Protozoa Overview and Classification

Study Notes

Protozoa are unicellular microscopic organisms, ranging in size from 2 to over 100 micrometers.
They are found in all moist habitats, including seawater, soil, and freshwater.
Protozoa primarily multiply through asexual reproduction.
In parasitic protozoa, developmental stages are often transmitted between hosts inside a cyst.

Protozoa are classified according to their habitat:
- Intestinal protozoa.
- Urogenital protozoa.
- Blood and tissue protozoa.

Pathogenic amoebas are parasitic protozoa causing disease. They have the ability to invade host cells and are often referred to as amphizoic (existing within a host or in the environment as "free-living").
Commensal amoebas are not believed to cause significant disease and are commonly found in the human gastrointestinal tract. Examples include Entamoeba dispar, Entamoeba hartmanni, Entamoeba coli, Entamoeba moshkovskii, Endolimax nana, Iodamoeba butschlii, and Entamoeba gingivalis.

Trophozoite: Size ranges between 20-60 µm. They move using unidirectional cytoplasmic protrusions (pseudopodia). Trophozoites have a clear ectoplasm and granular endoplasm, with a spherical nucleus containing a karyosome.
Cyst: Size ranges between 10–20 µm. They appear as spherical or oval bodies with a hyaline cyst wall. The mature cyst contains four visible nuclei, a rod-shaped chromatoid body, and rounded edges.

Infective stage: The mature 4-nucleated cyst.
Mode of infection: Ingestion of cysts in contaminated food, water, or hands.
Excystation: Within the small intestine, digestive juices and an alkaline medium cause the cyst to release its contents.
Nuclear and cytoplasmic division: This leads to 8 trophozoites.
Migration and multiplication: Trophozoites migrate to the large intestine, multiply by binary fission, and produce cysts, which are expelled in feces.
Diagnostic stage: Trophozoites and cysts.