The life cycle of Blastocystis spp is not yet understood, including the infectious stage and whether (and which of the) various morphologic forms of this polymorphic organism that have been identified in stool or culture constitute distinct biologic stages of the parasite in the intestinal tract of hosts. The cyst form (3 to 5 micrometers) is postulated to be an infectious stage, but not confirmed. The predominant form found in human stool specimens is referred to as the vacuolar (or central body) form and is of variable size (5 to 40 micrometers, occasionally much larger). Replication appears to occur via binary fission. Other morphologic forms (eg, ameboid and granular forms) also have been noted in stool samples and/or culture; their biological role and eventual developmental fate require further investigation.
* Various forms that may occasionally be seen in human stool samples and in culture. Their biological significance is not well understood.
