Aspergillus: Biology and Species
Aspergillus is a genus of ubiquitous, mostly saprophytic, filamentous fungi in the phylum Ascomycota, with about 350 described species and several hundred more that are known only by sequence.
They cause a range of diseases through three mechanisms: invasion (invasive aspergillosis in the immunocompromised), colonisation of cavities and chronic tissue damage (aspergilloma and chronic pulmonary aspergillosis), and allergy (allergic bronchopulmonary aspergillosis and fungal sinusitis).
They also produce mycotoxins (aflatoxin) and contaminate food, and are among the commonest fungal contaminants in clinical specimens, so interpretation needs the clinical setting.
Humans inhale hundreds of conidia daily. The conidia of Aspergillus fumigatus are small (2 to 3 micrometres), hydrophobic, and reach the alveoli, where they are cleared by macrophages and neutrophils in a person with normal immunity. Disease occurs when the host is unable to clear them.
The genus is divided into subgenera and sections. Cryptic species (morphologically similar species that need sequencing for identification) are within the main sections and often differ in antifungal susceptibility.
| Section | Species | Clinical importance |
|---|---|---|
| Fumigati | Aspergillus fumigatus (about 50 to 70 percent of invasive disease), and cryptic species Aspergillus lentulus, Aspergillus fischeri, Aspergillus felis, Aspergillus udagawae, Aspergillus thermomutatus | Most invasive and chronic disease, and allergic disease; some cryptic species have reduced susceptibility to azoles and amphotericin B |
| Flavi | Aspergillus flavus (second commonest cause), Aspergillus oryzae, Aspergillus parasiticus | Fungal sinusitis, keratitis, cutaneous infection, and invasive disease, particularly in hot and arid regions; aflatoxin production |
| Nigri | Aspergillus niger, Aspergillus tubingensis, Aspergillus welwitschiae, Aspergillus carbonarius | Otomycosis, colonisation of cavities (aspergilloma), occasional invasive infection; calcium oxalate crystals in tissue; ochratoxin |
| Terrei | Aspergillus terreus | Invasive disease with a poor outcome; intrinsic resistance to amphotericin B |
| Nidulantes | Aspergillus nidulans (Emericella nidulans), Aspergillus versicolor, Aspergillus sydowii | Aspergillus nidulans is a characteristic pathogen in chronic granulomatous disease; Aspergillus versicolor in onychomycosis |
| Usti | Aspergillus calidoustus | Multidrug resistant (intrinsic azole resistance, and reduced amphotericin B susceptibility); invasive disease in transplant |
| Circumdati | Aspergillus ochraceus, Aspergillus westerdijkiae | Contaminant; ochratoxin |
| Clavati | Aspergillus clavatus | Malt worker's lung, allergic alveolitis |
Cryptic species within the Fumigati section are separated from Aspergillus fumigatus by growth at 48 degrees Celsius (Aspergillus fumigatus grows at 48 degrees Celsius, and most cryptic species grow poorly) and by sequencing. Recognising them matters because their antifungal profiles differ.
| Feature | Description |
|---|---|
| Hyphae | Hyaline, septate, 3 to 6 micrometres wide, with parallel walls and dichotomous branching at acute angles of about 45 degrees |
| Conidiophore | An unbranched stalk that ends in a swollen vesicle |
| Phialides | Flask shaped cells that produce chains of conidia; uniseriate (one row of phialides directly on the vesicle) or biseriate (a row of metulae bearing phialides) |
| Conidia | Round, 2 to 4 micrometres, in dry chains |
| Colonies | Fast growing in 2 to 3 days on Sabouraud dextrose agar at 25 to 37 degrees Celsius; velvety to powdery |
| Thermotolerance | Aspergillus fumigatus grows from 12 to 55 degrees Celsius, an advantage in compost and in the human host |
| Species | Colony colour (front) | Conidiophore and vesicle |
|---|---|---|
| Aspergillus fumigatus | Blue green to grey green, with white margin | Uniseriate, short columnar conidial heads, flask shaped vesicle |
| Aspergillus flavus | Yellow green | Uniseriate and biseriate heads, rough walled stalks |
| Aspergillus niger | Black conidia on white to yellow mycelium | Biseriate, globose heads, carbon black |
| Aspergillus terreus | Cinnamon, sand, or brown | Biseriate, columnar heads; round accessory conidia (aleurioconidia) on submerged hyphae, which form in tissue and aid dissemination |
| Aspergillus nidulans | Green with yellow to red brown cleistothecia | Biseriate, with Hülle cells and ascospores |
In tissue, Aspergillus appears as uniform, septate hyphae with acute angle branching, stained by Grocott methenamine silver and periodic acid Schiff, and by calcofluor white in direct microscopy. Fruiting heads form in cavities exposed to air (aspergilloma and sinus fungus balls). Aspergillus niger shows calcium oxalate crystals. Hyphal appearance does not distinguish Aspergillus from Fusarium, Scedosporium, and other hyaline moulds, and culture or molecular identification is needed.
Aspergillus grows on decaying vegetation, compost, soil, stored grain, spices, potted plants, and air conditioning ducts. Concentrations of airborne conidia range from a few to several hundred per cubic metre of indoor air, and rise sharply with building work, demolition, gardening, handling of compost, and flooding and water damage. Hospital outbreaks are linked to construction, defective ventilation, and contaminated air filters, and to contaminated hospital water, devices, and food.
| Determinant | Role |
|---|---|
| Small conidia size | Penetration to alveoli |
| Hydrophobin RodA rodlet layer | Immune masking of conidia, hydrophobic dispersal |
| Dihydroxynaphthalene melanin | Protects against phagolysosomal acidification, oxidants, and ultraviolet light |
| Thermotolerance | Growth at 37 degrees Celsius and above, with regulators such as CgrA |
| Hypoxia adaptation (SrbA) | Growth in necrotic, hypoxic lung lesions; also linked to azole susceptibility |
| Siderophores (triacetylfusarinine C, ferricrocin) | Iron acquisition from transferrin, key in invasive disease |
| Gliotoxin | Epipolythiodioxopiperazine toxin that impairs neutrophil and macrophage function and causes apoptosis |
| Galactosaminogalactan | Adhesion, biofilm, immune modulation, and masking of beta glucan |
| Proteases and phospholipases | Tissue degradation and invasion |
| Catalases and superoxide dismutases | Resistance to oxidative burst |
| Allergens (Asp f 1 and others) | Drive IgE sensitisation and Th2 responses |
Host defence depends on resident alveolar macrophages (kill conidia), neutrophils (kill hyphae by oxidative burst and extracellular traps), Toll like receptors and dectin 1, and Th1 and Th17 responses. Defects that predispose to invasive disease are neutropenia, corticosteroids (impair macrophage and neutrophil function), chronic granulomatous disease (defective NADPH oxidase), and genetic defects in dectin 1 or plasminogen. Defects that predispose to allergic and chronic disease include Th2 skewing, cystic fibrosis, and structural lung damage.
- Aflatoxins from Aspergillus flavus and Aspergillus parasiticus contaminate maize, groundnuts, and other crops, and are potent hepatocarcinogens (aflatoxin B1 with hepatitis B), with acute aflatoxicosis in outbreaks, and stunting in children.
- Ochratoxin A from Aspergillus ochraceus and Aspergillus carbonarius is nephrotoxic.
- Gliotoxin is produced in the host during invasion.
- Occupational exposure: farmer's lung, malt worker's lung (Aspergillus clavatus), and allergic and toxic reactions to dust from grain and compost.
| Method | Use |
|---|---|
| Direct microscopy with potassium hydroxide or calcofluor white | Septate hyphae with acute angle branching; no species level information |
| Culture | Sabouraud dextrose agar at 30 to 37 degrees Celsius, with growth in 24 to 72 hours; cycloheximide containing media can inhibit some species; blood cultures are rarely positive |
| Section level identification | Colony colour, microscopy (vesicle, serial arrangement, conidia), growth at 45 to 48 degrees Celsius |
| Species identification | Sequencing of the beta tubulin (benA) or calmodulin (CaM) genes; the internal transcribed spacer region identifies the section but separates species poorly; matrix assisted laser desorption ionisation time of flight mass spectrometry with a good mould database |
| Susceptibility testing | Reference broth microdilution (European and United States methods) and commercial gradient strips and agar based screening with azole containing plates; testing is indicated for isolates from invasive disease and for treatment failure |
| Resistance genotyping | cyp51A sequencing and polymerase chain reaction for known mutations such as TR34/L98H and TR46/Y121F/T289A, and direct detection in specimens by commercial assays |
Resistance interpretation uses species specific breakpoints. For Aspergillus fumigatus, European breakpoints are voriconazole susceptible at 1 microgram per mL or less and resistant above 2, itraconazole susceptible at 1 or less and resistant above 2, posaconazole susceptible at 0.125 or less and resistant above 0.25, isavuconazole susceptible at 1 or less and resistant above 2, and amphotericin B susceptible at 1 or less and resistant above 1. These values are laboratory thresholds to confirm against the current standard in use.
| Pattern | Notes |
|---|---|
| Environmental azole resistance in Aspergillus fumigatus | Selected by demethylation inhibitor fungicides in agriculture and in flower bulb and compost environments. cyp51A tandem repeat mutations (TR34/L98H and TR46/Y121F/T289A) confer resistance or high MICs to voriconazole, itraconazole, and posaconazole, and lead to treatment failure. Found in azole naive patients, and on every inhabited continent |
| Clinical (patient acquired) resistance | Point mutations in cyp51A (at positions G54, G138, M220) arise during long term azole therapy in chronic pulmonary aspergillosis and cystic fibrosis, and give itraconazole and posaconazole cross resistance |
| Intrinsic amphotericin B resistance | Aspergillus terreus and Aspergillus nidulans (MIC raised), Aspergillus calidoustus |
| Intrinsic azole resistance | Aspergillus calidoustus |
| Reduced susceptibility of cryptic Fumigati species | Aspergillus lentulus (voriconazole and amphotericin B), Aspergillus fischeri |
| Echinocandin resistance | Rare; FKS1 hot spot mutations; echinocandins are only fungistatic (minimum effective concentration) against Aspergillus in any event |
Azole resistance in Aspergillus fumigatus varies from under 1 percent in some countries to 10 to 20 percent in parts of Europe and Asia. Where local prevalence of azole resistance exceeds 10 percent, guidance favours initial therapy with liposomal amphotericin B or a combination of voriconazole with an echinocandin, until susceptibility is known.
