Mould Identification: A Virtual Self Assessment
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Unknown 13 = Histoplasma capsulatum
|A 60 year-old renal transplant patient presented with an localised cellulitic lesion on his forearm. Direct examination of biopsy tissue revealed abundant small yeast like cells and the cultures grew the fungus shown below.|
|Direct Microscopy (GMS stained tissue section)|
Tissue morphology of H. capsulatum showing numerous small narrow base budding yeast cells (1-5um diam) inside macrophages.
On Sabouraud's dextrose agar at 25C, colonies are slow growing, white or buff-brown, suede-like to cottony with a pale yellow-brown reverse. Other colony types are glabrous or verrucose, and a red pigmented strain has been noted.
Microscopic morphology shows the presence of characteristic large (8-14 um in diameter), rounded, single-celled, tuberculate macroconidia formed on short, hyaline, undifferentiated conidiophores. Microconidia, if present, are small (2-4 um in diameter), round to pyriform and borne on short branches or directly on the sides of the hyphae. Confirmatory Exoantigen Test Exoantigen immunodiffusion plate showing positive identification of Histoplasma capsulatum. Note H and M bands of identification; EX = culture filtrate; H = Histoplasma antibody and antigen, C = Coccidioides antibody and antigen; B = Blastomyces antibody and antigen.
Histoplasma capsulatum has a world wide distribution, however, the Mississippi-Ohio River Valley in the U.S.A. is recognized as a major endemic region. Environmental isolations of the fungus have been made from soil enriched with excreta from chicken, starlings and bats.
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