''Exophiala hongkongensis'' is an

ascomycete Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The d ...

fungus species. Described as new to science in 2013, it was discovered on the

toenail A nail is a claw-like plate found at the tip of the fingers and toes on most primates. Nails correspond to the claws found in other animals. Fingernails and toenails are made of a tough protective protein called alpha-keratin, which is a polyme ...

clipping of a patient with the fungal disease

onychomycosis Onychomycosis, also known as tinea unguium, is a fungal infection of the nail. Symptoms may include white or yellow nail discoloration, thickening of the nail, and separation of the nail from the nail bed. Toenails or fingernails may be affec ...

Taxonomy

The fungus strain, originally named HKU32^T, was isolated from a toenail clipping of a 68-year-old female patient with onychomycosis at the Queen Mary Hospital in

Hong Kong Hong Kong ( (US) or (UK); , ), officially the Hong Kong Special Administrative Region of the People's Republic of China (abbr. Hong Kong SAR or HKSAR), is a city and special administrative region of China on the eastern Pearl River Delta i ...

. The overall

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...

of the fungus aligned it with '' Exophiala'', but it had

phenotypic In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...

characteristics that were not consistent with any other species of that genus.

Molecular A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

analysis of four independent DNA regions ( ITS, Rpb1, beta-tubulin, and beta-actin) confirmed that it was a new species. It is most closely related to '' Exophiala nishimurae'', with which it forms a clade that is

sister A sister is a woman or a girl who shares one or more parents with another individual; a female sibling. The male counterpart is a brother. Although the term typically refers to a familial relationship, it is sometimes used endearingly to refer to ...

to the species '' E. xenobiotica''.

Description

When grown on Sabouraud dextrose agar culture

media Media may refer to: Communication * Media (communication), tools used to deliver information or data ** Advertising media, various media, content, buying and placement for advertising ** Broadcast media, communications delivered over mass el ...

, the fungus grows slowly as black, slimy,

yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to consti ...

-like colonies that reach a diameter of 2 mm after 14 days at . Then the colony centers develop a velvety texture and become greenish-gray and dome-shaped, while retaining a slimy and yeast-like margin. After 14 days, the

ellipsoid An ellipsoid is a surface that may be obtained from a sphere by deforming it by means of directional scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface; that is, a surface that may be defined as the ...

al yeast-like cells are brown when stained with lactophenol cotton blue, and measure 6 by 3.5 µm. They have ringed zones that pinch off to create

budding Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is kno ...

cells. The

mycelia Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrates. A typical single spore germinates in ...

of the fungus are swollen and form torulose

hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one o ...

e (swollen and constricted at intervals) that are pinched at their ends. When the colonies become velvety, the mycelia are not swollen, and

conidia A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an asexual, non-motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also called mitospores due to th ...

are formed in clusters. The average length of the

conidiophore A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an asexual, non-motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also called mitospores due to the ...

s is about 10 µm. The conidia are oval to roughly

spherical A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ...

, and typically measure 3 by 2 µm.

Pathogenesis

''Exophiala hongkongensis'' is known only from Hong Kong. Known cases of onychomycosis caused by ''Exophiala'' are rare, although this genus commonly causes related skin infections such as

phaeohyphomycosis Phaeohyphomycosis is a diverse group of fungal infections, caused by dematiaceous fungi whose morphologic characteristics in tissue include hyphae, yeast-like cells, or a combination of these. It can be associated an array of melanistic filamento ...

chromoblastomycosis Chromoblastomycosis is a long-term fungal infection of the skin and subcutaneous tissue (a chronic subcutaneous mycosis). It can be caused by many different types of fungi which become implanted under the skin, often by thorns or splinters. C ...

, and

eumycetoma Eumycetoma, also known as Madura foot, is a persistent fungal infection of the skin and the tissues just under the skin, affecting most commonly the feet, although it can occur in hands and other body parts. It starts as a painless wet nodule ...

. Previous instances of onychomycosis had been previously traced to '' E. dermatiditis'' and '' E. jeanselmei''. In addition to ''Exophiala hongkongensis'', Woo and colleagues identified additional cases caused by '' E. oligosperma'' and '' E. bergeri''. All known cases are associated with an underlying disease that causes an

immunosuppressive Immunosuppression is a reduction of the activation or efficacy of the immune system. Some portions of the immune system itself have immunosuppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse react ...

state; in the case of ''E. hongkonensis'', the patient had hypertension.

References

{{Taxonbar, from=Q15632366 Eurotiomycetes Fungi described in 2013 Fungi of China