Blastocystis hominis is an eukaryotic microbe which is one of the most frequently encountered organisms in the intestinal tract of humans and various animals. It exhibits morphologic polymorphism consisting of a development cycle which is still controversial.
It is classified within the Stramenophiles, a heterogeneous group which includes diatoms, brown algae and oomycetes, the last of which have proven pathogenic potential. Blastocystis hominis is a commensal which may become pathogenic, especially in immunodeprived hosts. A total of 457 studies have been done on this organism from 1967 to the present; most of them have concentrated on pathology and treatment (168), or diagnosis and epidemiology (181).

Blastocystis hominis has an anaerobic life-style, but each organism has a large number of organelles which are similar to mitochondria. The question arises about whether this is a unique adaptation strategy for these organelles. Another question concerns apoptosis, which has been described in these organisms: is it caspase-dependent?
The haploid genome has a reduced size of about 10 Mb, distributed among 9 to 13 karyotypic bands depending on the strain, which are easy to separate. The estimated size of the chromosomes varies from 260 kb to 2.2 Mb.
In vitro culture using axenic media produces a large number of parasites, which is a source of DNA and proteins for functional genomics experiments.
The collection of several human strains as well as parasites from poultry from different regions of the world will make comparative studies possible which, when linked to clinical observations, will help refine the notion of virulence and that of a co-factor which could predispose or complicate some pathologies.