Microorganism of the Month: Aureobasidium

By Karen Santo-Pietro, EMLab P&K Analyst

Aureobasidium is a widely distributed fungal genus usually found in soil, fresh water, dead plant material, marine estuary sediments and wood. There are approximately 20 accepted species in this genus with the most commonly known being A. pullulans. This genus has also been observed to grow on textiles, foodstuffs, fruits and painted surfaces. In the indoor environment, Aureobasidium growth is commonly found in moist places such as bathrooms and kitchens, especially on shower curtains, tile grout and windowsills. The spores are usually disseminated by wind (when dry) and water.

Aureobasidium spores are difficult to identify on spore traps because of morphologic variation. Its most distinguishing feature is the production of primary blastospores (spores produced by a budding process) arising directly from pigmented, vegetative hyphae on short denticles (protuberances in the hyphae). The spores may be hyaline (colorless) or pigmented, variable in size, one-celled, ellipsoid or ovoid, and completely encased in a slimy coat. These primary spores can give rise to secondary or tertiary spores through yeast-like budding. The conidia (asexual spores) adhere together to form slimy heads. The brown hyphae can differentiate to form chlamydospores (resting spores) or arthroconidia (a spore type resulting from fragmentation of a hypha) at maturity. Generally, we report irregular clumps of dark brown hyphae, dividing in more than one plane to form chlamydospores, asA. pullulans. However, vegetative hyphae from other unrelated dematiaceous (pigmented) fungi, such as Cladosporium, may be indistinguishable from Aureobasidium when blastospores are absent. When chlamydospore-like structures are indistinguishable, we report them in the "other brown" category. Because this fungus is sticky and slimy, spores do not readily become airborne and are not commonly found on spore traps. In direct microscopic examination, it is recognizable if enough diagnostic structures have been preserved on tape lifts or swabs.

In culture, Aureobasidium species grow rapidly on Malt Extract Agar (MEA) and, at first, produce colonies that are yeast-like and cream or pink in color. As the colony ages, a slimy exudate appears and the coloration changes to dark brown or black on the surface. As seen from the reverse side of the agar plate the colony is a pale beige. The mycelium is characterized by irregular, dichotomous (two part) branching, with cells sometimes rounding off and separating, and is variable in thickness. Aureobasidium colonies exhibit distinct radial, "fan-shaped" growth that makes them recognizable among other colonies.

Aureobadisium pullulans has been used to produce pullulan, a biodegradable polysaccharide which, when processed, becomes a shiny and strong fiber used to package food and drugs. It has also been used industrially to remove unwanted components of raw textile materials. One of its negative economic impacts is that it has been associated with the deterioration of pears and oranges in storage or in transit.

Human pathogenicity is uncommon and Aureobasidum pullulans is generally considered a contaminant and not a primary human pathogen. Chronic human exposure via humidifiers/air conditioners can lead to hypersensitivity pneumonitis or "humidifier lung." It is also linked to a few other diseases such as keratomycosis, pulmonary mycosis with sepsis and other opportunistic infections such as cutaneous mycoses.