protozoa Article Contents: Types of Protozoa Pathogenic Protozoa Also known as: protozoan From: Encyclopedia of Life Science. According to the traditional five-kingdom classification system, the eukaryotic kingdom Protista consists of two major groups: protozoa and algae. Newer classification schemes do not combine all of the organisms that formerly belonged to the kingdom Protista into a single kingdom, and some propose as many as 18. Because classification of the domain Eukarya is in transition and not all biologists agree as to how this should be done, this entry will consider protists to be an informal category encompassing eukaryotic unicellular or colonial organisms that lack true tissues, thus including both protozoa and algae. The major difference between the two is that algae are photosynthetic whereas protozoa are heterotrophic, meaning they must obtain their nutrition from organic molecules in the environment. Though they are unicellular, protozoa exhibit a wide range of strategies for movement, feeding, and other behaviors. Of the 65,000 identified protozoan species, most are free-living and inhabit aquatic environments or moist soil, and only a few are pathogenic to humans. All protozoa can reproduce asexually and most can also undergo some form of sexual reproduction. The average size of a protozoan cell ranges from about 10 to 200 micrometers (µm, 1 µm = 10-6 m) though some are as long as 0.16 inches (4 mm). Protozoa possess all the typical eukaryotic cell structures except chloroplasts. Their cytoplasm consists of the ectoplasm and the endoplasm. The outer ectoplasm functions in feeding, motility, and protection. The endoplasm is the inner portion of cytoplasm that houses the nucleus and other organelles. Some protozoa have unique structures for feeding, while others absorb food directly through their cell membranes. Motility is accomplished through flagella, cilia, or pseudopods. Structures called contractile vacuoles remove excess water from the cell, especially in freshwater species. Many protozoa exhibit a life cycle that includes trophozoite and cyst stages. The trophozoite, or vegetative feeding form, rounds up and becomes immotile if adequate nutrients or moisture are not available. A tough wall forms around the cell, which becomes a dormant, resting structure called a cyst that persists until moisture and nutrient levels are sufficient. When the conditions return to favorable, the cyst wall breaks open and releases an active trophozoite. Some protozoa cannot
form cysts, and therefore are less likely to survive changes in environmental conditions.
Types of Protozoa Due to their tremendous diversity, classification of protozoa is complex. The traditional approach groups organisms together based on common means of motility, rather than evolutionary history or relationships. In fact, molecular data suggests that the evolutionary distance between some organisms grouped together based on motility mechanisms appears to be greater than the evolutionary distance between plants and animals. The Mastigophora, or the flagellates, share the distinguishing characteristic of having one or more flagella. Locomotion occurs when the flagella beat in an undulating motion, with a wave initiating at the junction of the cell body and the flagellum and traveling toward the end of the flagellum. The flagella of some mastigophora are arranged in a unique pattern. Flagellated protozoa that are sessile use their flagella to gather food, rather than to move from one location to another. Because the system for grouping protozoa based on locomotory organelles is not phylogenetic, the flagellates actually consist of two vaguely distinct groups, the plantlike flagellates (Phytomastigophora) and the animal-like flagellates (Zoomastigophora). The plantlike flagellates are considered algae because they have chloroplasts that carry out photosynthesis. Flagellates reproduce asexually by binary fission, in which a cell splits into two genetically identical, equally sized daughter cells, and some also reproduce sexually by syngamy, the fusion of two gametes. Most flagellates are free-living, but some are parasitic—for example, Trypanosoma (a blood pathogen), Giardia (an intestinal parasite), and Trichomonas (causes vaginitis). In a tripartite symbiosis, some flagellates (for example, Trichonympha) live inside the digestive tract of wood-eating termites and digest cellulose from the wood. Living within the flagellates are prokaryotic endosymbionts that probably perform tasks such as nitrogen fixation, ammonium assimilation, and hydrogen metabolism. Members of the phylum Sarcodina, or amoebas, possess unique structures called pseudopodia, meaning false feet, cytoplasmic protrusions that function in locomotion and in gathering food. The pseudopodia of some sarcodines are blunt, lobelike cytoplasmic projections, and others are long, firm, needlelike projections supported by microtubules. Amoebas that are encased in shells extend their pseudopodia through pores. The calcareous shells, or tests, from
amoebas such as foraminifers and radiolarians form layers of chalk deposits in the ocean. To move, an amoeba extrudes a pseudopodium from the cell body, anchors it to a surface, and then pulls the rest of the cell in that direction by contracting its body. When an amoeba senses nearby food such as bacteria, algal cells, or other protozoa, the pseudopodia reach around to envelope the particle, form a vacuole around it, and digest it into nutrients the organism can use. Indigestible material leaves the cell by exocytosis. Most amoebas are free-living, but a few parasitic forms exist, such as Entamoeba histolytica (causes dysentery). Protozoa reproduce by fission and can form cysts. The phylum Ciliophora includes the protozoa, such as Paramecium, that possess hundreds of cilia, short, dense, hairlike structures used for locomotion and feeding. Cilia usually occur in organized rows called kineties but also in tufts. The cilia beat in a coordinated fashion to propel through the environment and to move food particles toward the mouth. Most ciliates produce cysts, have a mouth, are harmless, and contain two types of nuclei: one large macronucleus and several micronuclei. The macronucleus contains thousands of short pieces of deoxyribonucleic acid (DNA) and is involved in regulation of the cell cycle. The diploid micronuclei contain two copies of each chromosome and divide by mitosis. Micronuclei are necessary for sexual reproduction by conjugation, a process during which individuals exchange genetic information but do not increase in numbers. Ciliates reproduce asexually by binary fission. The only ciliate known to be a human pathogen is Balantidium coli, which infects the intestine and causes diarrhea. Members of the protozoan phylum Apicomplexa are parasitic and contain no specialized locomotory structures but sometimes move by bending or creeping. This phylum was formerly called Sporozoa because many members produce spores, but Apicomplexa is preferred since they all have specialized organelles that form an apical complex at their anterior end. Microtubules extend from the apical complex to provide structural support for the cell, and enzymes produced in organelles of the complex assist in penetration of host tissues during infection. Apicomplexans have complicated life cycles that include both asexual and sexual stages and that involve at least one host species. Following sexual reproduction, apicomplexans form thick-walled zygotes called oocysts that undergo meiosis, giving rise to sporozoites, specialized sporelike cells that play a role in the transmission of infection. Asexual reproduction occurs by repeated fissions to form many cells. Human diseases caused by apicomplexans include malaria, caused by Plasmodium; toxoplasmosis, caused by Toxoplasma gondii; and cryptosporidiosis, caused by Cryptosporidium.
Pathogenic Protozoa Though relatively few species of protozoa are pathogenic to humans, the ones that are affect millions of people each year. The four mentioned groups of protozoa all contain species that are pathogenic to humans or animals, but all of the members belonging to the phylum Apicomplexa are parasitic. A common infective amoeba is Entamoeba histolytica, the causative organism of amebiasis and its more severe version, amoebic dysentery. An infected individual can be asymptomatic, or have mild symptoms including diarrhea, stomach aches, and cramping. More severe cases are called ameobic dysentery and involve bloody stools, intense stomach pain, and fever. A person becomes infected by ingesting resistant cysts, which germinate upon entering the small intestine. In the trophozoite stage, the amoebas migrate to the large intestine where they feed, grow, and divide. The trophozoites can penetrate the intestinal lining and travel to the liver, lungs, or brain, but this is rare. Some of the trophozoites in the intestine will form cysts that exit the body in feces. Treatment usually consists of prescribed antibiotics. Lack of sewage treatment facilities, improper treatment of sewage, and poor sanitation practices cause the spread of this disease. The cysts can persist in soil and water or on contaminated foods. Giardiasis and trichomoniasis are examples of diseases caused by flagellates that are common in the United States. Giardiasis is caused by Giardia lamblia, also called Giardia intestinalis, a protozoa that normally inhabits the digestive tract of many animals and even humans. When the organism overgrows, gastroenteritis results. Symptoms include diarrhea, fatigue, bloating, cramping, and flatulence, caused when a thick layer of Giardia coats the lining of the small intestine and prevents nutrient absorption. Many people never show any symptoms and fight the infection without treatment, but some cases require antibiotics. Transmission of Giardia occurs by the fecal-oral route. Eating anything that has come into contact with the organism or swallowing contaminated water (even from recreational sources such as pools or hot tubs) can cause infection. Trichomoniasis is a common sexually transmitted disease that affects the urethra or vagina and can be asymptomatic or can cause painful urination, itching, and discharge. The causative organism is the flagellate Trichomonas vaginalis, and treatment includes antibiotics.
Four different species of Plasmodium, belonging to the phylum Apicomplexa, cause malaria. More than 41 percent of the world's population lives in areas threatened by malaria (parts of Africa, Asia, the Middle East, Central and South America, Hispaniola, and Oceania). After becoming infected, a person develops malaise, nausea, and sometimes diarrhea, followed by chills, fever, and sweating. These symptoms cycle every 48 to 72 hours. Malaria causes anemia from lysis of red blood cells and leads to liver, spleen, and kidney damage resulting from the accumulation of cellular debris. In some types of malaria, vessels in the brain can become blocked, causing death. The life cycle of Plasmodium is complex, involving an asexual stage carried out in a human host and a sexual stage in the Anopheles mosquito. When a female Anopheles mosquito bites a human host, she injects an anticoagulant into the blood before sucking up her meal. If the mosquito is carrying the protozoa, sporozoites are also injected. The sporozoites enter the blood through a capillary and travel to the liver where they asexually reproduce to form thousands of merozoites per cell. After five to 16 days, infected liver cells burst open, releasing the merozoites into the bloodstream. The merozoites invade red blood cells, in which they develop into ring-shaped trophozoites and further asexual reproduction occurs, leading to the infection of more red blood cells.
Life Cycle of Plasmodium vivax
Enlarge Image A pathologist can easily identify the ring-shaped trophozoites in a blood smear under the microscope. Microgametocytes and macrogametocytes form from the merozoites, and persist in the person's bloodstream until another mosquito bites the human host. While feeding, the mosquito ingests both types of gametocytes, and inside the insect host the microgametocytes release spermlike gametes that fertilize the macrogametocytes, creating oocysts. After multiple mitotic divisions, the oocysts release sporozoites that can infect another human host when the mosquito seeks its next blood meal. The use of bed netting dipped in insecticide and the elimination of standing water where mosquitoes can breed are two effective preventative measures for the spread of malaria. Quinine and its
derivatives can be used to treat malaria caused by nonresistant strains of Plasmodium. The type of chemotherapeutic treatment prescribed depends on the strain Plasmodium, its drug-resistant status, the geographical region, the condition of the patient, and any other accompanying illnesses or conditions.
Further Information Cowan, Marjorie Kelly, and Kathleen Park Talaro. Microbiology: A Systems Approach. New York: McGraw Hill, 2006. International Society of Protistologists home page. Available online. URL: http://www.uga.edu/~protozoa. Accessed January 22, 2008. Lee, John J., Gordon F. Leedale, and Phyllis Bradbury, eds. An Illustrated Guide to the Protozoa: Organisms Traditionally Referred to as Protozoa, or Newly Discovered Groups. 2nd ed. 2 vols. Lawrence, Kans.: Society of Protozoologists, 2000. Rohde, Klaus, ed. Marine Parasitology. Wallingford, England: CABI, 2005. Sigee, David C. Freshwater Microbiology: Biodiversity and Dynamic Interactions of Microorganisms in the Aquatic Environment. Hoboken, N.J.: John Wiley & Sons, 2005.
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Citation Information Cullen, Katherine. "protozoa." Science Online. Facts On File, Inc. Web. 3 Feb. 2011. <http://fofweb.com/activelink2.asp? ItemID=WE40&SID=5&iPin=ELS0169&SingleRecord=True>.