Observations on the ultrastructure and viability of the cystic stage of Blastocystis hominis from human feces.

This report describes the ultrastructure and viability of cysts of Blastocystis hominis from feces of infected patients. The cysts were round to ovoid, measured 2-5 microns in size, and contained a condensed cytoplasm that had vacuoles of varying sizes, four nuclei, and as many as six cristate mitochondria. The cell wall was rather electron-lucent. Surprisingly, chromatoid-like structures were found in the cytoplasm and nucleus of some of the cysts. These have not previously been reported in Blastocystis. The cysts can survive in water for up to 19 days at normal temperatures but are fragile at extreme temperatures and in common disinfectants.