Aspergillus unguis

Aspergillus unguis is a member of the genus Aspergillus in the phylum Ascomycota, and the asexual state (anamorph) of Emericella unguis.^[1][2] Aspergillus unguis is a filamentous soil-borne fungus^[3] found on decomposing plant matter and other moist substrates including with building materials and household dust.^[4] Aspergillus unguis occurs mainly in tropical and suptropical soils but has also been isolated from various marine and aquatic habitats.^[5] The species was first isolated in 1935 by Weill and L. Gaudin.^[2] Historically, A. unguis was assigned to the A. nidulans group, a common group of soil-borne fungi due to the resemblance of its ascospores and cleistothecia to those of Emericella nidulans.^[6] Aspergillus unguis is distinctive, however, in possessing spicular hyphae.^[6] A number of synonyms have been collapsed into this species, including: Sterigmatocystis unguis, Aspergillus laokiashanensis and Aspergillus mellinus.^[7]

Growth and morphology

Colonies typically reach up to 2 cm in diameter at 10 days incubation, growing optimally at 30 °C (86 °F).^[5] A number of growth media are suitable for the cultivation of this species, including Sabouraud maltose agar, Czapek’s agar, Raulin’s agar.^[5] The colonies of A. unguis are first white in colour becoming greenish or chrome green in the centre,^[5] and ultimately dull brown colour similar to chocolate.^[6] The dark colour is due to the presence of melanin or melanin like-pigments within the cell wall.^[8] This dark brown colouration is unique to the species and has cause some researchers to include it in the dematiaceous fungi, a group of unrelated, darkly coloured moulds that includes some serious opportunistic pathogens of humans.^[8] Despite its outward similarity to the dematiaceous fungi, the latter group conventionally excludes members of the order Eurotiales.

The conidiophoress are smoothed walled, non-septate, reaching 45-65 µm in length.^[5] The mature sporulating heads are 250-300 µm in diameter and columnar.^[5] The conidiophores terminate in hemispherical vesicles 9-12 µm, wide, that become covered by primary and secondary metulae giving rise to phialides.^[6] The conidia are spherical in shape, roughly 3 µm in diameter and develop brownish vein-like structures on the spore walls over time.^[5] Conidia are borne in columns at the tips of aspergilloid conidiophores.^[6] The spicular hyphae of A. unguis are aseptate and become brown in colour, thick-walled and irregularly roughened and tapering with blunt apex.^[6] They reach a length of 1000 µm or greater.^[6]

Ecology

Aspergillus unguis is a soil-dwelling fungus that prefers high water activity (a_w).

Human disease

Disease of the nail and skin caused by A. unguis has been reported.^[4] This species was one of two most common fungi encountered in the homes of asthmatic children in Detroit, Michigan, reported from 72% of homes.^[9] It has also been reported as a colonist of water damaged construction materials in Finland where it was found to produce the mycotoxin, sterigmatocystin, a potent carcinogen and mutagen.^[10]

Metabolites

Beta-glucosidase

Production of beta-glucosidases is well known in members of the genus Aspergillus, however only few species can produce glucose tolerant isoforms. Beta-glucosidases produced by A. unguis are characteristically highly glucose-tolerant.^[3] For this reason, the fungus has been investigated industrially for use in the hydrolysis of cellulose hydrolysis and biomass conversion (cellulose to glucose to fuel ethanol).^[3]

Depsidones

Four bioactive, chlorinated depsidones have been isolated from strains of this species obtained from seaweed collected off the coasts of China and Thailand.^[11] These compounds exhibited potent inhibitory effects against methicillin-resistant Staphylococcus aureus (MRSA), brine shrimp larvae (test for compound cytotoxicity and pesticide-ability) and the human lung tumor line A-549.^[11] No activity was found against E. coli strain AB1157+; however depsidones 1, 3 & 4, and to a lesser extent 2, exhibited selective bioactivity on strain AB3027-.^[11] Some compounds introduced DNA damage.^[11] Only depsidone 1 showed significant anti-proliferatory activity on human cancer cells.^[11] Folipastatin, another depsidone is inhibitory to phospholipase A₂, an important enzyme in the lipolysis pathway that also is related to the expression of inflammatory mediators such as arachidonic acid.^[12] Exposure of hypertensive rats to this compound produced an anti-inflammatory effect with mild ]]edema]] at the injection site but without an increase in blood pressure edemas on the hypertensive rats.^[12]

Guisinol

Guisinol was isolated from a strain of the teleomorph, Emericella unguis derived from a mollusc collected from coastal Venezuela.^[13] This compound also inhibit the growth of MRSA in vitro.^[13]

Unguinol

The metabolite unguinol, produced uniuquely by A. unguis, is a growth promotor specific to monogastric animals such as chickens.^[14] It also been investigated as a potential herbicide due to its ability to selectively inhibit pyruvate, phosphate dikinase (PPDK), a key enzyme in gluconeogenesis and photosynthesis.^[15]

References