Dematiaceous Fungi

The term "dematiaceous" refers to the characteristic dark appearance of this group of fungi as it grows on agar. Colonies are dark gray, brown, or black and, importantly, have a black reverse when the bottom of the agar plate is examined. This distinguishes the dematiaceous fungi from fungi with black conidia but an otherwise pale mycelium, such as A. niger.

Nomenclature and Disease

Phaeohyphomycoses are clinical entities defined by the presence of pigmented hyphae, pseudohyphae, or yeast-like cells in tissue; this is in contrast with the hyalohyphomycoses, which lack pigmentation in tissue sections. Melanin stains can help to identify pigment in poorly-pigmented strains. Chromoblastomycoses, also known as chromomycoses, are chronic infections of the skin and subcutis developing after traumatic implantation of dematiaceous fungi; they are typically fungating, discolored, hemorrhagic lesions. Mycetomas (eumycotic mycetomas) are chronic, localized, progressive infections of the skin, subcutaneous tissue, muscle or bone that are caused by hyaline or dematiaceous fungi; their hallmark is the formation of granules of organized mycelial elements in tissue with the formation of abscesses and draining sinus tracts.
Note that some authors consider chromomycosis and older term for phaeohyphomycosis.

Dematiaceous Fungi: Direct Examination

Hyphae are usually regular, 3-6 μ, and have parallel walls, irregular branching, and septae
Hyphae are dark yellow or brown when unstained or poorly stained

Dematiaceous fungi can be conveniently divided into three groups based on morphology in slide culture:

macroconidia produced and are divided by longitudinal and transverse septae (these are referred to as muriform macroconidia
macroconidia produced and are divided transverse septae only
microconidia (one celled conidia) produced

Dematiaceous Fungi with Muriform Macroconidia

Alternaria

Usually considered a contaminant; may cause phaeohyphomycosis
Colonies mature rapidly, are wooly or cottony, and have a black reverse and a gray-white surface that becomes green-black or brown with a light border as the colony ages
Conidia form chains or occur singly and are divided by transverse and longitudinal septations; conidia usually have a club-shaped configuration
Identification beyond the genus level is difficult

FIG. 1. Note the color of the hyphae and the prominent, multicelled macroconidia. Longitudinal septae are difficult to appreciate at this power.

Ulocladium

Usually considered a contaminant
Colonies mature rapidly and are typically wooly or cottony and dark brown to black; the reverse is black
Conidia more spherical than Alternaria, do not form chains, and are borne on short, geniculate conidiophores

FIG. 1. Ulocladium in slide culture. Note muriform septation in the macroconidia indicated by the arrow.

FIG. 1. Ulocladium in slide culture. Again, note the color of the macroconidia and of the hyphae; while hyphae are stained with lactophenol cotton blue stain, macroconidia resist staining in this preparation and appear brown.

Epicoccum

Usually considered a contaminant
Colonies grow fairly rapidly and progress from cottony yellow or orange to brown or black. The reverse may be red and diffusible yellow, red, brown, or orange pigment may be present
Hyphae branch repeatedly in a focal pattern known as sporodochia
Short conidiophores arise from sporodochia
Muriform macroconidia are spherical to club shaped

FIG. 1. Epicoccum in slide culture. Sporodochia, conidiophores and small, muriform macroconidia are present.

Dematiaceous Fungi with Transversely Septated Macroconidia

Curvularia

May cause fungal sinusitis and keratitis, endocarditis, pulmonary infections, and subcutaneous phaeohyphomycosis; may also be contaminants
Colonies dark olive green to brown or black; surface pink-gray and wooly; the reverse is dark
Macroconidia characteristically 4-5 cells separated by transverse septae which arise sympodially from twisted conidiophores; macroconidia have a characteristic curved appearance secondary to disproportional enlargement of a central cell

FIG. 1. Curvularia in slide culture. Note the characteristic boomerang shaped macroconidia.

Drechslera

Not reported as a pathogen
Produce cylindrical, multicelled macroconidia from geniculate conidiophores
Resemble Bipolaris species; may be distinguished in saline mounts by germ tubes extending from the conidium between base and septum at right angle to the long axis of the macroconidium. In contrast, Bipolaris germ tubes arise from both ends of the base of the conidium and extend parallel to the long axis of the macroconidium

FIG. 1. Drechslera in slide culture. Macroconidia closely resemble those of Bipolaris

Dematiaceous Fungi with Microconidia

Cladosporium

May cause cutaneous, subcutaneous, and eye infections, but are among the most common dematiaceous mold contaminants
Nomenclature is changing, as with the rest of medical microbiology: Cladosporium carrionii may currently be better named Cladophialophora carrionii , but Koneman et al. elect to use the prior term in their popular textbook
Colonies are dark with a black reverse and may show velvety gray, gray-green, or purple-brown grown on a flat, dark base
Cladosporium-type conidiation displays freely branching hyphae that bear long chains of dark-staining, elliptical conidia that often contains scars or dysjunctors at the sites of attachment

FIG. 1. Cladosporium in slide culture. Elliptic microconidia are found in long chains.

Phialophora verrucosa

Causes chromoblastomycosis, phaeohyphomycosis, or mycetoma
Colonies are slow growing, dark gray or black, and hairlike
Characteristic urn-shaped phialides bear tight clusters of spherical to elliptical, hyaline conidia

FIG. 1. Phialophora verrucosa in slide culture. Flask-shaped phialides with cup-shaped collarettes and conidia in balls at the apices of the phialides are characteristic.

FIG. 2. Phialophora verrucosa in slide culture. This high power photomicrograph shows a double-flask-shaped phialide; conidia spill from its apex.

Fonsecaea pedrosoi

Causes chromoblastomycosis, phaeohyphomycosis
Colonies are typically slow-growing, dark brown, olive or black, and may be covered with a velvet-like, silver mycelium; the reverse is black
Conidiophores pale to brown, usually erect, apically swollen
Isolates are notoriously pleomorphic, with cladosporium-type, rhinocladiella-type, phialophora-type, and fonsecaea-type conidiation

FIG. 1. Fonsecaea pedrosoi in slide culture. This figure illustrates so-called fonsecaea-type conidiation: conidiophores are compactly sympodial and bear ovoid primary conidia upon denticles. Primary conidia may support secondary conidia, which in turn may support tertiary conidia, but long chains of conidia-denticle-conidia structures are not seen

Wangiella dermatitidis

Causes phaeohyphomycosis involving skin, subcutis, eye, brain, joints
Colonies mature slowly. Immature colonies resemble black yeasts but eventually display delicate hair-like or velvety mycelium. The reverse is white
Phialides without collarettes are a predominant and distinctive feature
Conidia are round to oval, unicellular, and up to 6μ in greatest dimension

FIG. 1. Wangiella dermatitidis in slide culture. Note the absence of collarette on the very slender phialide.

Dematiaceous Fungi that Defy the Above Grouping

Phaeoannellomyces werneckii

Colonies grow slowly and resemble pale yeast colonies before turning olive black; the reverse is black
May cause tinea nigra, a superficial, asymptomatic fungal skin infection, especially of the palms
A black yeast, formerly Exophiala werneckii; the genus Phaeoannellomyces was created in 1985 to accommodate black yeasts whose yeast cells function as annellides. Controversy still exists regarding this nomenclature.
Forms yeasts with 2 cells
Thick walled hyphae may form in aging colonies

FIG. 1. Phaeoannellomyces werneckii in slide culture. Note the 2 celled yeast forms.

FIG. 1. Phaeoannellomyces werneckii in slide culture. Note the thick walled hyphae seen in this aging culture.

Chaetomium species

An opportunistic moniliaceous mold; usually regarded a contaminant
Young colonies are cottony white or pale and usually darken with maturity to tan-gray or gray-olive
Single-celled ascospores are usually olive-brown (pigmented); Chaetomium is therefore considered here with dematiaceous fungi; it is, however, listed with other moniliaceous (hyaline or brightly colored) fungi in the ASM Manual of Clinical Microbiology 7th ed.
Distinctive round to flask-shaped perithecia with irregular olive to brown filamentous appendages (setae) are present; perithecia measure 100-225 μ in greatest dimension
Asci are club-shaped and contain 4-8 spores
Asci usually dissolve quickly after release from the ostiole of the perithecium, but oval to lemon-shaped, olive brown, unicellular spores remain

FIG. 1. Chaetomium in slide culture. Note the perithecia with radially extending irregular brown filamentous setae

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