Fungal Infections

Dimorphic fungi

The term dimorphic refers to having 2 fungal shapes, typically yeasts and filamentous fungi. In most human pathogenic species that take more than one form (morphology) such as Histoplasma capsulatum, temperature is the key environmental stimulus determining the fungal cell shape. Higher temperatures (ie >35ºC) shift the fungus towards the yeast phase, whereas typical environmental temperatures below 35ºC promote a filamentous fungal shape. Coccidioides immitis switches in tissue to form spherules, rather than yeasts. Pneumocystic jirovecii switches at the same temperature in the lung between cysts, which stain well with silver stains and trophozoites with are reminiscent of amoebae and poorly staining with silver. Most Candida species (with the notable exception of C. glabrata) are also dimorphic in tissue, exhibiting yeast forms and hyphal forms, but are placed within the yeasts section here as this is the predominant morphology seen in the mycology laboratory.

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