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Taxonomic Classifications

Kingdom: Fungi
Phylum: Ascomycota
Class: Euascomycetes
Order: Onygenales
Family: Onygenaceae
Genus: Blastomyces

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 phase.

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.

Information from: Mycology Online.

Description and Habitats

Blastomyces dermatitidis 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 this genus.
(Source of the descriptive information of Blastomyces dermatitidis:


Based on the mycological information provided  in the website,, Blastomyces dermatitidis is the only species included in the genus Blastomyces.

Health Effects


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.

blast7.jpg (505106 bytes)

South American Blastomycosis.  Paracoccidioidomycosis: Mouth Mucosa in man

blast3.jpg (23891 bytes)Nodular skin lesions ofblastomycosis,
one of which is a bullous lesion on top of a nodule.

Source of  Blastomycosis Information: Microbiology and Immunology Online

Macroscopic Features

Being a thermally dimorphic fungus, Blastomyces dermatitidis behaves diversely at different temperatures. The morphology of the fungus is mold-like at 25C and yeast-like at 37C. 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 25C
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 37C
Conversion to a yeast form at 37C 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 37C for 22 days. Following this step, a small amount of the solution (100 l) 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.

Microscopic Features

Similar to the colony morphology, microscopic appearance of the fungus also depends on the temperature of isolation:

1.At 25C
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 shape.

2. At 37C
After incubation at 37C on an enriched medium or in infected tissue sections, the fungus appears as budding yeast cells. The yeast cells (8-12 m in diameter) typically have double-contoured refractile walls and a broad base attaching the bud to the parent cell. These blastoconidia are globose in appearance.

Laboratory Precautions

Blastomyces dermatitidis is potentially 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, amphotericin B, ketoconazole and itraconazole are highly active. In contrast, fluconazole MICs against Blastomyces are unacceptably high. The novel agents, voriconazole, 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.

Browse these Webpages:

[Home] [Mold Species] [Absidia] [Acremonium] [Alternaria] [Aspergillus] [Aureobasidium] [Basidiobolus] [Beauveria] [Bipolaris] [Blastomyces] [Candida] [Chaetomium] [Chysosporium] [Cladophialophora] [Cladosporium] [Coccidioides] [Conidiobolus] [Cryptococcus] [Cunninghamella] [Curvularia] [Drechslera] [Emmonsia] [Engyodontium] [Epidermophyton] [Exophiala] [Exserohilum] [Fonsecaeae] [Fusarium] [Histoplasma] [Lecythophora Species] [Madurella] [Microsporum] [Mucor] [Paecilomyces] [Paracoccidioides] [Penicillium] [Phialophora] [Phoma] [Rhinocladiella] [Rhizomucor] [Rhizopus] [Scedosporium] [Scopulariopsis] [Scytalidium] [Sporothrix] [Stachybotrys] [Trichoderma] [Trichophyton] [Verticillium] [Wallemia] [Wangiella dermatitidis] [Yeast]

The mycological information gathered and organized in this extensive research on the
different Pathogenic Molds was  sourced out from the list of websites below:



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