Blastocystis hominis is classified as a protozoan and has 4 distinct stages: vacuolar, which is observed most commonly in clinical specimens; granular; ameboid; and the most recently discovered cystic stage. (Baron, 411; Suresh)
B. hominis displays marked morphologic variability and measures between 5 and 40 μm. It lacks a cell wall but contains mitochondria, Golgi apparatus, and smooth and rough endoplasmic reticula typical of protozoa. It reproduces asexually, usually by binary fission, and grows only under anaerobic conditions in culture (Parasite Image Library; Suresh). Extensive genetic diversity has been reported for B. hominis, which might account for differences in virulence (Kaneda).
The form used for transmission has not yet been defined. The vacuolated stage (with large central vacuole) is described most frequently in fecal samples, though recent literature suggests that the cystic form may in fact be more common (Blastocystis Hominis Infections). Some speculate that thick-walled cysts-- which infect epithelial cells of the digestive tract, asexually multiply, and are excreted in feces-- may be responsible for external transmission, while thin-walled cysts continually re-infect the host’s intestinal tract (Parasite Image Library; Yakoob). Others suggest that the ameboid form is more likely to be associated with disease, but this too has not been confirmed (Zierdt).