Structure, Pathogenesis, Lab Diagnosis of Aspergillus

Aspergillus

Aspergillus species are saprophytic molds. They exist only as molds. They have septate hyphae that from characteristic V-shaped (dichotomous branched) at 45⁰ angles. The walls are more or less parallel, in contrast to the walls of mucor and rhizopus which are irregular. They form asexual spores called conidia and the conidia of Aspergillus from radiating chains, in contrast to that of mucor and Rhizopus which are enclosed within a sporangium.

Among Aspergillus sp , Aspergillus fumigatus is the main opportunistic pathogen. Other species Aspergillus niger, Aspergillus flavus, associated with diseases.

Virulence factors and pathogenesis

Elastage production

Aspergillus Fumigatus produces 2 types of elastase enzymes which act on elastin protein that comprises approximately 30% of the lung tissue.

Catalase production

Aspergillus species also produces catalase enzymes that may be a virulence factor contributing to aspergillosis associated with chronic granulomatous diseases of childhood.

Aflatoxin

Aspergillus flavus produces aflatoxin and aflatoxin produced by this fungus are known carcinogenic hepatotoxin.

Diseases caused by Aspergillus species

Diseases caused by Aspergillus species is commonly called Aspergillosis. There are three clinical forms of aspergillosis.

• Respiratory diseases

1. Aspergillus asthma

It is an allergic bronchopulmonary aspergillosis due to hypersensitivity to Aspergillus antigen i.e. inhaled air-bone conidia. The fungus grows in the lumen of bronchioles and produces plugs of mycelium and mucous that may block the lumen.

2. Aspergilloma fungus ball

It is often called fungus ball in which fungus colonizes in the pre-existing cavities, often in the case of TB cavity i.e. in aspergilloma inhaled conidia germination in a pulmonary cavity and grows into fungus ball.

3. Disseminated (systemic) Aspergillosis

In this case, the fungus first establishes in the lungs tissues and then disseminates to different organs such as a brain, kidney, heart and other organs particularly in immunocompromised patients.

• Superficial infections

Aspergillus flavus and Aspergillus fumigatus colonize paranasal sinuses(sinusitis), external ear(otomycosis) and in some case eyes(mycotic keratitis)

Lab diagnosis

Specimens

Sputum and biopsy materials are generally used as preferred specimens.

Direct microscopy

Direct microscopy in KOH mount of exudates shows on pigments septate mycelium (3-5mm)in diameter of the fungus with characteristic branching(dichotomously branched) at 45⁰ angles. Gram`s staining may give gram positive reaction. In a case of a fluorescence microscope, calcofluor white stain may be used for direct observation of Aspergillus species.

Culture

Aspergillus fumigatus is a rapidly growing mold (2-6 days) that produces fluffy dextrose agar. Matured sporulation colonies most often exhibit the blue-green powdery appearance. Microscopically, A.fumigatus is characterized by a presence of septate hyphae with conidiophores on it. The tip of the conidiophores expands into large dome-shaped vesicles that have bottle shaped phialides covering the upper half or two-thirds of its surface. Long chains of small, spherical, rough-walled green conidia form a columnar mass to the vesicles , a culture of A.fumigatus is thermotolerant and is able to withstand temperature up to 45⁰C.

Aspergillus flavus produces yellow-green colonies on sabo urad's dextrose agar. However, A.niger beings initially as a yellow colony that soon develops a black dotted surface as conidia are produced.

Skin test

Intradermal skin test to Aspergillus antigen extracts is useful for patients suspected of allergic bronchial pulmonary Aspergillosis.

Prevention and treatment

There are no specific means of prevention. Invasive aspergillosis is treated with Amphotericin B but caspofungin may be effective in a case of invasive aspergillosis that does not respond to Amphotericin B. A.fungus ball growing in sinuses or in a pulmonary cavity can be surgically removed.