IN-VITRO ENCYSTMENT AND EXPERIMENTAL INFECTIONS OF BLASTOCYSTIS-HOMINIS

Cultures of Blastocystis hominis were induced to encyst using three en cystation media: (a) an encystation medium (EM) comprising yeast extra ct in buffered saline containing 50% horse serum, (b) an encystation m edium (CEM) comprising EM conditioned with bacterial soluble products and (c) an encystation medium (TEM) containing 0.5% trypticase in EM. Two isolates of B. hominis were studied, an axenized isolate C and a n on-axenized isolate MS. In EM, isolate C did not encyst, whereas 6.1% of isolate MS had encysted by day 1. However, in CEM and TEM, 17.4% an d 25.7% of isolate C, respectively, had encysted by day 5. In all thre e media, isolate MS encysted more readily than isolate C, with as much as 91.7% of the former encysting in TEM. As viewed by phase-contrast microscopy, cyst-like stages appeared highly refractile. Direct stool examination of juvenile Wistar rats infected with 10000 cyst-like stag es of both C and MS isolates showed Blastocystis at day 2 post-infecti on. At autopsy on day 7. large numbers of Blastocystis were seen in th e cecum, with smaller numbers being observed in the large intestine. I n contrast, rats fed with various inocula of the vacuolar stages of is olates C and MS did not become infected, indicating the importance of the encysted stages in the transmission of the parasite.