Tissue section showing large, broad-base, unipolar budding
yeast-like cells of Blastomyces dermatitidis.
On Sabouraud's dextrose agar at 25C, colonies are variable in both
morphology and rate of growth. They may grow rapidly, producing a fluffy
white mycelium, or slowly as glabrous, tan, non-sporulating colonies.
Growth and sporulation are enhanced by nitrogenous substances found in
starling dung and yeast extract. Most strains become pleomorphic with
age. Microscopically, hyaline, ovoid to pyriform, one-celled,
smooth-walled conidia (2-10 um in diameter) of the Chrysosporium
type, are borne on short lateral or terminal hyphal branches.
On blood agar at 37C, colonies are wrinkled and folded, glabrous and
yeast-like. Microscopically, the organism produces the characteristic
yeast phase as seen in tissue pathology. B. dermatitidis can be
described as a dimorphic fungus because it has both a mold and yeast
WARNING: Cultures of B. dermatiditis represent a severe
biohazard to laboratory personnel and must be handled with extreme
caution in an appropriate pathogen handling cabinet. In the past,
conversion from the mold form to the yeast form was necessary to
positively identify this dimorphic pathogen from species of
Chrysosporium or Sepedonium. However, culture identification
by the exoantigen test is now the method of choice.
Description and Habitats
is a thermally dimorphic fungus and a probable saprobe of the soil. It
specifically inhabits decaying wood material. Blastomyces dermatitidis
is very rarely isolated as a natural habitat. Isolation from the environment
is most likely when the sample contains soil and is rich in organic material
such as animal feces, plant fragments, insect remains, and dust. If the
substrate is moist, lacks exposure to direct sunlight, contains organic
debris, and has a pH of less than 6.0, isolation of Blastomyces
dermatitidis is probable. It is endemic in North America. Mississippi,
Ohio and Missouri valleys are the geographic locations with highest
incidence of infections due to Blastomyces dermatitidis. African type
Blastomyces dermatitidis strains isolated from cases in Africa also
exist. It was demonstrated that African type strains are not identical with
the North American strains. These two groups most probably constitute two
distinct serotypes of Blastomyces dermatitidis showing geographic
diversity. They have common and varying antigens.
The sexual state (teleomorph) of Blastomyces
dermatitidis belongs to the family Onygenaceae and is referred to as
Ajellomyces dermatitidis. Ascospores are the sexual spores produced by
(Source of the descriptive information of
the mycological information provided in the website,
dermatitidis is the only species included in the genus
Blastomycosis is a chronic granulomatous disease which means that it
progresses slowly. Although the pulmonary and skin involvement is the most
common, B. dermatitidis frequently affects bone, prostate and other
organs. More frequently blastomycosis presents as a cutaneous or a
respiratory disease. The cutaneous lesions may be primary (usually
self-limiting) or secondary (a manifestation of systemic disease). The
patient who presents with a complaint of respiratory symptoms will
frequently remark about loss of appetite, loss of weight, fever, productive
cough, and night sweats. While these symptoms resemble those of TB, it is
not this disease. The X-ray shows obvious pulmonary disease. To make the
specific diagnosis, the physician must be aware of blastomycosis. Sputum
sent to the lab for "culture" will not grow the organism. The lab must be
alerted to look for fungal organisms or to look specifically for blastomyces.
Some patients have a sub-clinical or "flu-like" response to infection. B.
dermatitidis can frequently be demonstrated in a KOH preparation of pus
from a skin lesion. A typical cutaneous lesion shows central healing with
microabscesses at the periphery. A pus specimen may be obtained by nicking
the top of a microabscess with a scalpel, obtaining the purulent material
and making the diagnosis in 5 min. by microscopic examination with KOH. This
organism has a characteristic appearance of a double contoured wall with a
single bud on a wide base. There are no specific virulence factors for B.
dermatitidis. Laboratory specimens depend on the manifestation of the
disease: If there are skin lesions, send skin scrapings or pus. If there is
pulmonary involvement, send sputum. Other specimens include biopsy material
and urine. Occasionally, the organism can be isolated from urine as it often
infects the prostate.
American Blastomycosis. Paracoccidioidomycosis: Mouth Mucosa in
Nodular skin lesions ofblastomycosis,
one of which is a bullous lesion on top of a nodule.
Source of Blastomycosis Information:
Microbiology and Immunology Online
a thermally dimorphic fungus, Blastomyces dermatitidis behaves
diversely at different temperatures. The morphology of the fungus is
mold-like at 25°C and yeast-like at 37°C. Although the demonstration of
conversion from the mycelial form to yeast form is favorable for definitive
identification of Blastomyces dermatitidis, it remains possible only
for few isolates pathogenic to humans. Other ways of verification of the
identification are by use of exoantigen test, direct flourescent antibody
and nucleic acid probes. Below are the macroscopic features of the fungus:
1. At 25°C
The growth rate is slow to moderately rapid. The colony diameter is 0.5 to 3
cm following incubation for 7 days on potato glucose agar. The texture is
membranous and downy to woolly. The surface color is white to beige and
reverse is pale to brownish.
2. At 37°C
Conversion to a yeast form at 37°C usually requires inoculation onto an
enriched medium. The growth rate is slow to moderately rapid. The colony
diameter is 0.5 to 3 cm following incubation for 7 days. The texture is
typically creamy and yeast-like. It appears granular to verrucose on the
surface. The color of the colony is white to beige.
The classical way to isolate Blastomyces
dermatitidis from environmental sources is by animal inoculation
techniques. More recently, an in vitro method has also been described and
used for isolation of the fungus from a woodpile in north central Wisconsin.
In this in vitro method, the environmental material or soil to be tested is
placed in a neutral aqueous solution containing allantoin, tween 80,
potassium phosphate, magnesium sulphate, penicillin and streptomycin and is
incubated at 37°C for 22 days. Following this step, a small amount of the
solution (100 µl) is plated onto yeast-extract phosphate agar and incubated
at 20°. After the isolation of Blastomyces dermatitidis mold
colonies on yeast-extract phosphate agar, the colonies are plated onto
brain-heart infusion agar to demonstrate the conversion to yeast phase.
Similar to the colony morphology, microscopic appearance of the fungus also
depends on the temperature of isolation:
Septate hyaline hyphae and unbranched short conidiophores are observed.
Conidiophores arise at right angles to the vegetative hyphae. The conidia
are hyaline and unicellular. They are solitary and pyriform to globose in
2. At 37°C
After incubation at 37°C on an enriched medium or in infected tissue
sections, the fungus appears as budding yeast cells. The yeast cells (8-12
µm in diameter) typically have double-contoured refractile walls and a broad
base attaching the bud to the parent cell. These blastoconidia are globose
dangerous and requires biological safety cabinet for all manipulations.
In vitro susceptibility testing procedures have not been yet standardized
for Blastomyces dermatitidis as well as other thermally dimorphic
fungi, except for mycelial phase of
Sporothrix schenckii. However, there are reports on in vitro
activity of some antifungal agents against Blastomyces. In general,
itraconazole are highly active. In contrast, fluconazole MICs against
Blastomyces are unacceptably high. The novel agents,
posaconazole, VER-002, and
caspofungin appear active against Blastomyces.
Amphotericin B, ketoconazole,
and itraconazole are effective in treatment of blastomycosis. Amphotericin B
should be preferred particularly in immunocompromised patients. On the other
hand, mild pulmonary blastomycosis may clear spontaneously and not require
antifungal therapy. Surgical excision of pulmonary lesions may be indicated
in some cases in addition to antifungal therapy.