Aspergillus speciesThe U.S. Government's Occupational Safety and Health Administration [OSHA] lists the following Aspergillus species as all being allergens and irritants and a cause of Hypersensitivity pneumonitis and Dermatitis: Aspergillus flavipes, Aspergillus flavus, Aspergillus fumigatus, Aspergillus glaucus, Aspergillus nidulans, Aspergillus niger, Aspergillus ochraceus, and Aspergillus versicolor. |
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Taxonomic Classifications
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For Aspergillus
mold pictures, please visit the Aspergillus species sections below--- |
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Description and Habitats Aspergillus species is cosmopolitan, filamentous, saprobic fungus. It is widely distributed in nature. It is primarily isolated from soils, especially cultivated soils, decomposing plant material, and indoor air environment.
Species There are around one hundred eighty – five species under the genus Aspergillus. Around twenty species have been reported so far as causative agents of opportunistic infections in humans. Among these, Aspergillus fumigatus is the most frequently isolated species, followed by Aspergillus flavus and Aspergillus niger. Among the other species not often isolated as opportunistic pathogens are Aspergillus clavatus, Aspergillus glaucus group, Aspergillus nidulans, Aspergillus oryzae, Aspergillus terreus, Aspergillus ustus and Aspergillus versicolor. |
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Aspergillus flavus |
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Culture of Aspergillus flavus.
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Conidial head of
A. flavus.
Note: conidial heads with both uniseriate and biseriate arrangement of phialides may be present. |
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Clinical Significance of Aspergillus flavus Aspergillus flavus has a world-wide distribution and normally occurs as a saprophyte in soil and on many kinds of decaying organic matter. A. flavus is the second most common species (next to A. fumigatus) to be isolated from human infections, and it is often associated with invasive aspergillosis seen in immunosuppressed patients and in paranasal sinus infections. |
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Source: Mycology Online (www.mycology.adelaide.edu.au) |
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Aspergillus fumigatus |
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Conidial head of
A. fumigatus
(Note: uniseriate row of phialides on the upper two thirds of the vesicle) |
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Clinical Significance of Aspergillus fumigatus Based on the research conducted by the National Center for Biotechnology Information (NCBI), Aspergillus fumigatus can also cause allergic bronchopulmonary and sinus infections. Patients with asthma and cystic fibrosis can frequently develop allergic broncho-pulmonary aspergillosis (ABPA), a hypersensitivity reaction to the fungus Aspergillus fumigatus, which frequently leads to a progressive loss in lung function. Morever, in developing countries patients with compromised immune response can develop keratitis, which usually leads to unilateral blindness. |
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Source: Mycology Online For more information on the different mold species, please visit this website: http://www.mycology.adelaide.edu.au |
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Pathogenicity and
Health Effects Aspergillus species are recognized to play a significant role in three different clinical settings in man namely, in opportunistic infections, allergic conditions, and in toxicoses. The major factor which led to the emergence of opportunistic infections is immunosuppression. Opportunistic infections are generally called aspergillosis. The most common forms of aspergillosis in humans are pulmonary in nature; on the other hand, other deep infections are also encountered particularly among immunocompromised patients. Among all filamentous fungi, Aspergillus is generally the most recovered one in invasive infections and is second to Candida as the most frequently isolated fungus in opportunistic mycoses.
Cerebral, cutaneous, hepatosplenic, pulmonary, and disseminated aspergillosis, endocarditis, endophthalmitis, meningitis, myocarditis, onychomycosis, osteomyelitis, otomycosis, sinusitis, and as well as Aspergillus fungemia may develop as almost any organ or system in the human body may be involved once Aspergillus - induced infection sets in.
Furthermore, there are numerous outbreaks of disseminated aspergillosis cases that have been recorded in neutropenic patients in concurrence with construction / renovation projects in hospital environments.
Aspergillus species may colonize lung cavities which may have previously been developed due to tuberculosis, sarcoidosis, bronchiectasis, pneumonia, ankylosing spondylitis or neoplasms. This whole distinct clinical identity is referred to as aspergilloma. Kidneys may also be targeted by aspergilloma.
Some Aspergillus species produce various mycotoxins which have proven to possess carcinogenic potential particularly in animals and these are acquired through chronic ingestion. Aspergillus flavus produces aflatoxin which is a well – known mycotoxin. Aflatoxin may induce hepatocellular carcinoma and it contaminates foodstuffs such as peanuts and grains. Ingestion of high amounts of aflatoxin may induce toxic effects in poultry animals fed with aflatoxin – contaminated grain.
Aspergillus species are known as occasional causative agent of respiratory infection in birds and mycotic abortion in certain animals, particularly cattle and sheep.
Aspergillus species are also encountered as common laboratory contaminants due to being ubiquitous in nature. Other Health Effects of Aspergillus mold Aspergillosis. Although metabolites of species of Aspergillus (Hyphomycetes) cause other health problems, such as acute and chronic aflatoxin poisoning, we are concerned here only with diseases caused by the growth of the fungus itself somewhere in the body. (1) Bronchopulmonary aspergillosis is usually caused by Aspergillus fumigatus, which colonizes mucus within the bronchi, evoking a severe allergic reaction. (2) In Aspergilloma, the fungus forms a mycelial ball in a lung cavity produced by an earlier attack of tuberculosis. The wall of the cavity may erode, causing the patient to spit blood, and necessitating surgical intervention. (3) Invasive aspergillosis is found only in patients who are severely debilitated, or are immunosuppressed, as in AIDS. The fungus grows outward from the lung, invading blood vessels and spreading to other organs through the bloodstream. This insidious disease is usually fatal, and is often diagnosed only when an autopsy is performed. [from Bryce Kendrick's The Fifth Kingdom]
Macroscopic Features Ø The major macroscopic characteristics that are essential in Aspergillus species identification are the growth rate, colony color and thermo – tolerance; Ø Texture of colonies varies from being downy to powdery; Ø The growth rate varies from slow to rapid depending on the species with diameter size ranging from 0.5 to 1 cm and 1 to 9 cm for Aspergillus species with slow growth and with moderately to rapid growth, respectively, after seven days of incubation at 25°C on Czapek – Dox agar; Ø Surface colony color may also vary depending on the species while the reverse is mostly uncolored to pale yellow, however, some strains of Aspergillus nidulans may be purple or olive while it is orange to purple in Aspergillus versicolor; Ø The only thermo – tolerant Aspergillus which can grow at temperature range of 20°C to 50°C is Aspergillus fumigatus. Microscopic Features Ø Hyphae are septate and hyaline; and Ø Conidiophores arise from the basal foot cell found at the supporting hyphae and terminate in a vesicle at the tip; Ø Vesicle is the typically formation for the genus Aspergillus; Ø Phialides are flask – shaped, may either cover the surface of the vesicle entirely called as “radiate head,” or partially at the upper surface only termed as “columnar head,” and may either be uniseriate – directly attached to the vesicle, or biseriate – attached to the vesicle via a supporting cell, metula; Ø Conidia are found over the phialides forming radial chains and with diameter size of 2 to 5 µm. Ø Other microscopic structures of the genus Aspergillus are: a. Aleuriconidia – observed with a truncated base and carries remnants of the lysed supporting cell; a type of conidium produced by lysis of the cell that supports it; b. Cleistothecia – produced during the sexual production of some Aspergillus species, are round and closed structures which enclose the ascospore – containing asci wherein the asci are distributed into the surrounding upon the bursting of cleistothecia; c. Hülle cells - are large sterile cell bearing a small lumen; and d. Sclerotia - are compact or hard masses of mycelium.
Table 1. Morphological Characteristics of Aspergillus species
Laboratory Precautions General laboratory precautions are required, no special safety measures needed.
Susceptibility Most of Aspergillus isolates yield acceptably low MICs for amphotericin B, itraconazole and voriconazole, however, a number of isolates show high MICs which indicates potential prediction of resistance. Such as the few isolates of Aspergillus fumigatus which are itraconazole – resistant and Aspergillus terreus resistant to amphotericin B. Fortunately, voriconazole appears effective in vitro against itraconazole – resistant Aspergillus fumigatus.
The novel azoles
which include voriconazole, posaconazole or ravuconazole, glucan inhibitors
such as caspofungin and V - echinocandins, and liposomal nystatin are active
in vitro against Aspergillus and remain promising for future
therapy treatment of aspergillosis.
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Clinical Laboratory Handbook: Identifying Filamentous
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