Blastomyces dermatitidis

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A Microbial Biorealm page on the genus Blastomyces dermatitidis

Classification

Fungi; Ascomycota; Ascomycetes; Incertae sedis; Incertae sedis; Incertae sedis; Blastomyces

Description and significance

Blastomyces dermatitidis is a dimorphic fungal pathogen, found in the Mid-West and Northern United States and Canada. Inhaling Blastomyces dermatitidis spores can cause blastomycosis, which commonly affects the lungs and skin. It also affects the bone, prostate and other organs (2). 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. All available clinical and epidemiological evidence indicates that humans and lower animals contract blastomycosis from some source in nature. However, the natural habitat of B. dermatitidis has yet to be clearly delineated, despite some reports of its isolation from soil (1).

Genome structure

Four strains have been sequenced by the Broad Insitute: B. dermatitidis SLH14081, B. dermatitidis ER-3, B. dermatitidis ATCC 18188, and B. dermatitidis ATCC 26199. The SLH-14081 strain is a highly virulent, clinical isolate that can cause disease in immunocompetent people. The results of automated genome annotation will be released later this year. The size of the SLH-14081 strain is 75.35 Mb (3).

Cell structure, metabolism & life cycle

Blastomyces dermatitidis grown on Sabouraud's dextrose agar at 25C, produces colonies that are variable in both morphology and rate of growth. They may grow rapidly, producing a fluffy white mycelium, or slowly as smooth, tan colored, non-sporulating colonies. Growth and sporulation are enhanced by nitrogenous substances found in starling dung and yeast extract. Strains usually become pleomorphic with age and take on multiple structural forms. Grown on blood agar at 37C, the colonies are wrinkled and folded, smooth 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 mould and yeast phase (1). It develops a typical yeast form of a thick wall and a single bud with a wide base. This wide base is characteristic of B. dermatitidis, and it is important to be able to recognize this. The cells are 12-15 microns in diameter. The yeast will convert to the mycelial form when incubated at 25 degrees C, taking from 3 to 4 days up to a few weeks. Similarly, the mycelial growth can be converted to yeast form when incubated at 37 degrees C. In the past, the only way to identify the dimorphic fungi was to convert from one form to the other, but now it is possible to take the mycelial growth (which is the easiest to grow), and confirm the isolate with a DNA probe in a matter of hours (2).

Ecology (including pathogenesis)

Blastomyces dermatitidis survives in soil that contains organic debris (rotting wood, animal droppings, plant material) and infects people collecting firewood, tearing down old buildings or engaged in other outdoor activities which disrupt the soil. In addition to an ecological niche, most fungi that cause systemic infections have a limited geographic distribution where they occur most frequently. Blastomycosis occurs in eastern North America and Africa. The vast majority of patients with blastomycosis in South Carolina are infected in the northern part of the state, above the Fall Line (Augusta, GA, Aiken, Columbia, Cheraw, Raleigh, NC). Blastomycosis is a rare infection. Symptoms include bone pain, chest pain, cough, fatigue, and fever. Since these symptoms are very similar to other illnesses, the physician needs to make careful diagnoses (2).

Interesting feature

At the elevated temperature of 37 °C in a host, the fungus undergoes a phase transition to the pathogenic yeast form. When inhaled, the yeast cells multiply in the lung and spreads to other parts of the body. It can cause skin lesions and abscesses (2).

References

1. Ellis D. “Blastomyces dermatitidis”. Mycology online at The University of Adelaide. 2011. Online at < http://www.mycology.adelaide.edu.au/Fungal_Descriptions/Dimorphic_Pathogens/Blastomyces/>

2. DiSalvo A. “Mycology chapter 6: blastomycosis (Blastomyces dermatitidis)”. Microbiology and Immunology On-line. 2009. online at <http://pathmicro.med.sc.edu/mycology/mycology-6.htm>

3. Klein B., Sullivan T., Heitman J., Li W., "Blastomyces dermatitidis database" Broad Insititue. 2010. Online at <http://www.broadinstitute.org/annotation/genome/blastomyces_dermatitidis/MultiHome.html>