Prototheca species are not fungi, but achlorophyllous algae with phylogenetic affinities to the genus Chlorella.
These organisms are commonly handled in mycology laboratories due to their macroscopic similarity to yeasts and susceptibility to antifungal agents. To date only P. wickerhamii and P. ciferrii (formerly P. zopfii) have been involved in human or animal infections (Lass-Florl and Mayr, 2007; Bakuła et al., 2021).
Colonies are smooth, moist, white to cream and yeast-like. Cultures are sensitive to cycloheximide (actidione) and optimal growth occurs at 25-30C. Mycelium and conidia are absent. Vegetative cells are globose to ovoid, hyaline, varying in size from approximately 3-30 µm, and have a relatively thick and highly refractile wall. No budding cells are present; reproduction is by the development of large sporangia (theca) which contain from 2-20 or more small sporangiospores (endospores or autospores) which are asexually produced by nuclear division and cleavage of the cytoplasm.
ITS and D1/D2 sequencing is recommended (Wang et al. 2014).
Achlorophyllous algae reproducing by sporangia (theca) and sporangiospores (autospores). Prototheca species which can be differentiated by assimilation tests and morphological criteria as outlined below. The API 20C yeast identification strip may be used for species identification.
Kaplan (1977), McGinnis (1980), Rippon (1988), Pore (1985), Ueno et al. (2005), Lass-Florl and Mayr (2007), Wang et al. (2014).
|P. wickerhamii||P. zopfii||P. stagnora|
|Colony morphology||Hemispheric, with
|Flat, with smooth
|Cell diameter µm||3-10||7-10||7-14|
|Growth at 37C||+||+||-|
|Acetate (pH 5)||-||+||+/-|