Distribution And Physiology

Members of the genus Aspergillus are found almost everywhere. They are truly ubiquitous and have been isolated from penguin dung in the Antarctic, binocular lenses in the tropics, and spoiled foodstuffs all over the planet. Asper-gilli can utilize an incredible variety of substrates, including plant debris, foods and feeds, fabrics, feathers, leather, and dung. Aspergilli have been associated with the biode-terioration of kerosene, paper, pesticides, plasticisers, and rubber and have been isolated from unlikely sources that range from biblical scrolls to sauna bath boards. They often grow on the surface of humid walls in cellars and stables, and their spores are found in high concentrations in attics, crawl spaces, air conditioning ducts, house dust, and insulation. Although the majority of species are saprophytic, the genus also includes opportunistic pathogens of mammals, insects, and plants.

The major natural habitats for aspergilli are soil and decaying vegetation. Water and temperature are the major factors influencing mycelial growth. Within the soil environment, aspergilli occur most frequently in subtropical/ warm temperate zones; however, certain species such as Aspergillus niveus, A. nidulans, and Aspergillus tamarii are more common in tropical zones. Most aspergilli prefer moist habitats, although several species are xerophilic. Whatever the optimal conditions for growth, all species of Aspergillus form abundant conidiospores that are resistant to harsh environments. These propagules bridge survival until hospitable conditions for germination prevail.

Aspergillus colonies are visible macroscopically as black, green, yellow, or brown mold. The colony color reflects the spore color of the particular species. Pigment production is also influenced by trace elements. In fact, the color of A. niger spores was once used as a bioassay to detect copper.

As with all filamentous fungi, the hyphae grow by apical extension, allowing for rapid exploitation of new habitats and facilitating the colonization ofsolid and semisolid substrates. The ramifying hyphal filaments are profusely branched and have a high surface-to-volume ratio in intimate contact with their substrate. As mycelia colonize, they extrude enzymes that break down complex carbon sources such as cellulose, hemicellulose, pectin, and starch, not only changing their substrate, but literally becoming part of it. Simultaneously, mycelia influence the growth of other organisms, especially bacteria, and create complex degradative consortia.

Because of their robust physiology, aspergilli are easy to grow in the laboratory and can be cultured with simple carbon and nitrogen sources, mineral salts, water, and oxygen. Mutations blocked in various catabolic and anabolic pathways are also easily isolated. Biochemical and molecular genetic analyses in A. nidulans have revealed considerable detail about the physiology and genetics of these processes. In addition, the weedy nature of these molds means they often cause trouble as common contaminants in tissue culture and bacteriology laboratories. Techniques for handling aspergilli as well as common media for cul-turing species are given by Raper and Fennell (3) and Onions et al. (11).

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