Protozoa Cell Wall Made Of

General Concepts

Protozoa

Protozoa are one-celled animals constitute worldwide in most habitats. Most species are free living, just all higher animals are infected with one or more species of protozoa. Infections range from asymptomatic to life threatening, depending on the species and strain of the parasite and the resistance of the host.

Structure

Protozoa are microscopic unicellular eukaryotes that have a relatively complex internal structure and deport out complex metabolic activities. Some protozoa have structures for propulsion or other types of movement.

Classification

On the basis of light and electron microscopic morphology, the protozoa are currently classified into half dozen phyla. Most species causing human illness are members of the phyla Sacromastigophora and Apicomplexa.

Life Wheel Stages

The stages of parasitic protozoa that actively feed and multiply are frequently called trophozoites; in some protozoa, other terms are used for these stages. Cysts are stages with a protective membrane or thickened wall. Protozoan cysts that must survive outside the host usually take more resistant walls than cysts that form in tissues.

Reproduction

Binary fission, the most common course of reproduction, is asexual; multiple asexual division occurs in some forms. Both sexual and asexual reproduction occur in the Apicomplexa.

Diet

All parasitic protozoa crave preformed organic substances—that is, diet is holozoic as in higher animals.

Introduction

The Protozoa are considered to be a subkingdom of the kingdom Protista, although in the classical system they were placed in the kingdom Animalia. More than 50,000 species have been described, most of which are free-living organisms; protozoa are establish in almost every possible habitat. The fossil record in the course of shells in sedimentary rocks shows that protozoa were nowadays in the Pre-cambrian era. Anton van Leeuwenhoek was the starting time person to see protozoa, using microscopes he constructed with simple lenses. Between 1674 and 1716, he described, in improver to free-living protozoa, several parasitic species from animals, and Giardia lamblia from his own stools. Almost all humans take protozoa living in or on their torso at some fourth dimension, and many persons are infected with i or more than species throughout their life. Some species are considered commensals, i.e., normally non harmful, whereas others are pathogens and usually produce disease. Protozoan diseases range from very mild to life-threatening. Individuals whose defenses are able to control but not eliminate a parasitic infection become carriers and constitute a source of infection for others. In geographic areas of high prevalence, well-tolerated infections are often not treated to eradicate the parasite because eradication would lower the individual's amnesty to the parasite and result in a high likelihood of reinfection.

Many protozoan infections that are inapparent or mild in normal individuals tin can be life-threatening in immunosuppressed patients, particularly patients with caused allowed deficiency syndrome (AIDS). Evidence suggests that many good for you persons harbor depression numbers of Pneumocystis carinii in their lungs. Nevertheless, this parasite produces a frequently fatal pneumonia in immunosuppressed patients such as those with AIDS. Toxoplasma gondii, a very common protozoan parasite, usually causes a rather mild initial illness followed by a long-lasting latent infection. AIDS patients, notwithstanding, can develop fatal toxoplasmic encephalitis. Cryptosporidium was described in the 19th century, but widespread human infection has only recently been recognized. Cryptosporidium is another protozoan that tin produce serious complications in patients with AIDS. Microsporidiosis in humans was reported in only a few instances prior to the appearance of AIDS. It has at present become a more common infection in AIDS patients. Every bit more thorough studies of patients with AIDS are made, it is likely that other rare or unusual protozoan infections volition be diagnosed.

Acanthamoeba species are free-living amebas that inhabit soil and h2o. Cyst stages can be airborne. Serious heart-threatening corneal ulcers due to Acanthamoeba species are beingness reported in individuals who utilise contact lenses. The parasites presumably are transmitted in contaminated lens-cleaning solution. Amebas of the genus Naegleria, which inhabit bodies of fresh h2o, are responsible for almost all cases of the usually fatal disease chief amebic meningoencephalitis. The amebas are thought to enter the trunk from water that is splashed onto the upper nasal tract during swimming or diving. Man infections of this type were predicted before they were recognized and reported, based on laboratory studies of Acanthamoeba infections in cell cultures and in animals.

The lack of effective vaccines, the paucity of reliable drugs, and other issues, including difficulties of vector control, prompted the World Health Organization to target six diseases for increased research and training. Three of these were protozoan infections—malaria, trypanosomiasis, and leishmaniasis. Although new information on these diseases has been gained, virtually of the problems with command persist.

Structure

Most parasitic protozoa in humans are less than 50 μm in size. The smallest (mainly intracellular forms) are 1 to ten μm long, but Balantidium coli may measure 150 μm. Protozoa are unicellular eukaryotes. As in all eukaryotes, the nucleus is enclosed in a membrane. In protozoa other than ciliates, the nucleus is vesicular, with scattered chromatin giving a diffuse appearance to the nucleus, all nuclei in the individual organism appear alike. One type of vesicular nucleus contains a more or less central body, called an endosome or karyosome. The endosome lacks DNA in the parasitic amebas and trypanosomes. In the phylum Apicomplexa, on the other mitt, the vesicular nucleus has one or more than nucleoli that contain Deoxyribonucleic acid. The ciliates have both a micronucleus and macronucleus, which appear quite homogeneous in limerick.

The organelles of protozoa have functions similar to the organs of higher animals. The plasma membrane enclosing the cytoplasm also covers the projecting locomotory structures such every bit pseudopodia, cilia, and flagella. The outer surface layer of some protozoa, termed a pellicle, is sufficiently rigid to maintain a distinctive shape, as in the trypanosomes and Giardia. However, these organisms tin can readily twist and bend when moving through their environs. In well-nigh protozoa the cytoplasm is differentiated into ectoplasm (the outer, transparent layer) and endoplasm (the inner layer containing organelles); the structure of the cytoplasm is most easily seen in species with projecting pseudopodia, such as the amebas. Some protozoa have a cytosome or cell "oral cavity" for ingesting fluids or solid particles. Contractile vacuoles for osmoregulation occur in some, such as Naegleria and Balantidium. Many protozoa take subpellicular microtubules; in the Apicomplexa, which have no external organelles for locomotion, these provide a means for boring movement. The trichomonads and trypanosomes have a distinctive undulating membrane between the trunk wall and a flagellum. Many other structures occur in parasitic protozoa, including the Golgi apparatus, mitochondria, lysosomes, food vacuoles, conoids in the Apicomplexa, and other specialized structures. Electron microscopy is essential to visualize the details of protozoal structure. From the point of view of functional and physiologic complication, a protozoan is more like an animal than like a unmarried cell. Figure 77-1 shows the construction of the bloodstream grade of a trypanosome, as determined by electron microscopy.

Figure 77-1

Fine structure of a protozoan parasite, Typanosoma evansi, as revealed by transmission electron microcopy of thin sections. (Adapted from Vickerman 1000: Protozoology. Vol. 3 London Schoolhouse of Hygiene and Tropical Medicine, London, 1977, with permission.) (more than...)

Classification

In 1985 the Society of Protozoologists published a taxonomic scheme that distributed the Protozoa into six phyla. Two of these phyla—the Sarcomastigophora and the Apicomplexa--contain the most important species causing human disease. This scheme is based on morphology equally revealed past light, electron, and scanning microscopy. Dientamoeba fragilis, for example, had been thought to exist an ameba and placed in the family Entamoebidae. However, internal structures seen by electron microscopy showed that it is properly placed in the order Trichomonadida of flagellate protozoa. In some instances, organisms that appear identical under the microscope have been assigned unlike species names on the footing of such criteria as geographic distribution and clinical manifestations; a expert example is the genus Leishmania, for which subspecies names are oft used. Biochemical methods have been employed on strains and species to make up one's mind isoenzyme patterns or to identify relevant nucleotide sequences in RNA, Deoxyribonucleic acid, or both. Extensive studies have been made on the kinetoplast, a unique mitochondrion establish in the hemoflagellates and other members of the order Kinetoplastida. The DNA associated with this organelle is of great interest. Cloning is widely used in taxonomic studies, for example to study differences in virulence or disease manifestations in isolates of a single species obtained from different hosts or geographic regions. Antibodies (particularly monoclonal antibodies) to known species or to specific antigens from a species are being employed to place unknown isolates. Eventually, molecular taxonomy may prove to exist a more reliable basis than morphology for protozoan taxonomy, but the microscope is still the well-nigh applied tool for identifying a protozoan parasite. Tabular array 77-ane lists the medically important protozoa.

Tabular array 77-1

Classification of Parasitic Protozoa and Associated Diseases.

Life Cycle Stages

During its life cycle, a protozoan generally passes through several stages that differ in structure and activeness. Trophozoite (Greek for "fauna that feeds") is a general term for the active, feeding, multiplying stage of almost protozoa. In parasitic species this is the phase usually associated with pathogenesis. In the hemoflagellates the terms amastigote, promastigote, epimastigote, and trypomastigote designate trophozoite stages that differ in the absence or presence of a flagellum and in the position of the kinetoplast associated with the flagellum. A variety of terms are employed for stages in the Apicomplexa, such every bit tachyzoite and bradyzoite for Toxoplasma gondii. Other stages in the complex asexual and sexual life cycles seen in this phylum are the merozoite (the form resulting from fission of a multinucleate schizont) and sexual stages such equally gametocytes and gametes. Some protozoa form cysts that contain ane or more than infective forms. Multiplication occurs in the cysts of some species and so that excystation releases more than one organism. For case, when the trophozoite of Entamoeba histolytica starting time forms a cyst, it has a single nucleus. Every bit the cyst matures nuclear division produces iv nuclei and during excystation 4 uninucleate metacystic amebas appear. Similarly, a freshly encysted Giardia lamblia has the aforementioned number of internal structures (organelles) as the trophozoite. Nevertheless, as the cyst matures the organelles double and two trophozoites are formed. Cysts passed in stools have a protective wall, enabling the parasite to survive in the outside environment for a period ranging from days to a year, depending on the species and environmental conditions. Cysts formed in tissues do not usually have a heavy protective wall and rely upon carnivorism for manual. Oocysts are stages resulting from sexual reproduction in the Apicomplexa. Some apicomplexan oocysts are passed in the feces of the host, but the oocysts of Plasmodium, the agent of malaria, develop in the body crenel of the mosquito vector.

Reproduction

Reproduction in the Protozoa may be asexual, equally in the amebas and flagellates that infect humans, or both asexual and sexual, as in the Apicomplexa of medical importance. The most common blazon of asexual multiplication is binary fission, in which the organelles are duplicated and the protozoan then divides into two complete organisms. Segmentation is longitudinal in the flagellates and transverse in the ciliates; amebas have no apparent anterior-posterior axis. Endodyogeny is a grade of asexual division seen in Toxoplasma and some related organisms. 2 daughter cells grade within the parent cell, which then ruptures, releasing the smaller progeny which abound to total size before repeating the procedure. In schizogony, a common form of asexual sectionalization in the Apicomplexa, the nucleus divides a number of times, and so the cytoplasm divides into smaller uninucleate merozoites. In Plasmodium, Toxoplasma, and other apicomplexans, the sexual bicycle involves the production of gametes (gamogony), fertilization to form the zygote, encystation of the zygote to grade an oocyst, and the germination of infective sporozoites (sporogony) inside the oocyst.

Some protozoa have complex life cycles requiring two different host species; others require simply a single host to complete the life wheel. A unmarried infective protozoan entering a susceptible host has the potential to produce an immense population. Nevertheless, reproduction is express by events such as death of the host or by the host's defense mechanisms, which may either eliminate the parasite or residuum parasite reproduction to yield a chronic infection. For example, malaria tin result when only a few sporozoites of Plasmodium falciparum—perchance 10 or fewer in rare instances—are introduced by a feeding Anopheles mosquito into a person with no immunity. Repeated cycles of schizogony in the bloodstream can upshot in the infection of x percent or more than of the erythrocytes—about 400 meg parasites per milliliter of claret.

Nutrition

The nutrition of all protozoa is holozoic; that is, they require organic materials, which may exist particulate or in solution. Amebas engulf particulate food or droplets through a sort of temporary mouth, perform digestion and absorption in a food vacuole, and eject the waste substances. Many protozoa have a permanent mouth, the cytosome or micropore, through which ingested food passes to get enclosed in nutrient vacuoles. Pinocytosis is a method of ingesting nutrient materials whereby fluid is drawn through small, temporary openings in the body wall. The ingested material becomes enclosed inside a membrane to form a food vacuole.

Protozoa accept metabolic pathways like to those of higher animals and crave the same types of organic and inorganic compounds. In recent years, significant advances have been made in devising chemically divers media for the in vitro tillage of parasitic protozoa. The resulting organisms are gratuitous of various substances that are present in organisms grown in complex media or isolated from a host and which can interfere with immunologic or biochemical studies. Research on the metabolism of parasites is of immediate interest considering pathways that are essential for the parasite merely not the host are potential targets for antiprotozoal compounds that would block that pathway but be condom for humans. Many antiprotozoal drugs were used empirically long earlier their mechanism of action was known. The sulfa drugs, which block folate synthesis in malaria parasites, are 1 instance.

The rapid multiplication rate of many parasites increases the chances for mutation; hence, changes in virulence, drug susceptibility, and other characteristics may accept identify. Chloroquine resistance in Plasmodium falciparum and arsenic resistance in Trypanosoma rhodesiense are two examples.

Contest for nutrients is not normally an important factor in pathogenesis because the amounts utilized by parasitic protozoa are relatively small. Some parasites that inhabit the small-scale intestine can significantly interfere with digestion and assimilation and bear on the nutritional status of the host; Giardia and Cryptosporidium are examples. The destruction of the host'southward cells and tissues every bit a effect of the parasites' metabolic activities increases the host'southward nutritional needs. This may be a major gene in the outcome of an infection in a malnourished individual. Finally, extracellular or intracellular parasites that destroy cells while feeding tin lead to organ dysfunction and serious or life-threatening consequences.

References

Englund PT, Sher A (eds): The Biology of Parasitism. A Molecular and Immunological Approach. Alan R. Liss, New York, 1988 .
Goldsmith R, Heyneman D (eds): Tropical Medicine and Parasitology. Appleton and Lange, Eastward Norwalk, CT, 1989 .
Lee JJ, Hutner SH, Bovee EC (eds): An Illustrated Guide to the Protozoa. Guild of Protozoologists, Lawrence, KS, 1985 .
Kotler DP, Orenstein JM. Prevalence of Intestinal Microsporidiosis in HIV-infected individuals referred for gastrointestinal evaluation. J Gastroenterol. 1994;89:1998. [PubMed: 7942725]
Neva FA, Brown H: Basic Clinical Parasitology, 6th edition, Appleton & Lange, Norwalk, CT, 1994 .