Prototheca wickerhamii is a ubiquitous alga that does not have chlorophyll. It is widely present in the environment and is a rare cause of infection in humans (protothecosis). Infection usually results by direct traumatic inoculation, which most commonly presents as nodules of the skin. Most cases reported have a suppressed immune system (from drugs or disease).
Diagnosis can be made through culture of infected fluid in Sabouraud dextrose agar or by visualization of sporangia containing sporangiospores on tissue biopsy (using hematoxylin/eosin, GMS, or PAS histochemical stains). The organism incites a chronic granulomatous inflammation with infiltrate of histiocytes, lymphocytes, giant cells and occasional eosinophils. The organism has thick wall, internal septations, measures 3-11 µm in diameter. The sporangia have very small wedge-shaped endospores arranged radially and moulded (morula-like form).
Differential diagnosis: Protothecal sporangia may be confused with Coccidioides immitis, which are much larger