm formation | Increases persistence |
| Cell wall lipids | Prevent phagocytosis |
Pathogenesis
The infection usually begins after inhalation of contaminated dust.
- Inhalation of Nocardia organisms
- Entry into the lungs
- Phagocytosis by alveolar macrophages
- Survival within macrophages due to mycolic acids
- Multiplication inside tissues
- Formation of abscesses
- Tissue destruction
- Spread through the bloodstream
- Brain, skin, kidneys, bones, or other organs may become infected
Patients with impaired cell-mediated immunity are at greatest risk of disseminated disease.
Clinical Manifestations
1. Pulmonary Nocardiosis
Most common presentation.
Symptoms include:
- Chronic cough
- Fever
- Chest pain
- Shortness of breath
- Weight loss
- Blood-stained sputum
Chest X-ray may show:
- Consolidation
- Lung nodules
- Cavitary lesions
- Pleural effusion
2. Cutaneous Nocardiosis
Occurs after skin injury.
Features include:
- Cellulitis
- Nodules
- Abscesses
- Ulcers
- Draining sinuses
3. Mycetoma
Usually caused by Nocardia brasiliensis.
Clinical triad:
- Swelling
- Sinus tract formation
- Discharge containing granules
Feet are most commonly affected.
4. Central Nervous System Infection
Brain abscess is the most serious complication.
Symptoms:
- Headache
- Fever
- Seizures
- Confusion
- Weakness
- Neurological deficits
5. Disseminated Nocardiosis
Infection spreads via blood to:
- Brain
- Kidney
- Liver
- Bones
- Skin
- Eyes
Laboratory Diagnosis
Accurate laboratory diagnosis is essential because nocardiosis can mimic tuberculosis or fungal infections.
1. Specimen Collection
Depending on the site of infection:
- Sputum
- Bronchoalveolar lavage (BAL)
- Pus
- Skin biopsy
- Tissue biopsy
- Blood (rare)
- Cerebrospinal fluid (CSF)
2. Direct Microscopy
Gram Stain
Shows:
- Gram-positive
- Branching filamentous rods
Modified Acid-Fast Stain (Modified Ziehl-Neelsen)
- Weakly acid-fast
- Uses 1% sulfuric acid as decolorizer
This test helps differentiate Nocardia from many other bacteria.
3. Culture
Media used:
- Blood agar
- Chocolate agar
- Sabouraud dextrose agar
- Brain Heart Infusion agar
Incubation:
- 35–37°C
- Aerobic
- May require 2–3 weeks
4. Colony Identification
Typical colonies:
- Dry
- Chalky
- Wrinkled
- White to orange
- Adherent
5. Biochemical Tests
- Catalase positive
- Urease positive
- Casein hydrolysis
- Xanthine hydrolysis
- Tyrosine hydrolysis
These tests help identify species.
6. Molecular Methods
Modern laboratories use:
- PCR
- 16S rRNA sequencing
- MALDI-TOF Mass Spectrometry
- Whole Genome Sequencing
These provide rapid and accurate species identification.
Differential Diagnosis
Nocardiosis should be differentiated from:
| Disease | Similar Features |
|---|---|
| Tuberculosis | Chronic cough, lung cavities |
| Actinomycosis | Branching filamentous bacteria |
| Aspergillosis | Lung nodules |
| Lung cancer | Pulmonary mass |
| Fungal mycetoma | Chronic foot swelling |
Treatment
Treatment usually requires prolonged antibiotic therapy.
First-line Drug
- Trimethoprim-Sulfamethoxazole (TMP-SMX)
Other Antibiotics
- Amikacin
- Imipenem
- Linezolid
- Ceftriaxone
- Minocycline
- Meropenem
Severe infections often require combination therapy.
Treatment duration:
- 6–12 months
- Longer for brain involvement or immunocompromised patients
Prevention
There is no vaccine against Nocardia.
Preventive measures include:
- Avoid exposure to contaminated soil and dust if immunocompromised.
- Wear gloves while gardening.
- Use protective masks in dusty environments.
- Maintain good wound care after injuries.
- Control underlying diseases such as diabetes.
- Use immunosuppressive drugs cautiously under medical supervision.