Nature
Aureobasidium pullulans (also known as Pullularia pullulans, black yeast) is a yeast-like ubiquitous saprophyte mold (1, 2). This fungal species is characterized by highly variable phenotype and physiology; three sub varieties have been identified: A. pullulans var. pullulans, A. pullulans var. melanogenum and A. pullulans var. aubasidani (1, 2), some of which were later re-classified as separate species following genomic analyses (3, 4). The hyphae are hyaline (transparent), smooth and single celled, variable in size and shape, form chlamydospores in places and the colony color can turn dark due to melanin production. Budding of conidia (a type of spore) is frequent. A. pullulans is a fast-growing mold, able to withstand extreme osmotic environments (2, 5) and freezing temperatures (6). The variations observed in the types of colony depend on factors such as the type of carbon substrate used in the medium, age of the colony, temperature and light conditions (7).
In a US study, the researchers found that hyaline conidia presence peaked during periods of rainfall and high winds in the fall and winter (8). However, certain fungi are able to produce spores all year round. There is considerable overlap between peak fungal activity season and other allergens such as grass and weed pollens, often masking the pathology caused by fungal spores in people multi-sensitized (9).
A. pullulans can be found as epiphyte on different fruits and crops (apples, grapes, strawberries, citrus fruits, cabbages, wheat, barley, nuts) and other foods (meat, shrimp, flour, frozen foods); it affects the esthetic appearance of food but is not considered a common cause of spoilage (6). Other sources are plant leaves, wood, damp indoor environments (including cement walls, window frames, glass and latex-coated surfaces), animal feed and even rocks (1, 2, 6, 10). Strains that formed part of a genotyping study were also isolated from human sputum and peritoneal dialysis (1).
A. pullulans is important in the field of microbiology and industrial processes because of its ability to produce pullulan (a biodegradable polysaccharide used for coatings and wrappings). Other metabolites and enzymes of Aureobasidium spp. are widely used in the food and medical industry (11).
Taxonomy
Aureobasidium pullulans (Pullularia pullulans) belongs to the Family Dothioraceae, Order Dothideales, Class Dothideomycetes, Phylum Ascomycota (12, 13).
Taxonomic tree of Aureobasidium genus (12, 13)
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Domain
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Eukaryota
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Kingdom
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Fungi
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Phylum
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Ascomycota
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Subphylum
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Pezizomycotina
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Class
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Dothideomycetes
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Order
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Dothideales
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Family
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Dothioraceae
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Genus
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Aureobasidium
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Taxonomic tree of Aureobasidium genus (12, 13)
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Tissue
Hyaline unicellular conidia (an asexual, colorless microscopic component released in the environment in lieu of sexual reproduction), which can reach high concentrations in the air and represent the main fungal allergen detected (90%; (8)).
