Cytology and physiological features
Protozoa belong to the domain Eukarya. Previously, all such microorganisms were grouped into the kingdom Protozoa; however, new taxonomical data have allowed the establishment of a separate kingdom — Chromista. It includes some organisms previously considered protozoa or fungi, such as blastocysts (Blastocystis), oomycetes, and Rhinosporidium seeberi.
The protozoan cell contains a defined nucleus with a nuclear envelope and nucleolus. The cytoplasm contains the endoplasmic reticulum, mitochondria, lysosomes, and numerous ribosomes. The cell surface is covered by a pellicle.
Nutrition occurs via phagocytosis — the microorganism engulfs solid particles with the formation of food vacuoles. Metabolic waste products are excreted through contractile vacuoles. Protozoa exhibit two main forms of existence:
- Trophozoite — the growing, motile vegetative form. It feeds actively but has low resistance in the external environment.
- Cyst — the resting stage with a thick wall. Formed from the trophozoite under unfavorable conditions (temperature fluctuations, dryness).
Locomotor organelles
The morphology of locomotor organelles is a key feature used to divide protozoa into classes:
- Pseudopodia (false feet) — cytoplasmic extensions that provide amoeboid movement and phagocytosis.
- Flagella — originate from a basal body (blepharoplast). A flagellum consists of a membrane and an internal microtubular skeleton (nine peripheral pairs and two central pairs). In some parasites, the flagellum connects to the cell body, forming an undulating membrane. Special fibrils may perform a supportive function.
- Cilia — structurally similar to flagella, but differ by being significantly shorter, thinner, and present in large numbers on the cell surface.
Classification of protozoa
Medical microbiology divides the kingdom Protozoa into four main groups based on their mode of locomotion and lifestyle.
1. Amoebae (Sarcodina) Move using pseudopodia. Reproduce asexually (by binary fission). Include both free-living species and harmless commensals (Entamoeba coli), as well as dangerous pathogens. The main representative is Entamoeba histolytica, which causes amoebic dysentery.
2. Flagellates (Mastigophora) Possess one or more flagella. Classified by their parasitic habitat:
- Blood and tissues: Leishmania (Leishmania tropica) with a single flagellum and trypanosomes (Trypanosoma brucei), which cause sleeping sickness and Chagas disease.
- Intestines: Giardia (Lamblia intestinalis).
- Urogenital tract: Trichomonas vaginalis (Trichomonas vaginalis). Possesses four free flagella and one connected to a short undulating membrane.
3. Sporozoans (Sporozoa) Exclusively obligate intracellular parasites. Adult forms lack locomotor organelles. They use an apical complex to invade the host cell. Characterized by a complex life cycle with alternation of hosts and reproductive modes (asexual schizogony and sexual sporogony).
| Localization | Representatives | Disease |
|---|---|---|
| Blood | Plasmodium vivax, Babesia bovis | Malaria, piroplasmosis |
| Tissues and intestines | Toxoplasma gondii, Cryptosporidium parvum | Toxoplasmosis, cryptosporidiosis |
This same group includes the causative agents of sarcocystosis (Sarcocystis), isosporiasis (Isospora), and cyclosporidiosis (Cyclospora).
4. Ciliates (Ciliata) Locomotor organelles are numerous cilia. A typical human pathogen is Balantidium coli, which infects the large intestine and causes balantidial dysentery.
Briefly: answers to topical questions
What specific stages does the life cycle of the malaria parasite include in the human and mosquito organisms?
The life cycle of the malaria parasite includes sexual development in the mosquito and asexual reproduction in the human.
The mosquito organism hosts:
- Sporogony — fusion of male and female gametocytes, formation of a zygote, and development of sporozoites that migrate to the salivary glands.
The human organism hosts the following cycle:
- Tissue schizogony — sporozoites inside hepatocytes pass through trophozoite and schizont stages, forming tissue merozoites.
- Erythrocytic schizogony — merozoites in erythrocytes transform into trophozoites (ring-form, young, semi-mature, mature), then into schizonts, from which erythrocytic merozoites are formed.
- Gametogony — some merozoites transform into immature sex cells (microgametocytes and macrogametocytes).
Which Plasmodium species cause malaria in humans?
Four main Plasmodium species cause malaria in humans.
- Plasmodium falciparum (Plasmodium falciparum) — the causative agent of falciparum malaria, which causes the most severe clinical course.
- Plasmodium vivax (Plasmodium vivax) — the causative agent of tertian malaria.
- Plasmodium ovale (Plasmodium ovale) — the causative agent of ovale malaria (a type of tertian malaria).
- Plasmodium malariae (Plasmodium malariae) — the causative agent of quartan malaria.
Which amoeba species are harmless commensals of the human intestine?
Non-pathogenic commensals of the human large intestine include:
- Entamoeba coli (Entamoeba coli).
- Entamoeba hartmanni (Entamoeba hartmanni).
- Iodamoeba bütschlii (Iodamoeba buetschlii).
A total of 7 amoeba species parasitize the human large intestine; most of them are non-pathogenic. The exception is Entamoeba histolytica (the dysenteric amoeba). Occasionally, non-pathogenic amoebas can provoke disease.
Protozoan classes are easy to remember by their organelles: Amoebas "flow" (pseudopodia), Flagellates "beat" (flagella), Ciliates "shiver" (cilia), and Sporozoans "sleep" inside cells (no locomotor organelles).
Class and exam questions
What is the difference between a trophozoite and a cyst?
A trophozoite is the active, vegetative stage of a protozoan that feeds and moves, but is easily killed in the external environment. A cyst is a resting stage covered by a protective wall that allows the microorganism to survive unfavorable conditions.
What is a blepharoplast?
A blepharoplast is a basal body, a structure in the protozoan cytoplasm from which the flagellum originates. It serves as the microtubule-organizing center of the locomotor apparatus.
What are the features of the sporozoan life cycle using Plasmodium as an example?
In sporozoans, the developmental cycle includes alternation of hosts. For example, the malaria parasite reproduces sexually in the mosquito (definitive host), while in human cells (intermediate host) it undergoes multiple asexual division — schizogony.
What else to study on this topic
- Morphology and pathogenesis of Trichomonas vaginalis
- Life cycle and mechanism of action of the apical complex in sporozoans
- Causative agents of sleeping sickness and Chagas disease (Trypanosoma)
- Balantidial dysentery: features of large intestine lesions
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