Prototheca (Algae)

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).

RG-1 organisms.

Prototheca

Prototheca wickerhamii thecae and autospores.

Morphological description: 
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.

Molecular identification: 
ITS and D1/D2 sequencing is recommended (Wang et al. 2014).

Key features: 
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.

  P. wickerhamii P. zopfii P. stagnora
Colony morphology Hemispheric, with
smooth margin		 Flat, rough
corrugated margin		 Flat, with smooth
margin
Cell diameter  µm 3-10 7-10 7-14
Growth at 37C + + -
Glucose + + +
Trehalose + - -
L-propanol - + +/-
Acetate (pH 5) - + +/-
Galactose + - +
Capsule - - +
Antifungal susceptibility: Prototheca wickerhamii (Australian national data); MIC µg/mL.
Antifungal No ≤0.03 0.06 0.125 0.25 0.5 1 2 4 8 ≥16
AMB 16     2 5 6 3        
ISAV 7 1 2 4              
VORI 14       2 2 9 1      
POSA 14       1 4 7 2      
ITRA 16         3 10   1 2  

References: 

  • Ahrholdt, J., Murugaiyan, J., Straubinger, R.K., et al. (2012) Epidemiological analysis of worldwide bovine, canine and human clinical Prototheca isolates by PCR genotyping and MALDI-TOF mass spectrometry proteomic phenotyping. Medical Mycology, 50, 234-243.
  • Bakuła, Z., Siedlecki, P., Gromadka, R., et al. (2021) A first insight into the genome of Prototheca wickerhamii, a major causative agent of human protothecosis. BMC Genomics, 22, 168-168.
  • Jagielski, T., Bakuła, Z., Gawor, J., et al. (2019) The genus Prototheca (Trebouxiophyceae, Chlorophyta) revisited: Implications from molecular taxonomic studies. Algal Research, 43, 101639.
  • Kaplan, W. (1977) Protothecosis and infections caused by morphologically similar green algae. The black and white yeasts. Proceedings of the Fourth International Conference on the Mycoses. Scientific Publication No. 356. Pan American Health Organization. Washington, DC, USA.
  • Lass-Florl C. and Mayr, A. (2007) Human protothecosis. Clinical Microbiology Reviews, 20, 230-242.
  • McGinnis, M.R. (1980) Laboratory handbook of medical mycology. Academic Press, New York.
  • Murugaiyan, J., Ahrholdt, J., Kowbel, V., et al. (2012) Establishment of a matrix-assisted laser desorption ionization time-of-flight mass spectrometry database for rapid identification of infectious achlorophyllous green micro-algae of the genus Prototheca. Clinical Microbiology and Infection, 18, 461-467.
  • Pore, R.S. (1985) Prototheca taxonomy. Mycopathologia, 129, 129-139.
  • Rippon, J.W. (1988) Medical mycology: the pathogenic fungi and the pathogenic actinomycetes, 3rd edition. W,B. Saunders Co, Philadelphia, USA.
  • Ueno, R., N. Hanagata, N. Urano et al. (2005) Molecular phylogeny and phenotypic variation in the heterotrophic green algal genus Prototheca. Journal of Phycology, 41, 1268-1280.
  • Wang, X., Fu, Y-F., Wang, R-Y., et al. (2014) Identification of clinically relevant fungi and Prototheca species by rRNA gene sequencing and multilocus PCR coupled with electrospray ionization mass spectrometry. PLoS One, 9, e98110-e98110.