Protozoa: Structure, Classification, Growth, and Development
General Concepts
Protozoa
Protozoa
are one-celled animals found worldwide in most habitats. Most species are free
living, but 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 carry out complex metabolic activities. Some protozoa have
structures for propulsion or other types of movement.
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Classification
On the basis of light and electron microscopic
morphology, the protozoa are currently classified into six phyla. Most species causing human disease are members of the
phyla Sacromastigophora and Apicomplexa.
Life Cycle
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 have
more resistant walls than cysts that form in tissues.
Reproduction
Binary fission, the most common form of reproduction, is asexual;
multiple asexual division occurs in some forms. Both
sexual and asexual reproduction occur in the Apicomplexa.
Nutrition
All parasitic protozoa require preformed
organic substancesthat is, nutrition 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 found in almost every
possible habitat. The fossil record in the form of shells in sedimentary rocks
shows that protozoa were present in the Pre-cambrian era. Anton van Leeuwenhoek
was the first person to see protozoa, using microscopes he constructed with
simple lenses. Between 1674 and 1716, he described, in addition to free-living
protozoa, several parasitic species from animals, and Giardia lamblia from his own stools. Virtually
all humans have protozoa living in or on their body at some time, and many
persons are infected with one or more species throughout their life. Some
species are considered commensals,
i.e., normally not 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 immunity to the parasite and
result in a high likelihood of reinfection.
Many protozoan infections that are inapparent
or mild in normal individuals can be life-threatening
in immunosuppressed patients, particularly patients with acquired immune
deficiency syndrome (AIDS). Evidence suggests that many healthy persons harbor
low numbers of Pneumocystis carinii in their lungs. However, 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, however, 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 can 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 now become a more common infection in AIDS
patients. As more thorough studies of patients with AIDS are made, it is likely
that other rare or unusual protozoan infections will be diagnosed.
Acanthamoeba species are free-living amebas that inhabit soil and
water. Cyst stages can be airborne. Serious eye-threatening corneal ulcers due
to Acanthamoeba species are being reported in individuals who use
contact lenses. The parasites presumably are transmitted in contaminated
lens-cleaning solution. Amebas of the genus Naegleria, which inhabit
bodies of fresh water, are responsible for almost all cases of the usually
fatal disease primary amebic meningoencephalitis. The amebas are thought to
enter the body from water that is splashed onto the upper nasal tract during
swimming or diving. Human 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 problems, including difficulties of vector
control, prompted the World
Health Organization to target six diseases for increased research and training.
Three of these were protozoan infectionsmalaria, trypanosomiasis, and
leishmaniasis. Although new information on these diseases has been gained, most
of the problems with control persist.
Structure
Most parasitic protozoa in humans are less than 50 µm in size. The smallest
(mainly intracellular forms) are 1 to 10 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 hand, the vesicular nucleus has one or more nucleoli that contain DNA. The ciliates
have both a micronucleus and macronucleus, which appear quite homogeneous in
composition.
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 as 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 can readily twist and bend when moving through their
environment. In most 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 "mouth" for ingesting fluids or solid particles. Contractile
vacuoles for osmoregulation occur in some, such as Naegleria and Balantidium.
Many protozoa have subpellicular microtubules; in the Apicomplexa, which have
no external organelles for locomotion, these provide a means for slow movement.
The trichomonads and trypanosomes have a distinctive undulating membrane
between the body 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 complexity, a
protozoan is more like an animal than like a single cell. Figure 77-1 shows the
structure of the bloodstream form of a trypanosome, as determined by electron
microscopy.
FIGURE Fine structure of a protozoan parasite, Trypanosoma
evansi, as revealed by transmission electron microcopy of thin sections. (Adapted
from Vickerman K: Protozoology. Vol. 3 London School of Hygiene and Tropical
Medicine, London, 1977, with permission.)
Classification
In 1985 the Society of Protozoologists
published a taxonomic scheme that distributed the Protozoa into six phyla. Two of these phylathe
Life Cycle
Stages
During its life cycle, a protozoan generally
passes through several stages that differ in structure and activity.
- Trophozoite (Greek for "animal that
feeds") is a general term for the active, feeding, multiplying stage
of most protozoa. In parasitic species this is the stage 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 as 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 as
- gametocytes and gametes. Some protozoa form
- cysts that contain one or more infective
forms. Multiplication occurs in the cysts of some species so that
excystation releases more than one organism. For example, when the trophozoite
of Entamoeba histolytica first forms a cyst, it has a single
nucleus. As the cyst matures nuclear division produces four nuclei and
during excystation four uninucleate metacystic amebas appear. Similarly, a
freshly encysted Giardia lamblia has the same number of internal
structures (organelles) as the trophozoite. However, 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
transmission. 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 cavity of the mosquito vector.
Reproduction
Reproduction in the Protozoa may be asexual, as in
the amebas and flagellates that infect humans, or both asexual and sexual, as in the Apicomplexa of medical importance. The most common type of asexual
multiplication is binary fission,
in which the organelles are duplicated and the protozoan then divides into two
complete organisms. Division is
longitudinal in the flagellates and transverse in the ciliates; amebas have
no apparent anterior-posterior axis.
- Endodyogeny is a form of asexual division seen in Toxoplasma
and some related organisms. Two daughter cells
form within the parent cell, which then ruptures, releasing the smaller
progeny which grow to full size before repeating the process. In
- schizogony, a common form of asexual division in the Apicomplexa, the nucleus
divides a number of times, and then the cytoplasm
divides into smaller uninucleate merozoites. In Plasmodium, Toxoplasma,
and other apicomplexans, the sexual cycle involves the production of
gametes
- (gamogony), fertilization
to form the zygote, encystation of the zygote to form an oocyst, and the
formation of infective sporozoites (sporogony) within the oocyst.
Some protozoa have complex life cycles
requiring two different host species; others require only a single host to
complete the life cycle. A single infective protozoan entering a susceptible
host has the potential to produce an immense population. However, reproduction
is limited by events such as death of the host or by the host's defense
mechanisms, which may either eliminate the parasite or balance parasite
reproduction to yield a chronic infection. For example, malaria can result when
only a few sporozoites of Plasmodium falciparumperhaps ten or fewer in
rare instancesare introduced by a feeding Anopheles mosquito into a
person with no immunity. Repeated cycles of schizogony in the bloodstream can
result in the infection of 10 percent or more of the erythrocytesabout 400
million parasites per milliliter of blood.
Nutrition
The nutrition of all protozoa is holozoic;
that is, they require organic materials, which may be 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 become enclosed in food 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 within a membrane to form a food vacuole.
Protozoa have metabolic pathways similar to
those of higher animals and require the same types of organic and inorganic
compounds. In recent years, significant advances have been made in devising
chemically defined media for the in vitro cultivation of parasitic protozoa.
The resulting organisms are free 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 because pathways that are essential for
the parasite but not the host are potential targets for antiprotozoal compounds
that would block that pathway but be safe for humans. Many antiprotozoal drugs
were used empirically long before their mechanism of action was known. The
sulfa drugs, which block folate synthesis in malaria parasites, are one
example.
The rapid multiplication rate of many
parasites increases the chances for mutation; hence, changes in virulence, drug
susceptibility, and other characteristics may take place. Chloroquine
resistance in Plasmodium falciparum and arsenic resistance in Trypanosoma
rhodesiense are two examples.
Competition for nutrients is not usually an
important factor in pathogenesis because the amounts utilized by parasitic
protozoa are relatively small. Some parasites that inhabit the small intestine
can significantly interfere with digestion and absorption and affect the
nutritional status of the host; Giardia and Cryptosporidium are
examples. The destruction of the host's cells and tissues as a result of the
parasites' metabolic activities increases the host's nutritional needs. This
may be a major factor in the outcome of an infection in a malnourished
individual. Finally, extracellular or intracellular parasites that destroy
cells while feeding can 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, East Norwalk, CT, 1989
Lee JJ, Hutner SH, Bovee EC (eds): An
Illustrated Guide to the Protozoa. Society of Protozoologists, Lawrence, KS,
1985
Kotler DP, Orenstein JM: Prevalence of
Intestinal Microsporidiosis in HIV-infected individuals referred for
gastrointestinal evaluation. J Gastroenterol 89: 1998, 1994
Neva FA, Brown H: Basic Clinical Parasitology,
6th edition, Appleton & Lange, Norwalk, CT, 1994
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