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Term Paper on Protozoa


Term Paper # 1. Origin of Protozoa:

April 12, 1993 should have been a festive day in Milwaukee, Wisconsin. The baseball home opener was scheduled for that day, and fans were eager to see the Brewers play the California Angels. But the scoreboard contained an ominous message – “For your safety, no city of Milwaukee water is being used in any concession item.” The city was in the throes of an epidemic, and a protozoan was to blame.

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The protozoan was Cryptosporidium coccidi, an intestinal parasite that causes mild to serious diarrhea, especially in infants and the elderly. As the protozoa attach themselves to the intestinal lining, they mature, reproduce, and encourage the body to release large volumes of fluid. The infection is accompanied by abdominal cramps, extensive water loss, and in many cases, vomiting and fever.

Even as the first ball was being thrown out at the stadium, health inspectors were checking Milwaukee’s two water purification plants to see how a protozoan could be pumped into the city’s water supply. Cryptosporidium is a waterborne parasite commonly found in the intestines of cows and other animals.

Perhaps, they guessed, the heavy rain and spring thaw had washed the protozoan from farm pastures and barns into the Milwaukee River. The river might have brought Cryptosporidium into Lake Michigan from which the city drew its water. Indeed, the mouth of the river was unusually close to the intake pipe from the lake.

Moreover, they added, Cryptosporidium can resist the chlorine treatment used to control bacteria in water; and the tests to detect bacterial contamination do not detect protozoa, such as Cryptosporidium.

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As researchers worked to unravel the mystery, the game went on. Soda was available, but only from bottles. Drinking fountains were turned off. Two huge U.S. Army water tanks stood by to provide a reserve for the 50,000 fans in attendance. And in the city, tens of thousands of Milwaukeeans made the mildly embarrassing trip to the drug store to stock up on toilet paper and antidiarrheal medications. Back at the ballgame, things were not going much better-the Brewers lost to the Angels 12 to 5.

Cryptosporidium coccidi will be one of the protozoa. We shall encounter other protozoa that infect the human intestine, as well as several protozoa that live primarily in the blood and other organs of the body.

Many of the diseases we encounter (for example, malaria) will have familiar names, but others, such as Cryptosporidium infections, are emerging diseases in our society (indeed, Cryptosporidium was not known to infect humans before 1976). Our study will begin with a focus on the characteristics of protozoa.


Term Paper # 2. Characteristics of Protozoa:

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Protozoa are a group of about 30,000 species of single-celled organisms. They take their name from the Greek words protos and zoon, which translate literally to “first animal.” This name refers to the position many biologists believe protozoa occupy in the evolution of living things.

Though often studied by zoologists, protozoa are also of interest to microbiologists because they are unicellular, have a microscopic size, and are involved in disease. The discipline of parasitology is generally concerned with the medically related protozoa and multicellular parasites.


Term Paper # 3. Structure and Growth of Protozoa:

Protozoa are among the largest organisms encountered in microbiology, some forms reaching the size of the period at the end of this sentence. With only a few exceptions, protozoa have no chlorophyll in their cytoplasm and thus cannot produce carbohydrates by photosynthesis. Although each protozoan is composed of a single cell, the functions of that cell bear a resemblance to the functions of multicellular animals rather than to an isolated cell from that animal.

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Most protozoa are free-living and thrive where there is water. They may be located in damp soil and mud, in drainage ditches and puddles, and in ponds, rivers, and oceans. Some species of protozoa remain attached to aquatic plants or rocks, while other species swim about. The film of water on an ordinary dirt particle often contains protozoa.

Protozoal cells are sur­rounded only by a membrane. However, outside the membrane, some species of protozoa possess a rigid structure called a pellicle. The cytoplasm contains eukaryotic features, each cell having a nucleus and nuclear membrane. In addition, fresh­water protozoa continually take in water by the process of osmosis and eliminate it via organelles called contractile vacuoles.

These vacuoles expand with water drawn from the cytoplasm and then appear to “contract” as they release water through a temporary opening in the cell membrane. Many protozoa also contain locomotor organelles, which permit independent motion.

Protozoa obtain their nutrients by engulfing food particles by phagocytosis or through special organs of ingestion. A membrane then encloses the particles to form an organelle called a food vacuole. The vacuole joins with another organelle known as the lysosome, and digestive enzymes from the lysosome proceed to break down the particles. Nutrients are absorbed from the vacuole, and the remaining material is eliminated from the cell.

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Nutrition in protozoa is primarily heterotrophic, since chlorophyll pigments are generally lacking. Except for the parasitic organisms of disease and the species that feed on bacteria, protozoa are saprobic. All protozoa are aerobic, obtaining their oxygen by diffusion through the cell membrane.

The feeding form of a protozoan is commonly known as the trophozoite (troph- is the Greek stem for food). Another form, the cyst, is a dormant, highly resistant stage that develops in some protozoa when the organism secretes a thick case around itself during times of environmental stress.

Reproduction in protozoa usually occurs by the asexual process of mitosis, although many protozoa also have a sexual stage. Whittaker’s classification scheme places the organisms together with certain algae in the kingdom Protista.

Within the kingdom, further classification tends to be controversial although three phyla are generally recognized – Sarcomastigophora (the ameboid and flagellated protozoa). Ciliophora (the ciliated protozoa), and Apicomplexa (the sporozoan protozoa).

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Within these three phyla are four major groups, which we shall refer to as “classes” (although general agreement may be lacking on this concept). The classes are distinguished from one another by how the members of the class move.


Term Paper # 4. Classifications of Protozoa:

1. Sarcodina:

Sarcodina is the class of amoeboid protozoa. These organisms move as their cell contents flow into temporary formless projections called pseudopodia (“false-feet”). The amoeba is the classic example of the group, and thus the motion is called amoeboid motion. Pseudopodia also capture small algae and other protozoa in the process of phagocytosis.

An amoeba may be as large as a millimeter in diameter. It usually lives in fresh water and reproduces by binary fission. Amoebas may be found in home humidifiers, where they have been known to cause an allergic reaction called humidifier fever. Far more serious are the parasitic amebas that cause amoebiasis and a form of encephalitis.

Two large groups of marine amebas are included in the class Sarcodina. The first group, the radiolaria, are abundant in the Indian and Pacific oceans. These amebas have spherical shells with highly sculptured glassy skeletons, reminiscent of vintage Christmas ornaments.

When the protozoa die, their skeletal remains litter the ocean floor with deposits called radiolarian ooze. The second group, the foraminifera, have-chalky skeletons, often in the shape of snail shells with openings between sections (“foraminifera” means “little window”). Foraminifera flourished during the Paleozoic era about 225 million years ago.

Their shells in ocean sediments therefore serve as depth markers for oil drilling rigs and as estimates of the age of the rock. Geologic up-thrust has brought the sediments to the surface in several places around the world such as the White Cliffs of Dover.

Reports first published in 1987 indicated that amoebas in the genus Acanthamoeba (e.g., A. castellani) can cause corneal infection in persons who wear contact lenses. Wearers were reminded to adhere to recommended lens wear and care procedures, and ophthalmologists and optometrists were advised to increase patient education. Recent research indicates that bacterial infection can be a cofactor in Acanthamoeba infection of the eye.

2. Mastigophora:

Protozoa of the class Mastigophora often have the shape of a vase. All members move by means of one or more whip-like, undulating flagella (mastig- is Greek for whip). The flagellum can either push or pull the organism, depending on the species. Flagella occur singly, in pairs, or in large numbers.

Each flagellum has the characteristic 9 + 2 arrange­ment of microtubules found in all eukaryotic flagella. Undu­lations sweep down the flagella to the tip, and the lashing motion forces water outward to provide locomotion. The movement resembles the activity of a fish sculling in water. Flagella also occur in bacteria but their structure, size, and type of movement differ.

Almost half the known species of protozoa are classified as Mastigophora. An example is the green flagellate Euglena often found in freshwater ponds. This organism is unique because it is one of the few types of protozoa that contain chloroplasts with chlorophyll, and is thus capable of photosynthesis.

Some botanists claim it to be a plant, but zoologists point to its ability to move and suggest that it is more animal-like. Still other biologists point out that it may be the basic stock of evolution from which animal and plant forms once arose.

Some species of flagellated protozoa are free-living, but most live together with plants or animals. Several species, for example, are found in the gut of the termite, where they participate in a symbiotic relationship. Other species are parasitic in humans and cause disease of the nervous, urogenital, or gastrointestinal systems.

3. Ciliophora:

Members of the class Ciliophora are among the most complex cells on Earth. They range in size from a microscopic 10 μm to a huge 3 mm (about the same relative difference between a football and a football field). All members are covered with hair like cilia (sing., cilium) in longitudinal or spiral rows.

The movement of the cilia is coordinated by a network of fibers running beneath the surface of the cell. Cilia beat in a synchronized pattern much like a field of wheat bending in the breeze or the teeth on a comb when the thumb is passed down the row. The organized rowing action that results speeds the ciliate along in one direction. By contrast, flagellar motion tends to be jerky and much slower.

The complexity of ciliates is illustrated by the slipper-shaped Paramecium. This organism has a primitive gullet, as well as a “mouth” into which food particles are swept, a single large macronucleus, and one or more micronuclei. During sexual conjugations, two cells make contact and a cytoplasmic bridge forms between them.

A micronucleus from each cell undergoes two divisions to form four micronuclei, of which one remains alive and undergoes division. Now a “swapping” of micronuclei takes place followed by a union to re-form the normal micronucleus.

This genetic recombination is somewhat analogous to that occurring in bacteria. It is observed during periods of environmental stress, a factor that suggests the formation of a genetically different and perhaps, better adapted organism. Reproduction at other times is by mitosis.

Another feature of Paramecium is the kappa factors. These nucleic acid particles appear responsible for the synthesis of toxins that destroy ciliates lacking the factors. Paramecium species also possess trichocysts, organelles that discharge filaments to trap prey.

A third feature is the contractile vacuole used to “bail out” excess water from the cytoplasm. These organelles are present in freshwater ciliates but not in saltwater species because little excess water exists in the cells.

Ciliates have been the subject of biological investigation for many decades. They are readily found in almost any pond or gutter water – they have a variety of shapes; they exist in several colours, including light blue and pink; they exhibit elaborate and controlled behaviour patterns; and they have simple nutritional requirements, which makes cultivation easy.

4. Sporozoa:

The class Sporozoa includes a number of parasitic protozoa with complex life cycles that include alternating sexual and asexual reproductive phases. These life cycles include intermediary forms that resemble bacterial or fungal spores, a factor from which the class takes its name. However, the spores lack the resistance of other spores, and so the name sporozoa is probably a misnomer.

Sporozoa are notable for the absence of locomotor organelles in the adult form. Two members of the class, the organisms of malaria and toxoplasmosis, are of special significance, the first because it is one of the most prolific killers of humans, the second because of its association with the disease AIDS.

Other notable members of the class include Isospora belli, a cause of the human intestinal disease coccidiosis, and Sarcocystis species, which live in the intestine as well as the muscle tissue of humans and animals.