Actinomycetes Flashcards
The actinomycetes are gram-positive organisms that tend to grow slowly as branching elements. They are prokaryotes but have fungus-like characteristics. Actinomycetes are the most abundant organisms in the soil.
What are two most relevant actinomycetes?
Actinomycosis
Nocardia
Other common actinomycetes - Streptomyces Rhodococcus Mycobacteriaan Corynebacteria
How is actinomycosis transmitted?
Actinomycetes are the most abundant organisms in the soil.
Actinomycosis generally arise from endogenous inhabitants of the oral cavity.
How is nocardia transmitted?
inhalation of soil organisms
Farmers lung allergic pneumonitis has been linked to various species of actinomycetes
Actinomycosis israelli is usually responsible for disease in man. It is part of the normal oral flora; it can be cultured from the majority of human tonsils and is nearly always found in scrapings of gums and teeth.
Unclear how becomes invasive.
What clinical picture does it cause?
- chronic destructive abscesses of connective tissues
- abdominal/ lungs/ thorax/ face/ foot. Most common presentation is slow neck/ facial swelling
- actinomycetes (actinomycosis/ nocardia) are one the many bacterial/ fungal causes of madura foot
- Abscesses expand into contiguous tissues and eventually form burrowing, tortuous sinuses to the skin surface, where they discharge purulent material.
How to diagnosis actinomycosis?
Aspirate pus - microscopy and culture
Microscopy - sulfur granules, gram positive branching rods
What is treatment of actinomycosis?
Benzathine penicillin
or
ceftriaxone
May need IV for 4-8 weeks, then up to 12 months oral to prevent relapse
In contrast to Actinomyces, species of Nocardia are inhabitants of the soil rather than commensals in animals and they are aerobic. Ubiquitous throughout world. Nocardia species are gram-positive and two species are pathogenic for man.
What are two most common species?
N. asteroides
N. brasiliensis
What is nocardia mechanism of infection?
What clinical picture does it cause?
Skin contamination - ulcers or mycetoma (usually N brasiliensis). Chronic subcutaneous absesses (mycetomas) arise from contamination of skin wounds, usually on the feet, and hands of labourers. Can infect wounds (nosocomial) after surgery
Inhaled - pulmonary disease (usually N asteroides). Usually immunocompromised
Central nervous system - brain abscess
Involvement of the eyes
Disseminated nocardiosis - infection in two non-contiguous sites
Pulmonary nocardia, on CXR, what disease can it mimic?
When to suspect?
- Can look similar to TB/ CAP/ fungal or malignancy on CXR
- Suspect if subacute presentation e.g cough, weight loss, night sweats.
- Suspect if patient immunosuppressed, and not responding to antibiotics
- suspect if skin/ cerebral lesions, in context of someone with pneumonia
Who is at risk of nocardia infection?
Immunosuppressed, including steroids/ diabetes - pulmonary/ disseminated
Laborer - wound infection
Nocardia has various unique virulence factors, allowing it to evade the normal human defence mechanisms
What are examples of this?
Release of cord factor, which prevents nocardia from being phagocytosed by macrophages.
Catalase production, which inactivates oxygen metabolites which would normally be toxic to bacteria
How to diagnose nocardia?
Sputum/ BAL samples
growth of nocardia from tissue samples sent for staining and culture.
Nocardia is a strict aerobe and, on culture, colonies are seen with hyphae. Can take up to 4 weeks for growth
No serological methods are presently available for diagnosis
CXR - may look similar to TB/ CAP/ malignancy
What is treatment of nocardia?
co-trimoxazole and amikacin
treatment duration 6-12 months
abscesses may require drainage, debridement of madura foot
How to prevent mycetoma due to actinomycetes?
Avoid walking barefoot in high-risk areas and also ensure that cuts and grazes are appropriately covered
When to suspect CNS nocardia?
CNS lesion with recent pulmonary infection
Differentials include - CNS lymphoma or metastases brain abscess Cryptococcus Aspergillus Coccidioides mucormycosis tuberculoma toxoplasmosis cysticercos
