The following points highlight the top nine examples of protozoa. The examples are: 1. Giardia 2. Trypanosoma 3. Trichonympha 4. Leishmania 5. Entamoeba 6. Plasmodium 7. Toxoplasma 8. Paramecium 9. Tetrahymena.
Protozoa: Example # 1. Giardia:
The genus belongs to the Phylum Sarcomastigophora, Sub-phylum Mastigophora and class Zoomastigophora. In the classification based on r-RNA homology, the genus is placed in the Archaezoa. The organisms are amitochondriate.
Giardia intestinalis (= Giardia lamblia) is an intestinal parasite causing diarrhoeal diseases in man. It exists in a feeding vegetative form, known as trophozoites or as cysts. The trophozoites measure about 14 μm in length and 7 μm in breadth and have eight flagella and two prominent nuclei (Fig. 5.49). 
There is also a large characteristic sucking organ by which they attach to the intestinal wall. They grow generally in the small intestine of humans and other animals. Cysts are slightly smaller, oval and thick walled. Infection occurs by ingestion of cysts through food and water.
The cysts migrate with ingested food to the small intestine where they produce trophozoites. After 4 to 7 days, the trophozoites are transformed into cysts and are excreted in feces. Thus, Giardia has a simple life-cycle having only two types of growth form — an active trophozoite stage and an inactive cyst stage.
Protozoa: Example # 2. Trypanosoma:
Trypanosomes are flagellated protozoa placed classically in the Zoomastigophora, but similarity of r-RNA homology with Euglenoids shows their affinity. In the phylogenetic classification (Table 5.5), they have been placed in the phylum Euglenozoa.
Trypanosoma spp. are haemoflagellates thriving in the circulatory system of the host. For mobility in the viscous plasma, the cells are provided with an undulating membrane which is externally bordered by a flagellum. There is another flagellum which remains free and attached at the anterior end of an elongated leaf-like cell. Mitochondria are absent and the mitochondrial DNA is located in an organelle, called kinetoplast.
Infections caused by Trypanosoma are known as trypanosomiasis and commonly as sleeping sickness. Two different types of sleeping sickness diseases are recognized — the African sleeping- sickness caused by Trypanosoma brucei and American sleeping sickness caused by Trypanosoma cruzi.
The American type, prevalent in central and south America, is also known as Chagas’ disease (named after the discoverer Carlos Juan Chagas). African sleeping sickness is due to two varieties of T. brucei. One, known as T. brucei gambiense is common in Central Africa and the other T. brucei rhodesiense is restricted to east and south-east Africa.
Trypanosomes pass their life-cycle in two hosts, one a vertebrate mammal (specially human being) and an invertebrate host. Infection of African sleeping sickness in man is caused through bite of tsetse fly (Glossina palpalis).
Infection of Chagas’ disease is spread through bite of so-called “kissing bugs” (Triatoma). In the invertebrate hosts trypanosomes are present as long slender flagellated organisms (Fig. 5.49). In humans, the organisms thrive in blood and cerebrospinal fluid and assume a non-flagellated form.
Protozoa: Example # 3. Trichonympha:
Trichonympha campanula is a multi-flagellate symbiotic protozoan which inhabits the intestines of termites. The protozoa ingest particles and convert cellulose to soluble carbohydrates which are taken up by the termites for their nourishment. The protozoa get in return a safe habitat and food in the form of cellulose present in the wood eaten by the termites. The gross structure of T. campanula is shown in Fig. 5.49.
Protozoa: Example # 4. Leishmania:
Leishmania includes several species of flagellated protozoa belonging to the class Zoomastigophora which cause different types of leishmaniasis in humans. The diseases caused by different species are spread through bites of sand fly (Phlebotomus spp.).
The pathogen exists in two forms in its life-cycle. In the sand fly as well as in culture, the organisms are flagellated elongated structures (promastigote). This is the infective form of leishmanias. In this form they remain in the saliva of sand flies. As they enter through a bite in the skin, they lose their flagella and become ovoid non-flagellate bodies (amastigotes) measuring 2-4 μm x 1-2 μm. They proliferate in the phagocytic leucocytes of the affected person.
Leishmaniasis can be of different types caused by different species of Leishmania. A visceral leishmaniasis, known as kala-azar is a serious type of infection, once widely prevalent in many parts of Asia including India, Africa and also Mediterranean countries. The causal organism is L. donovani.
The pathogen grows in liver and spleen causing their enlargement. L. tropica infects mostly skin causing a lesion. L. brasiliensis and L. Mexicana are the causes of American leishmaniasis. They also affect skin and additionally the mucous membrane. L. brasiliensis causes disfiguration of nose, mouth and throat.
Protozoa: Example # 5. Entamoeba:
Entamoeba belongs to the sub-phylum Sarcodina of the Phylum Sarcomastigophora and in the classification based on r-RNA studies, to the Phylum Rhizopoda. The organisms are non-flagellated amoebae. E. histolytica is the causal agent of amoebiasis in humans. It is an intestinal parasite which is transmitted passively through food and water. Other species, like Entamoeba coli and E. gingivalis are harmless commensal organisms.
The active feeding stage of E. histolytica are known as trophozoites. These measure 15-40 pm in diameter and are motile with the help of pseudopodia. The cells have a wide clear ectoplasm which is more retractile than the more or less homogeneous endoplasm.
There is commonly a single nucleus and the cells in parasitic stage often contain ingested red blood cells (Fig. 5.49). E. histolytica forms thick- walled cysts and the pathogen is transmitted in the form of cysts. Cysts are highly resistant to anti- amoebic drugs and for that reason cause a chronic infection. Each cyst on germination produces several trophozoites.
After entering into the body with food or water, the cysts pass through the gastrointestinal tract until they reach the distal portions of the small intestine and the large intestine. The trophozoites .are formed from the cysts. They possess proteolytic enzymes with the help of which they can penetrate the mucous layers producing lesions. Amoebiasis is a common intestinal infection in the tropical countries.
Protozoa: Example # 6. Plasmodium:
Plasmodia are sporozoa belonging to the Phylum Apicomplexan. Like other genera of sporozoa, they are obligate parasites and the cells are with an apical complex of several organelles. In the mature stage they do not have any locomotion organelles and are non-motile.
Several species of the genus Plasmodium, like P. vivax, P. ovale, P. malariae and P. falciparum are etiological agents of malaria, one of the greatest killer diseases that affects 200 to 300 million people yearly. Plasmodium species have a complex life-cycle involving two widely different hosts. One of these is invariably the female mosquitos belonging to the genus Anopheles and the other, vertebrate mammals including man.
The life-cycle has four important stages — the sporozoite, the merozoite, the gametocyte and the gamete. Plasmodia reproduce both asexually and sexually. Sexual reproduction takes place in the mosquito. Asexual reproduction occurs in the mammalian host. Mosquitoes are known as the definitive host and the mammal as the intermediate host.
Infection in humans is effected through the bite of a female anopheline mosquito carrying the sporozoites in its saliva. The injected sporozoites are carried by the blood stream to the liver cells where each sporozoite produces up to 25,000 merozoites by schizogamy (multiple fission).
The liberated merozoites then infect the red-blood cells and develop into a ring-like structure which enlarges and eventually breaks up to produce numerous merozoites, as well as several gametocytes. The gametocytes present in the blood are sucked in by the mosquito when it bites an infected person.
The gametocytes are of two types, micro- and macro gametocytes, from which motile male gametes and non- motile female gametes are formed. Sexual union takes place in the intestine of mosquitoes resulting in the formation of a diploid zygote.
The zygote is transformed into a thick-walled oocyst. Cell division occurs in the oocyst resulting in the formation of large number of sporozoites which are liberated by rupture of the oocyst wall. They move to the salivary gland of the mosquito and can be injected into another person.
The merozoites liberated by rupture of red blood cells of an infected person can invade new red blood cells to repeat an asexual cycle of multiplication. The merozoites take about 48 to 72 hours to complete the asexual cycle depending on the species of Plasmodium.
When the merozoites are released by rupture of the blood cells, there is also a simultaneous release of toxic compounds which causes the rise of body temperature and other symptoms associated with malarial attack. Of all species of Plasmodium, P. falciparum is the most dangerous and is the cause of ‘malignant’ malaria which is often fatal. The life-cycle of Plasmodium is depicted in Fig. 5.50.
Protozoa: Example # 7. Toxoplasma:
Toxoplasma gondii, the cause of toxoplasmosis is an Apicomplexan sporozoan. It is a widespread parasite of humans and other vertebrates and it spreads through domestic cats. T. gondii is an intracellular parasite capable of existing in three forms — trophozoites, cysts and oocysts.
The trophozoites representing the active form are crescent-shaped (Fig. 5.49) without flagella or cilia. In human, the trophozoites are the active form of the parasite which penetrate into cells, except erythrocytes (red blood cells).
Trophozoites are transformed into cysts within host cells. The cysts are converted to oocysts in the body of the cat by an asexual and sexual process. The oocytes germinate to produce eight sporozoites which give rise to the trophozoites (also called tachyzoites).
In nature, T. gondii remains in the form of cysts and oocysts. These pass into the grazing animals and to humans through consumption of undercooked beef, pork or mutton. Birds and rodents also are infected and the domestic cats may acquire the infection from them.
In humans, the trophozoites develop from cysts or oocysts in the intestines and they penetrate the cells to spread throughout the body via blood stream. Toxoplasmosis develops with fever, sore throat and enlargement of spleen, liver and lymph nodes. The symptoms are usually mild in normal human individuals, but may be dangerous in AIDS patients with impaired immune system.
Protozoa: Example # 8. Paramecium:
Paramecium is a ciliate protozoan belonging to the Phylum Ciliophora. The cells have a characteristic shape, looking like a slipper having a broad anterior rounded end and a comparatively narrower rounded posterior end. The surface (pellicle) is covered by a large number of short cilia arranged in a precise order. All the cilia beat rhythmically to propel the organism in liquid medium.
The cilia also help in collecting solid food particles into the mouth. At the base of each cilium, there is a basal granule situated in the cytoplasm. The basal granules are interconnected by a system of fine fibrils to form a ciliary complex.
The cytoplasm is covered by a flexible pellicle. The organisms feed on bacteria and other microorganisms present in the habitat. The food particles are collected by the cilia and taken up in the cytostome. The cytostome consists of an oral aperture and a mouth cavity. The captured food particles are transferred to food-vacuoles.
These vacuoles can move passively in the cell by cytoplasmic streaming. The food is digested with the help of enzymes and the undigested materials are excreted from the cells through a posteriorly situated cytoproct (an anal opening). The cells also contain contractile vacuoles which store water and increase in size.
After reaching a certain size, the vacuole contracts to discharge the liquid contents through a pore situated in the pellicle. Contractile vacuoles serve to expel excess of water from the cytoplasm, thus functioning as osmoregulatory organelles.
The cytoplasm having a higher osmotic pressure than the surrounding water absorbs water by osmosis. This might lead to bursting of the protozoal cell, unless excess water is actively expelled through contractile vacuoles. Paramecia also contain the other eukaryotic cell organelles, like mitochondria, Golgi bodies, nucleus etc. as well as eukaryotic ribosomes.
A characteristic feature is the presence of two types of nuclei — a macronucleus and one or two micronuclei. The macronucleus is involved in cellular functions, while the micronucleus takes part in reproduction. The pellicle is a more or less firm covering, often variously ridged and sculptured. It may be divided into several layers.
Another feature of Paramecium is the presence of Kappa factors which are nucleic acid particles. These particles are thought to be involved in synthesis of toxins which can kill other ciliates without the kappa factors, thereby eliminating competitors for food.
Paramecium can reproduce asexually by means of transverse binary fission. The nuclei divide and each daughter cell receives one copy of the macronucleus which divides amitotically and a copy of micronucleus which divides by regular mitosis. The cytostome is inherited by one of the daughter cells. The other cell has to form a new cytostome.
Sexual reproduction takes place by conjugation of two paramecia belonging to different mating types. Two cells come together and make contact along their oral grooves. Sexual reproduction is well studied in another ciliate, Tetrahymena, and is described next.
Protozoa: Example # 9. Tetrahymena:
Tetrahymena is a ciliated protozoan belonging to the Phylum Ciliophora. Sexual reproduction of this organism also takes place by conjugation. The process begins when two cells of compatible mating types come in close contact and a cytoplasmic bridge is established between them.
This is followed by division of the micronucleus in each partner. The macronucleus does not take any part in the conjugation process. The micronuclei of the conjugate divide meiotically to form four haploid nuclei in each cell. Of these nuclei, three degenerate, and the remaining nucleus divides mitotically to produce two nuclei. One of these remains within the cell, while the other passes into the partner cell, i.e., both cells exchange one nucleus each.
The exchanged nucleus of each then fuses with the other one present in the cell forming a diploid nucleus in both. After the exchange of nuclei has taken place, the two conjugating cells separate. The single macronucleus in the cells then degenerates. The diploid micronucleus then divides three times by mitosis to form eight nuclei.
Three of the eight nuclei degenerate. One functions as the micronucleus of the Tetrahymena cell, and the remaining four nuclei form a macronucleus. Thus, each cell has one micronucleus and one macronucleus. The protozoan then divides by binary fission to produce progenies, each of which has a diploid micronucleus and a polyploid macronucleus. Through sexual reproduction genetic materials are exchanged between two compatible partners.
The conjugation process of Tetrahymena is diagrammatically represented in Fig. 5.51:
