Sporobolomyces salmonicolor

Sporoidiobolus salmonicolor is a yeast-like fungus in the Basidiomycota more commonly known by the name of its asexual yeast-like state, Sporobolomyces salmonicolor. It is generally considered a Biosafety Risk Group 1 fungus; however isolates of S. salmonicolor have been recovered from cerebrospinal fluid, infected skin, a nasal polyp, lymphadenitis and a case of endophthalmitis. It has also been reported in AIDS-related infections. The fungus exists predominantly in the asexual state as a unicellular, haploid yeast yet this species can sometimes produce a sexual state when conjugation of compatible yeast cells occurs. The asexual form consists of a characteristic, pink, ballistosporic yeast. Ballistoconidia are borne from slender extensions of the cell known as sterigma, and are forcibly ejected into the air upon maturity. Levels of airborne yeast cells peak during the night and are abundant in areas of decaying leaves and grains. Three varieties of Sporobolomyces salmonicolor have been described; S. salmonicolor var. albus, S. salmonicolor var. fischerii, and S. salmonicolor var. salmoneus. In 1924, Kluyver and van Niel coined the genus Sporobolomyces and classified it under the Basidiomycota. They recognized that the yeast phase produced by Sporobolomyces exhibited the same forcible discharge mechanism as the basidiospores of the Basidiomycota. They therefore hypothesized that the asexual ballistoconidia of Sporobolomyces are homologues with the basidiospores of the Basidiomycota. Their hypothesis however was questioned by many who did not consider the asexual nature of the ballistoconidia as a basidiomycetous trait. Its classification as a basidiomycetous yeast was further demonstrated by Nyland (1949) with the discovery of its teleomorph; the genus Sporidiobolus. The teleomorph presented basidiomycetous traits such as the presence of dikaryotic hyphae with clamp connections and the formation of resting spores known as teliospores. In the past, Sporidiobolus salmonicolor was thought to be conspecific with Sporidiobolus johnsonii; however it is now well established that they are distinct taxa. Sporidiobolus salmonicolor is distinguished from S. johnsonii by the absence in the former of assimilation of maltose, methyl-a-D-glucoside, cellobiose or salicin. Colonies of Sporobolomyces salmonicolor produce visible, liposoluble carotenoid pigments, resulting in salmon-pink colored colonies. The colony surface is smooth and has a pasty texture. There is considerable cell and colony morphology when S. salmonicolor is grown in culture. The budding yeast-like cells produced during the asexual stage are ellipsoidal to subcylindrical in shape and 8–25 × 2–5.5 μm in size. They can occur singly or in pairs. The ballistoconidia produced are kidney-shaped and can range in size from 6–18 × 2.5–7.0 μm. The characteristic ballistoconidia are borne by extension of the sterigmata which can reach up to 50 μm in length. Both pseudohyphae and true hyphae may also be present. In its sexual state, Sporidiobolus salmonicolor produces dikaryotic hyphae with clamp connections. At the terminal end of the hyphae, thick-walled teliospores are produced. Teliospores are 9–15 μm in diameter, brown in color, spherical in shape and contain lipid-rich globules. Upon germination of the teliospore, basidia with basidiospores are produced. Basidia are transversely septate, two-celled and 4–6 x 20–25 μm in size. Each basidium will generally produce two large basidiospores that are 5–6 × 7–10 μm in size. Prior to the production of the basidium, endospores have also been known to form in the interior of teliospores. This phenomenon is associated with the production of meiospores within the teliospore cytoplasm, ultimately released by rupture of the teliospore wall. In 1969, Van der Walt and Pitout elucidated the life cycle of S. salmonicolor. They performed DNA analysis on a colony of S. salmonicolor grown from a single cell in culture. After several generations, they observed a 2:1 ratio of diploid and haploid cells, respectively. The diploid cells were recovered from thick-walled resting spores known as teliospores. Meiosis was occurring within the teliospore, followed by germination of the teliospore and beginning of the haploid yeast state. Sporobolomyces salmonicolor is predominantly a yeast-like fungus. It is typically found in the asexual state as a unicellular haploid yeast, however a sexually reproducing state can sometimes form. The name Sporobolomyces salmonicolor refers to the anamorph state whilst the name Sporidiobolus salmonicolor refers to its teleomorph state. Sporidiobolus salmonicolor is a heterothallic species; two mating types are known. Induction of the sexual stage begins with anastomosis of compatible yeast cells to form dikaryotic hyphae with clamp connections. Hyphae have “simple” septal pores that allow continuity of the cytoplasm between cells. At the terminal end of the hypha, thick-walled, resting spores called teliospores form. These germinate to form a transversely-septate basidia that bears two large basidiospores. The optimum temperature for growth of Sporobolomyces salmonicolor is between 25–35 °C (77–95 °F), being the highest tolerable temperature. Growth does not occur at 37 °C (99 °F). This species does not undergo fermentation. Additionally, Sporobolomyces salmonicolor shows positive urease activity and a positive staining response when stained with diazonium blue B. Diazonium blue B is a technique used to classify asexual yeasts as members of the Zygomycota, Basidiomycota or Ascomycota. The major ubiquinone present is Q-10. The cell wall of S. salmonicolor contains fucose, mannose, glucose and galactose, however xylose is absent. Colonies grow in the presence of glucose, sucrose, maltose, cellobiose, α,α-trehalose, melezitose, D-arabinose, ethanol, glycerol, D-mannitol, D-glucitol, D-gluconate, succinate, nitrate and urease. This species does not assimilate myo-inositol or D-glucuronate, and do not form extracellular starch-like compounds. Sporobolomyces salmonicolor has a broad geographical distribution. It has been isolated from many areas across the world including Europe, North and South America, Asia, Africa and Antarctica where it is known from a broad spectrum of substrates. It is principally characterized as a phyllosphere fungus and is commonly found in areas of decaying organic material such as leaves and grains as well as ripening grapes. Isolates of this species however have been recovered from freshwater, marine water and clinical specimens. It has also been isolated from agricultural areas and indoor built environments. In agricultural environments, Sporobolomyces salmonicolor can pose a respiratory hazard for agricultural workers. Agricultural workers are subject to increased exposure when they partake in activities involving the handling of grains. Sporobolomyces salmonicolor has additionally been isolated from straw in hay barns or hay lofts. Workers in these settings should consider the proper use of masks to avoid infection. If individuals show atopic symptoms, a change in occupation might be considered. In indoor environments, S. salmonicolor has been associated with severe water and mould damage. Flooded basements and utility rooms are places where S. salmonicolor may be recovered. This fungus will also commonly associate with standing water films, although not very much has been documented on this. It can form a pink water film around stagnant toilet water. The most efficient way to avoid exposure in the home is to eliminate moisture sources and keep bathrooms clean, dry and ventilated.