Cousetive Agent Of Cilia

The phylum Ciliophora, or the ciliates, includes organisms that have cilia. The cilia are similar in construction to the flagella and usually completely cover the surface of an organism. Most often, they are arranged in distinct rows and are connected to one another by fibrils known as kinetodesma. Cilia beat in a coordinated fashion in waves across the body of the protozoan. A beat of one cilium affects the cilia immediately around it, but there is no evidence that the connecting fibrils aid in this coordination. The cilia found near the oral cavity propel food into the opening. Paramecia are members of the Ciliophora. Balantidium coli is the only known ciliate to cause human disease. It produces ulcers in the large intestines, and pigs are its major reservoir.

Organisms in the phylum Apicomplexa, also referred to as sporozoa, cause some of the most serious protozoan diseases of humans. Malaria is caused by any of four Plasmodium species. It is transmitted by the female Anopheles mosquito. Cats are the primary host for Toxoplasma gondii, with humans serving as secondary hosts. Another Apicomplexa is Cryptosporidium parvum, which causes the diarrheal disease cryptosporidiosis. ■ malaria, p. 731 ■ toxoplasmosis, p. 755 ■ cryptosporidiosis, p. 626

The phylum Microspora includes the intracellular protozoa that infect immunocompromised humans, especially persons with AIDS. There are other protozoan phyla such as Labyrinthomorpha, Ascetospora, and Myxozoa, but they are not implicated in human disease and so we will not consider them here. They are most often found in marine habitats and are parasitic on fish and other sea life.

Protozoan Habitats

A majority of protozoa are free-living and found in marine, freshwater, or terrestrial environments. They are essential as decomposers in many ecosystems. Some species, however, are parasitic, living on or in other host organisms. The hosts for protozoan parasites range from simple organisms, such as algae, to complex vertebrates, including humans. All protozoa require large amounts of moisture, no matter what their habitat.

In marine environments, protozoa make up part of the zooplankton, where they feed on the algae of the phytoplank-ton and are an important part of the aquatic food chains. On land, protozoa are abundant in soil as well as in or on plants and animals. Specialized protozoan habitats include the guts of termites, roaches, and ruminants such as cattle.

Protozoa are an important part of the food chain. They eat bacteria and algae and, in turn, serve as food for larger species. The protozoa help maintain an ecological balance in the soil by devouring vast numbers of bacteria and algae. For example, a single paramecium can ingest as many as 5 million bacteria in one day. Protozoa are important in sewage disposal because most of the nutrients they consume are metabolized to carbon dioxide and water, resulting in a large decrease in total sewage solids. ■ sewage treatment, p. 786

Structure of Protozoa

Cell Wall

Protozoa lack the rigid cellulose cell wall found in algae. Most protozoa do, however, have a specific shape determined by the rigidity or flexibility of the material lying just beneath the plasma membrane. Foraminifera have distinct hard shells composed of silicon or calcium compounds (figure 12.8). The foraminiferans, which secrete a calcium shell, have through the course of millions of years formed limestone deposits such as the white cliffs of Dover on England's southern coast.

306 Chapter 12 The Eukaryotic Members of the Microbial World

306 Chapter 12 The Eukaryotic Members of the Microbial World

Figure 12.8 A Group of Protozoa with Hard Silicon Shells

Eukaryotic Cell Structures

Protozoa are eukaryotic organisms and as such have a membrane-bound nucleus as well as other membrane-bound organelles such as mitochondria. Protozoa are not photosynthetic and thus lack chloroplasts. ■ eukaryotic organelles, p. 75

Protozoa have specialized structures for movement such as cilia, flagella, or pseudopodia. As described in chapter 3, eukaryotic flagella and cilia are distinctly different in construction from prokaryotic flagella (see figures 3.40 and 3.54). Protozoa are grouped by their mode of locomotion. For example, the Mastigophora have flagella, and Ciliophora have cilia during at least some part of their life cycle, and Sarcodina use pseudopodia for movement (see table 12.2). ■ flagella, p. 74

Feeding in Protozoa

Since protozoa live in an aquatic environment, water, oxygen, and other small molecules readily diffuse through the cell membrane. In addition, as described in chapter 3, protozoa use either pinocy-tosis or phagocytosis to obtain food and water (see figure 3.50).

Protozoan Reproduction

The life cycles of protozoa are sometimes complex, involving more than one habitat or host. Morphologically distinct forms of a single protozoan species can be found at different stages of the life cycle. Such organisms are said to be polymorphic (figure 12.9). This polymorphism is comparable in some respects to the differentiation of various cell types that form plant and animal tissues.

The ability to exist in either a trophozoite (vegetative or feeding form) or cyst (resting form) is characteristic of many protozoa. Certain environmental conditions, such as the lack of nutrients, moisture, oxygen, low temperature, or the presence of toxic chemicals may trigger the development of a protective cyst wall within which the cytoplasm becomes dormant. Cysts provide a means for the dispersal and survival of protozoa under adverse conditions and can be compared to the bacterial endospore. Protozoan cysts, however, are not as resistant to heat and other adverse conditions as are bacterial endospores. When the cyst encounters a favorable environment, the trophozoite emerges. Thus, a number of parasitic protozoa are disseminated to new hosts during their cyst stage. ■ endospores, p. 67

Figure 12.9 Polymorphism in a Protozoan The species of Naegleria may infect humans. (a) In human tissues, the organism exists in the form of an ameba (10-11 mm at its widest diameter). (b) After a few minutes in water, the flagellate form appears. (c) Under adverse conditions, a cyst is formed.

Figure 12.9 Polymorphism in a Protozoan The species of Naegleria may infect humans. (a) In human tissues, the organism exists in the form of an ameba (10-11 mm at its widest diameter). (b) After a few minutes in water, the flagellate form appears. (c) Under adverse conditions, a cyst is formed.

Both asexual and sexual reproduction are common in protozoa and may alternate during the complicated life cycle of some organisms. Binary fission takes place in many groups of protozoa (figure 12.10). In the flagellates, it usually occurs longitudinally, and in the ciliates, it occurs transversely. Since some protozoa possess both cilia and flagella, their method of asexual reproduction determines in which group they are classified.

Some protozoa divide by multiple fissions, or schizogony, in which the nucleus divides a number of times and then the cell produces many small single-celled organisms, each one capable of infection. Multiple fission of the asexual forms in the human host results in large numbers of parasites released into the host's circulation at regular intervals, producing the characteristic cyclic symptoms of malaria. A more detailed account of this process is described in chapter 28.

Protozoa and Human Disease

The major threat posed by protozoa results from their ability to parasitize and often kill a wide variety of animal hosts. Human infections with the protozoans Toxoplasma gondii, which causes toxoplasmosis, Plasmodium species, which are responsible for malaria, Trypanosoma species, which cause sleeping sickness, and Trichomonas vaginalis, which causes vaginitis, are common in many parts of the world. Malaria has been one of the greatest

Figure 12.10 Various Forms of Asexual Reproduction in Protozoa

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Responses

  • loretta
    Can humans become infected and a host to ascetospora ?
    3 years ago

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