Introduction
This topic builds the systematic ladder that the next ones hang from: Protozoa → Phylum Sarcomastigophora → Subphylum Mastigophora (flagellates) → Order Kinetoplastida → Family Trypanosomatidae → Genus Trypanosoma. Learn the general features at each level, then the two human trypanosomes: T. brucei (African sleeping sickness) and T. cruzi (American trypanosomiasis, Chagas disease).
Protozoa: general characteristics
Protozoa are unicellular eukaryotes: a single cell performs every function of life (movement, feeding, excretion, reproduction, protection). More than 50,000 species are known; most are free-living in water and moist soil, and fewer than 100 regularly infect humans.
Cell organisation
- Pellicle: the plasma membrane reinforced by a layer of microtubules; it gives a constant shape (flagellates, ciliates, Apicomplexa). Amoebae have only a flexible membrane and change shape constantly.
- Cytoplasm: a clear outer ectoplasm (movement, protection) and a granular inner endoplasm (nucleus, organelles, food vacuoles).
- Nucleus: one or more; vesicular (with a central karyosome, as in Entamoeba) or compact. Ciliates have two kinds (macronucleus and micronucleus).
- Organelles of locomotion: pseudopodia, flagella or cilia; Apicomplexa glide using their pellicle.
- Organelles of feeding and excretion: cytostome (cell mouth), food vacuoles, contractile vacuoles (osmoregulation in fresh-water species; parasitic species usually lack them).
- Special organelles: kinetoplast (Kinetoplastida), apical complex (Apicomplexa), axostyle and costa (trichomonads), adhesive disc (Giardia).
Nutrition
- Most are heterotrophs: holozoic (phagocytosis of particles or other cells, e.g. Entamoeba eating erythrocytes) or saprozoic (absorption of dissolved nutrients across the surface, e.g. trypanosomes in blood).
- A few free-living flagellates are autotrophs with chloroplasts (Euglena); some are mixotrophs.
Life-cycle stages
- Trophozoite: the active, feeding and multiplying stage; usually the stage that causes disease.
- Cyst: a resting stage with a resistant wall that survives outside the host; it is usually the infective stage of intestinal protozoa (Entamoeba, Giardia). Forming a cyst is encystation; leaving it in a new host is excystation. Protozoa without cysts (Trichomonas, trypanosomes) must pass directly from host to host or through a vector.
Reproduction
Medical importance
- Protozoa live in the blood and tissues (trypanosomes, Leishmania, Plasmodium, Toxoplasma), the intestine (Entamoeba, Giardia, Cryptosporidium) or the urogenital tract (Trichomonas vaginalis).
- Ways in: swallowed cysts or oocysts (faecal–oral), a blood-sucking vector, sexual contact, the placenta, blood transfusion and organ transplantation.
- Their short generation time lets them multiply inside the host (unlike most helminths), so a single infection can produce an overwhelming disease.
Classification
The classical classification (Levine et al., 1980), which the topic titles use, groups protozoa mainly by their organelles of locomotion: