DESCRIPTIONS OF MEDICAL FUNGI

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Descriptions of Medical Fungi
Blastomyces dermatitidis Gilchrist & Stokes
At present the genus Blastomyces contains two species, Blastomyces dermatitidis
and Blastomyces gilchristi, which are morphologically identical but distinguishable
by sequence analysis of the ITS region (Brown et al. 2013). B. dermatitidis lives in
soil and in association with decaying organic matter such as leaves and wood. It is
the causal agent of blastomycosis a chronic granulomatous and suppurative disease,
having a primary pulmonary stage that is frequently followed by dissemination to other
body sites, typically the skin and bone. Although the disease was long thought to be
restricted to the North American continent, in recent years autochthonous cases have
been diagnosed in Africa, Asia and Europe.
WARNING: RG-3 organism. Cultures of B. dermatitidis represent a biohazard to
laboratory personnel and must be handled in a Class II Biological Safety Cabinet
(BSCII).
Morphological Description: Colonies at 25 O C have variable morphology and growth
rate. They may grow rapidly, producing a fluffy white mycelium or slowly as glabrous, tan,
nonsporulating colonies. Growth and sporulation may be enhanced by yeast extract.
Most strains become pleomorphic with age. Microscopically, hyaline, ovoid to pyriform,
one-celled, smooth-walled conidia (2-10 µm in diameter) of the Chrysosporium type,
are borne on short lateral or terminal hyphal branches.
Colonies on blood agar at 37 O C are wrinkled and folded, glabrous and yeast-like.
Microscopically, the organism produces the characteristic yeast phase seen in tissue
pathology; ie. B. dermatitidis is a dimorphic fungus.
Comment: In the past, conversion from the mould form to the yeast form was
necessary to positively identify this dimorphic pathogen from species of Chrysosporium
or Sepedonium. However, culture identification by exoantigen test and/or molecular
methods is now preferred to minimise manipulation of the fungus.
Key Features: Clinical history, tissue pathology, culture identification by positive
exoantigen test and/or by molecular methods.
a
b
Blastomyces dermatitidis (a) culture and (b) one-celled, smooth-walled
conidia borne on short lateral or terminal hyphal branches.