Dimorphic fungi - Talaromyces marneffei

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The page below is a sample from the LabCE course Fungal Infections in Humans. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Dimorphic fungi - Talaromyces marneffei

Talaromyces marneffei produces elongated, ovoid, small (3-5 µm) yeast that divide by fission rather than by budding. They are often found within histiocytes. T. marneffei forms rapidly growing tan mold colonies that are initially powdery or velvety on the surface and become colored with maturity (typically blue or green centrally) when it is cultured at 25-30ºC. Additionally, a red pigment diffuses into the agar around and underneath the colonies.
Microscopically, the mold colonies are composed of hyaline septate hyphae with conidiophores and metulae producing brush-like clusters of phialides. There are chains of small oval conidia that form at the terminal ends of the phialides. Thermal dimorphism should be demonstrated for correct identification.
The yeast colonies are off-white to pink and consist of small (3-5 µm) oval yeast-like cells that reproduce by fission rather than by budding.
The organism is endemic in Southeast Asia and is not found elsewhere. Those at risk are HIV/AIDS patients. It is acquired via the respiratory route and most cases lead to involvement of the bone marrow, skin, lymphatics, liver, and spleen.
The top right image shows a T. marneffei culture plate demonstrating diffuse red pigment. The bottom right image shows a lactophenol cotton blue preparation of T. marneffei. The fungal conidiophore is a stalked structure, which at the distal end produces the asexual spores known as conidia, through a process of budding. Conidia are apparent as very small round structures clustered like grapes at the end of these conidiophores.
Images courtesy of the CDC.