Aspergillus versicolor

Aspergillus versicolor is a slow-growing filamentous fungus commonly found in damp indoor environments and on food products. It has a characteristic musty odor associated with moldy homes and is a major producer of the hepatotoxic and carcinogenic mycotoxin sterigmatocystin. Like other Aspergillus species, A. versicolor is an eye, nose, and throat irritant. The fungus was first described by Jean-Paul Vuillemin in 1903 under the name Sterigmatocystis versicolor, and was later moved to the genus Aspergillus by Carlo Tiraboschi in 1908. Presently, the genus Sterigmatocystis is obsolete. Aspergillus versicolor is a highly ubiquitous species commonly isolated from soil, plant debris, marine environments, and indoor air environments. It is among the most common of indoor molds, often reported in dust and in water-damaged building materials, such as wallboards, insulation, textiles, ceiling tiles, and manufactured wood. Aspergillus versicolor is a highly resilient fungus, explaining its wide global distribution in a variety of environmental conditions. Although it grows optimally between 22 and 26 °C, A. versicolor can grow at a larger temperature range from 4–40 °C. The fungus can also tolerate a wide pH range, and is particularly resistant to alkaline conditions. The soil depth at which the fungus can be found is variable (down to 50 cm), but it appears to be particularly abundant in deeper soils. Like other members of its genus, A. versicolor displays moderate xerophillic characteristics, meaning that it can grow in conditions with low water activity (down to aW of 0.75–0.81 in the optimal temperature range). A. versicolor is also considered to be osmophilic as it is able to survive in solutions that are up to 30% NaCl or 40% sucrose. This makes the fungus an economically important spoilage organism for stored grains, rice, tea, and spices. Additionally, A. versicolor has been isolated from areas with high saline levels including the Dead Sea. Other extreme habitats from which the fungus has been reported include peat bogs, deglaciated Arctic soil, and uranium mines. Colonies vary greatly in colour, growth rate, and surface characteristics depending upon growth conditions. By contrast, microscopic morphology tends to be consistent independent of growth parameters. Colonies are typically white at the start of development, and change to yellow, orange, and green, often with pink or flesh hues intermixed, as they mature. Reverse pigmentation is often variable as well, especially for incubation periods greater than two weeks in duration. Aspergillus versicolor has long, septate hyphae that appear glassy and transparent. Conidiphores, which are specialized hyphal stalks for asexual reproduction, typically measure 120–700 µm in length. Conidiophores terminate in small vesicles (10–15 µm in diameter) that are biseriate (i.e., with two successive layers of cells interposing the vesicle and conidia). The first layer of cells are called metulae upon which phialides are borne. The vesicles are variable in shape but are often described as 'spoon-shaped'. Conidia are spherical, approximately 2.5–3.5 µm in diameter, and may have smooth or slightly roughened surfaces. Aspergillus versicolor is able to grow on a variety of surfaces, including those that are nutrient-deficient, because it is autotrophic for most growth substances and the macronutrient riboflavin. Additionally, A. versicolor has high activity levels of xylanase, an enzyme that breaks down hemicellulose in plant cell walls. Xylanase is a secondary metabolite controlled through gene-specific induction and carbon catabolite repression.