Supplemental Lecture (97/04/27 update) by Stephen T. Abedon (email@example.com)
- Chapter title: Protozoa
- A list of vocabulary words is found toward the end of this document
- The protists are best defined in terms of what they are not: They are not procaryotes. They are mostly free-living and unicellular. They are not animals. They are not plants. They are not algae (though many argue that algae and protozoa ought to be grouped together). They are not fungi. They are not viruses.
- Put another way: They typically do not have cell walls. They are not multicellular (or, if so, then colonial with minimal cellular differentiation). And, they are heterotrophs, not autotrophic (as are plants) but instead must eat other organisms (photosynthetic algae are an exception but we're avoiding lumping algae in with the protista in this discussion).
- The need for the negative definition is to immediately focus your attention away from any consideration that protozoa should be considered a monophyletic taxon. This is because the protozoa consist of a number (actually the majority) of deeply rooted lineages found among the eucaryotes, including those that lead to the fungi, the plants, and the animals. Indeed, the take home message from the discussions presented thus far are that the niches open to unicellular eucaryotes must be numerous and long lived.
- The majority of 16S rRNA genetic divergence (universal tree) found among eucaryotes is found within Protista. The evolutionary radiance (which presumably was associated with the16S rRNA genetic divergence) into the various niches occupied by protozoa therefore presumably occurred at or near the dawn of the eucaryotic cellular architecture. These niches have been sufficiently stable that they have maintained diverse protozoan lineages to the present.
- As noted, some authors group together protozoa and microscopic algae. Think in this case of the algae as protozoa with chloroplasts. In this scheme the protista are "unicellular or colonial microorganisms that lack specialization into tissues." (p. 146, Talaro and Talaro, 1996) Here we will consider algae in a separate lecture, however.
- In this lecture we will walk through descriptions of features associated with various protozoa. We will then progress to a discussion of diseases caused by protozoa. Since protozoan diseases can be very complicated and relevant, we will go through the life cycles of a few of the more important protozoan parasites including those causing malaria and giardiasis.
- Some human diseases caused by protozoa
- African trypanosomiasis
- Amoebic dysentery
- balantidial dysentery
- Chagas' disease
- giardial enteritis
- Overview (of protozoa
- The following is quoted from Prescott et al., 1996 (p. 520):
- Protozoa are protists exhibiting heterotrophic nutrition and various types of locomotion. They occupy a vast array of habitats and niches and have organelles similar to those found in other eucaryotic cells, and also specialized organelles.
- The protozoa are divided into seven phyla: Sarcomastigophora, Labryinthomorpha, Apicomplexa, Microspora, Ascetospora, Myxozoa, and Ciliophora. These phyla represent four major groups: flagellates, amoebae, ciliates, and sporozoa.
- Protozoa usually reproduce asexually by binary fission. Some have sexual cycles, involving meiosis and the fusion of gametes or gametic nuclei resulting in a diploid zygote. The zygote is often a thick-walled, resistant, and resting cell called a cyst. Some protozoa undergo conjugation in which nuclei are exchanged between cells.
- All protozoa have one or more nuclei; some have a macro- and micronucleus.
- Various protozoa feed by holophytic, holozoic, or saprozoic means; some are predatory or parasitic.
- Protist [protista]
- By the five-kingdom system of phylogenetic classification, as well as on the universal tree, reference is made to protistaor protists. This is simply another way of saying protozoa.
- Negative definition:
- Protists are best defined as what they are not: eucaryotes that are mostly unicellular and not:
- "Protists cannot be considered plants or animals because these multicellular organisms develop from embryos. Protists are not fungi because fungi develop from spores and lack flagella at any stage of their life cycle." (p. 460, Postlethwait and Hopson, 1995)
- Do not form clade:
- Note that in no way do the protists represent a clade (i.e., a monophyletic taxon).
- The protists are an ancient group which, in a monophyletic sense, ought to include the fungi, plants, and animals (i.e., all eucaryotes).
- The absence of these latter taxa, makes the protists a polyphyletic taxon, one from which fungi, plants, and animals arose (e.g., animals are modified protozoa in the same sense that mammals are modified fish).
- Protozoa are:
- highly varied
- mostly aerobic (there are some examples of facultatively anaerobic protozoa)
- water living
- typically cell-wall-less
- that feed upon bacteria as well as particulate (as opposed, usually, to dissolved) nutrients
- Approximately 65,000 protozoa are known. By way of comparison, note that only about 4500 bacteria are known.
Variations on the protozoa theme
- Study of protozoa, etc.:
- Parasitology is the study of disease causing protozoa.
- This is traditionally done by the same field of science as that which studies helminths (parasitic worms), i.e., parasitology.
- Numerous variations on protozoa ecology have been documented including variations in:
- nutritional source
- mechanisms of reproduction
- growth habitat
- mechanism of locomotion
- Varied nutrient sources:
- Protozoa have varied nutrient sources, depending on type.
- Descriptions of protozoa nutrient acquisition types include:
- saprozoic (or saprophytic)
Adaptation to wet environments:
- Holozoic describes the obtainment of nutrients by engulfing.
- Generally engulfers:
- Free living protozoa tend to consume:
- animal debris
- plant debris
- or other protozoa
- The majority of protozoa are holozoic.
Engulfment is probably a good strategy particularly in wet, well mixed environments.
Here exoenzymes would probably be unsuitable due to the large diffusion factor.
Engulfment results in intracellular (though not cytoplasmic) digestion thus limiting the volume to which digestive enzymes are exposed.
- Holophytic describes the obtainment of nutrients by the same method as algae, cyanobacteria, and green plants (i.e., photoautotrophic).
- Some, but only a minority of protozoa are holophytic (depending, of course, on how one defines protozoa).
In addition, there exist organisms which are classified as protozoa which fill fungi-like niches (e.g., slime and water molds) beyond simply acting as absorbers.
- Saprozoic (or saprophytic) protozoa absorb nutrients from the extracellular environment.
- This contrasts with taking up chunks of nutrients (i.e., as in holozoic).
- Note that the engulfing strategy of protozoa also contrasts with that fungi which are also nutrient absorbers.
- Feature of parasitic protozoa:
- It is parasitic protozoa which tend to be nutrient absorbers.
- Note the similarity of this protozoa niche to that occupied by the many gram-negative bacteria.
- Protozoa reproduce asexually by:
- These are all variations on mitotic division.
- Many protozoa reproduce sexually as well.
Three binomials to remember include:
- Moist environments:
- Protozoa generally require moisture and have adapted to most if not all environments in which moisture and nutrients are present.
- Note how this contrasts with fungi which generally inhabit dry environments.
- Prevalence as parasites:
- Some protozoa of which are part of the normal flora of animals.
- Only a few Protozoa cause disease.
- Protozoa infections among the healthy tend occur with greatest frequency in tropical, third world nations. This is especially true for malaria.
- In the U.S., protozoa infections tend to occur to the greatest extent among AIDS patients.
The hosts protozoa parasitze are typically distinguished as:
- Giardia lamblia
- Plasmodium spp.
- Toxoplasma gondii
- the intermediate host
- the definitive host
- For parasitic protozoa (and other parasites such as helminths) the intermediate hostis that in which asexual reproduction occurs.
- Immature form:
- Alternatively, the intermediate hostis that which harbors the immature form.
- Note that immature form and asexual reproduction are typically not mutually exclusive.
- For parasitic protozoa (and other parasites such as helminths) the definitive host is that in which sexual reproduction occurs.
- Alternatively, the definitive host is that which harbors the mature form.
- For Plasmodium spp., the etiological agent of malaria, the definitive host is the mosquito while the intermediate host is man and other primates.
- Below we discuss distinct aspects of protozoa anatomy including:
- cell size
- cell shape
- cell envelope
- Protozoa cells generally range in size from about 1 to 300 µm, the size of a bacteria at the small end and a good size eucaryotic cell at the large end.
- There exist, however, certain protozoa which sport cell sizes in the range of 3 to 4 mm (3000-4000 µm). This is large enough to viewed with the naked eye.
- Well defined cell shape:
- Many protozoa have very well defined cell shapes.
- In fact, a fairly dominant characteristic of protozoa is a complex cellular morphology and anatomy.
- Some protozoa called amoebas, on the other hand, display a constantly changing cell shape.
- Protozoa typically lack cell walls.
- Note that a flexible cell covering called a pellicle is found on some protozoa.
- The pellicle serves as an osmotic shield.
Some protozoa have cells.
Shells can consist of calcium carbonate as well as silicon.
- The cells of many protozoa consists in part of a clear outer layer called the ectoplasm.
- The ectoplasm contains the organelles associated with:
- Typical cytoplasm:
- Within the ectoplasm of protozoa cells lies the endoplasm.
- The endoplasm has a granular appearance and houses the nucleus, mitochondria, and various food and contractile vacuoles.
- Protozoa employ a variety of mechanisms of locomotion and may be classified according to these mechanisms of locomotion:
- Additionally there exist fungus-like protozoa.
- Motile feeding stage:
- The trophozoite is the motile feeding stage of protozoa.
- Some protozoa exhibit no other stage.
- Usually a sufficient moisture levels and food supplies are necessary to maintain this stage of the protozoa life cycle.
- The flagellates are protozoa which move by means of flagellar action.
- Some flagellates have their flagella attached in a structure called an undulating membrane.
- Interestingly, flagellates tend to have symbiotic relationships with multicelled organisms.
- Cilia moving:
- The ciliates are protozoa which move by means of cilia action.
- Recall that the difference between eucaryotic flagella and cilia is one of size and number, the latter being small and numerous, the former large and few.
- "Because of the tremendous variety in ciliary arrangements and functions, ciliates are among the most diverse and awesome cells in the biological world." (p. 147, Talaro and Talaro, 1996)
- Example: The paramecia are ciliates.
- Advanced protozoa:
- The ciliates, interestingly, represent a branch of the protozoa particularly close evolutionarily to the fungi, plants, and animals.
- They are also among the more advanced of protozoa.
- Pseudopod (pseudopodia):
- Amoebas are protozoa which move by employing pseudopodia, which are membrane covered cytoplasmic extensions.
- Many amoebas employ their pseudopodia also to engulf food.
- Amoebas live in moist terrestrial or aquatic environments.
- Amoebas include among their members various protozoa which form calcium-based or silicon-based shells (the foraminiferans and the radiolarians, respectively).
- Chalk deposits such as the White Cliffs of Dover formed from the shells of numerous, long dead foraminiferans.
- The sporozoans are parasitic spore formers which do not locomote under their own power.
- Plasmodium spp., the cause of malaria, are sporozoans.
- Microspora are an extremely ancient eucaryote lineage also classified as sporozoans.
True slime molds [Myxomycota]
- Exoenzymes and nutrient absorption:
- Protozoa exist which occupy a fungus-like niche and employ the very fungal nutrient acquiring strategy of secreting exoenzymes and absorbing the solubilized nutrients.
- These include the following:
- slime molds
- cellular slime molds
- water molds
- These organisms differ from fungi in terms of:
- the motility of their spores
- their cellulose containing cell walls
- their pattern of mitosis
- their diploid hyphae
- Difficult to catalog:
- Note that not all authors group the fungus-like protozoa among the protists preferring, instead, to group them among the fungi.
- However, Postlethwait and Hopson (1996; p. 460) argue that the slime molds would form a polyphyletic taxon if included among the fungi and therefore are more appropriately included among the already polyphyletic protozoa.
Cellular slime molds [Acrasiomycota]
- True slime molds are found as giant motile cells. That is, these cells are surrounded by a single plasma membrane and contain multiple, diploid nuclei.
- "These bizarre, plasmodial slime molds stream along the damp forest floor in a mass of brightly colored protoplasm called a plasmodium in which individual cells are indistinguishable. Plasmodia are coenocytic because their nuclei are not separated by cell walls, and they resemble a moving mass of slime. Remember not to confuse the plasmodium of a slime mold with the sporozoan genus with the same name. Often plasmodia of slime molds live beneath detached bark on decomposing tree trunks. Two species occasionally occur on lawns or on mulch beneath shrubs. The moving protoplasm of the plasmodia conspicuously pulsates back and forth as they engulf and digest bacteria, yeasts, and other organic particles . . . If environmental conditions become less than optimal, e.g., if food or moisture decrease, then one of two things happen. The plasmodium may dry into a hard resistant structure called a sclerotium and remain dormant until conditions improve. Or, if light is available the diploid plasmodium will move to the exposed area and coalesce. The condensed structure will grow sporangia and meiosis will produce spores for dispersal. Light is associated with an open environment for successful reproductive dispersal rather than under or with a log where dispersal would be difficult. Haploid spores produced by meiosis in the sporangia germinate as amoeboid or flagellated organisms. These haploid stages may later fuse as gametes and grow into new plasmodium." (pp. 221-222, Vodipich & Moore, 1992)
Water molds [Oomycetes]
- Multiple cells:
- Cellular slime molds can also be found in a similar slug-like form (though one consisting of multiple cells rather than, essentially, only a single, albeit multi-nucleated cell).
- The slug form among cellular slime molds, however, represents only a portion of its life cycle, one entered into only when environmental conditions are no longer conducive to growth.
- Both types of slime molds reproduce by differentiating into fruiting bodies from which spores are released.
- Water molds (Oomycetes) are an even more fungus-like protozoa.
- Phytophthora infestans, for example, is a parasitic water mold that was responsible for the Irish Potato Blight.
- Endospore analog:
- Protozoa enter into a state analogous to the bacterial endospore when moisture and food supplies dwindle.
- Similar to the bacterial endospore, in the cyst stage protozoa are tougher, dormant, and consequently better able to survive adverse conditions.
- For protozoa, the cyst stage particularly contrasts with the motile trophozoite stage.
- For some parasitic protozoa, such a Giardia lamblia, the cyst stage represents the means by which progeny are packaged for dissemination to new hosts.
- During the cyst stage protozoa are capable of being transported on the wind thus aiding in their dispersion. This includes dispersion to new hosts in the case of amebic dysentery.
Schizogony (multiple fission):
- Asexual replication:
- All protozoa employ asexual methods of replication.
- Usually this is mitotic cell division.
- Sexual replication:
- Most protozoa also reproduce via a sexual cycle.
- Sex among protozoa, though it can, does not always (depending on species) involve reproduction (i.e., fertilization, fusion, and meiosis occur with no intervening mitosis).
Schizogony (or multiple fission) is the multiple division of cell nuclei (and DNA) prior to partitioning of cytoplasm into multiple, separate, much smaller daughter cells.
In other words, in these protozoa, mitosis and cytokinesis are not directly temporally linked.
Giardiasis [Giardia lamblia]
- Most cases of protozoa-borne disease occur in the tropic and subtropics where they are very common.
- Three stages:
- "Most human parasites go through three general stages:
- The microbe is transmitted to the human host from a source such as soil, water, food, other human, or animals.
- The microbe invades and multiplies in the host, producing more parasites that can infect other suitable hosts.
- The microbe leaves the host in large numbers by a specific means and, to survive, most find and enter a new host.
- There are numerous variations on this simple theme. For instance, the microbe can invade more than one host species (an alternate host), and undergo several changes as it cycles through these hosts, such as sexual reproduction or encystment. Some microbes are spread from human to human by means of vectors, defined as animals such as insects that carry diseases. Others can be spread through bodily fluids and feces." (p. 152, Talaro and Talaro, 1996)
Infection by cysts:
- Simple parasitic disease:
- Giardiasis, caused by the protozoa Giardia lamblia, is an example of a relatively simple parasitic disease.
- Giardia, particularly, are very primitive (or degenerative) eucaryotes, lacking even in mitochondria.
- Intestinal disease:
- Giardiasis is an intestinal disorder which leads to diarrhea, flatulence, weight loss, and anorexia.
- The parasite takes up residence in the small intestine and symptoms are a consequence of interference with host nutrient adsorption.
- Diagnosis of giardiasis is notoriously difficult.
- Treatment is relatively easy.
The G. lamblia life cycle thus is one relatively typical of an enteric infection, though infections can be chronic. That is, infections tend to occur as a consequence of consumption of a fecally contaminated vehicle (water typically in this case).
Infection is via the ingestion of cysts and, to a lesser extent, trophozoites.
Cysts are both highly infectious and highly durable.
Chlorine is not effective in killing cysts (though iodine as well as boiling and ozone are).
Source of infection:
Ingestion leads to infection and typically occurs through:
Typically it is impossible to tell whether a water supply is contaminated on appearance alone.
G. lamblia has been isolated from numerous mammals. These animals defecate in water sources thus contaminating those sources.
Humans tend not to be the reservoir for the disease though, once acquired, humans can transmit the pathogen via intimate contact.
Outbreaks of giardiasis have been traced to community water supplies in the U.S. as well as to daycare centers.
Toxoplasmosis [Toxoplasma gondii]
- untreated water
- intimate contact
Toxoplasmosis, caused by the protozoa Toxoplasma gondii, has a life style that is a step up in complexity from that exhibited by Giardia lamblia.
Definitive and intermediate hosts:
Particularly, T. gondii has both a definitive host and various possible intermediate hosts.
T. gondii displays little host specificity though its primary reservoir is both wild and domestic cats.
T. gondii goes through its sexual stage in cats (i.e., cats are the definitive host).
Its portal of exit is in feces.
As oocysts (zygotic cysts) it can survive for months in moist soil.
Disease is initiated either upon ingestion of oocysts or upon consumption of asexual stage-harboring flesh (i.e., intermediate host).
In cats as well as other animals the asexual stage is capable of causing disease in numerous tissues.
It is thought that humans are constantly exposed to T. gondii often through:
"There is no such thing as a safe form of raw meat, even salted or spiced. Adequate cooking or freezing below -20°C destroys both oocysts and tissue cysts. Oocysts can also be avoided by washing the hands after handling cats or soil that is possibly contaminated with cat feces, especially sandboxes and litter boxes. Pregnant women should be especially attentive to these rules and should never clean out the cat's litter box." (p. 719, Talaro and Talaro, 1996)
- the consumption of raw meat
- the ingestion of oocysts excreted by cats
- the inhalation of oocysts
Toxoplasmosis shows a wide distribution among humanity and it is thought that the majority of humans become infected with T. gondii at some time in their lives.
Fortunately, in most cases the disease is either subclinical or not severe.
While T. gondii can make an otherwise healthy person sick, it can kill an immunodepressed individual.
T. gondii can also kill or severely damage a fetus if transmitted from the mother (which occurs with a probability of about 33% given infection of the mother).
Chagas' disease [Trypanosoma cruzi]
- Arthropod vector:
- Chagas' disease shows a step up in complexity from that of toxoplamosis because the Chagas' disease parasite, Trypanosoma cruzi, is spread via an arthropod vector.
- Trypanosoma cruzi parasitizes numerous mammalian hosts including dogs, cats, opossums, armadillos, and foxes. These presumably serve as the reservoir for T. cruzi from the point of view of humanity.
- An insect vector parochially referred to as the "kissing bug" picks up the protozoa when acquiring a blood meal from an infected animal.
- T. cruzi replicates in this arthropod's intestinal lumen and accumulates in its feces.
- Upon acquiring a blood meal the bug simultaneously deposits feces on or near the bite.
- Scratching of the bite by the mammalian host contributes to the entry of the parasite.
Within mammalian hosts, T. cruzi initially has a white blood cell and muscle tropism.
Tropism can broaden over time ultimately leading to extensive systemic infection and somtimes death.
There is no cure or effective treatment for Chagas' disease and it is endemic to South and Central America.
Malaria [Plasmodium spp.]
- Highly complex life history:
- Malaria means "bad air" in Italian (based upon the disproven hypotheses that malaria is caused by bad air).
- Malaria is a step or two up in complexity from that exhibited by Trypanosoma cruzi.
- Particularly, the malaria parasites (four species all of the Plasmodium genus) have (or undergo):
- a definitive host
- an intermediate host.
- an arthropod vector.
- multiple tissue tropisms which they explore sequentially
- distinct cellular phases depending on current tropism
- intracellular replication
- Globally important:
- Malaria is humanity's dominant protozoan disease.
- Malaria kills two million people each year while infecting hundreds of millions.
- The Plasmodium species together have a range which overlaps that of one-third of humanity.
- Historically malaria has had a worldwide distribution. However, as a consequence of mosquito control measures, malaria today is restricted to the tropics. Note that this is a significant achievement stemming from the germ theory of disease.
- As a wonderfully perverse example of natural selection and adaptation in action (i.e., evolution), cases of malaria are apparently on the rise resulting on the one hand from Anopheles mosquitoes developing resistance to mosquito control measures (insecticides) and on the other hand Plasmodium spp. developing resistance to anti-malarial chemotherapeutics.
Four species of Plasmodium are implicated in human disease:
They all are spread primarily by mosquito vectors, all of the genus Anopheles.
Asexual phase (sporozoites):
- P. malariae
- P. vivax
- P. falciparum
- P. ovale
The asexual human stage of malaria is initiated upon injection of saliva by an Anopheles mosquito.
She does this in order to inject a coagulant (and she is a she since the purpose of the blood meal is to nourish her developing eggs).
The Plasmodium cells called sporozoites (asexual cells) are injected along with the saliva.
This begins the human infection.
Exoerythrocytic development (merozoites):
The sporozoites initially take up residence in the liver.
There they enter liver cells. They do this because Plasmodium are obligate intracellular parasites: they do not replicate outside of cells.
In these liver cells they undergo schizogony, packing these cells with progeny called merozoites.
In 5 to 16 days these infected liver cells are lysed releasing from 2,000 to 40,000 merozoites per infected cell.
Once released from liver cells merozoites take up residence in red blood cells where they feed on hemoglobin.
They can go through multiple round of red blood cell infection, progeny production, burst, and infection of new red blood cells.
Meanwhile during erythrocytic development, some of the merozoite progeny differentiate into male and female gametes.
Sexual phase (sporogony):
Fertilization with these gametes occurs only upon the taking of a blood meal by a female Anopheles mosquito, and occurs within her stomach.
The diploid, fertilized cells (oocysts), begin to undergo meiosis upon implantation into the mosquitos stomach wall.
The meiotic products (called sporozoites) migrate to the mosquito salivary gland where, of course, they become available for injection upon the next taking of a blood meal.
"Malaria is a perfect model to demonstrate the problems of vaccine development. A successful malaria vaccine must be capable of striking a diverse and rapidly changing target. Not only are there four different species, each having different sporozoite, merozoite, and gametocyte (the male and female gametes) stages, but each species can also have different antigenic types of sporozoites and merozoites. A sporozoite vaccine would prevent liver infection, while a merozoite vaccine would prevent infection of red blood cells and diminish the pathology that is most responsible for morbidity and mortality. The best vaccines would provide long-term protection for all the phases and variants, so that if one immunity were to fail, another could come into play." (p. 717, Talaro and Talaro, 1996)
- No entry.
- "The protozoa have not escaped problems in taxonomy. They, too, are very diverse, and frequently frustrate attempts to generalize or place them in neat groupings. The most recent system divides them into seven phyla with several subphyla and classes, but this method may be more complex than is necessary for our survey. We will simplify this system by dividing the Subkingdom Protozoa into four medically important groups, based on method of motility, mode of reproduction, and stages in the life cycle, summarized as follows:" (all below---including the Mastigophora, the Sarcodina, the Cilophora, and the Sporozoa---are quoted, as is the previous quote, from Talaro and Talaro, 1996, pp. 150-152)
- "Motility is primarily by flagella alone or both flagella and amoeboid motion; single nucleus; sexual reproduction, when present, by syngamy; division by longitudinal fission. Several parasitic forms lack mitochondria and Golgi apparatus; most species form cysts and are free-living; the group also includes several parasites. Some species are found in loose aggregates or colonies, but most are solitary. Examples: Trypanosoma and Leishmania, important blood pathogens spread by insect vectors; Giardia, an intestinal parasite spread in fecally-contaminated water; Trichomonas, a parasite of the reproductive tract of humans spread by sexual contact." (pp. 150-152, Talaro and Talaro, 1996)
- "Cell form is primarily an amoeba; major locomotor organelles are pseudopodia, although some species have flagella during reproductive states. Asexual reproduction by fission; two groups have an external shell; mostly uninucleate; usually encyst. Examples: Most amoebas are free-living and not infectious; Entamoeba is a pathogen or parasite of humans; shelled amoebas called foraminifera and radiolarians are responsible for chalk deposits in the ocean." (pp. 150-152, Talaro and Talaro, 1996)
- "Trophozoites are motile by cilia; some have cilia in tufts for feeding and attachment; most develop cysts; have both macronuclei and micronuclei; division by transverse fission; most have a definite mouth and feeding organelle; show relatively complicated and advanced behavior. Example: the majority of ciliates are free-live and harmless. One important pathogen, Balantidium coli, lives in vertebrate intestines and can infect humans." (pp. 150-152, Talaro and Talaro, 1996)
- "Life cycles are complex, with well-developed asexual and sexual stages. Motility is absent in most cells except male gametes. Sporozoans produce special sporelike cells called sporozoites following sexual reproduction, which are important in transmission of infections; most form thick-walled zygotes called oocysts; entire group is parasitic. Examples: Plasmodium, the most prevalent protozoan parasite, causes 100 million to 300 million cases of malaria per year worldwide. It is an intracellular parasite with a complex cycle alternating between humans and mosquitoes. Toxoplasma gondii causes acute infection (toxoplasmosis) in humans, which is transmitted primarily by cats." (pp. 150-152, Talaro and Talaro, 1996)
- Amoebic dysentery
- Cellular slime molds
- Cell shape
- Cell size
- Chagas' disease
- Definitive host
- Fungus-like protozoa
- Giardia lamblia
- Human diseases caused by protozoa
- Intermediate host
- Malaria asexual stage
- Malaria erythrocytic development
- Malaria exoerythrocytic development
- Malaria gamete formation
- Malaria sexual phase
- Multiple fission
- Nutritional source
- Plasmodium spp.
- Toxoplasma gondii
- True slime molds
- Water molds
Practice question answers
- Which of the following is acquired from cat litter (circle only one correct answer)? [PEEK]
- Giardia lamblia
- Toxoplasma gondii
- Plasmodium spp.
- Candida albicans
- all of the above
- none of the above
- Which likely causes more plant disease (circle only one correct answer)? [PEEK]
- all of the above
- none of the above
- For which of the following are humans the intermediate host (circle only one correct answer)? [PEEK]
- Giardia lamblia
- Toxoplasma gondii
- Plasmodium spp.
- Candida albicans
- all of the above
- none of the above
- In terms of nutrient acquisition, describe an environment in which a particle engulfment strategy is superior to one employing exoenzymes followed by the adsorption of digested material. [PEEK]
- Define trophozoite. [PEEK]
- Describe the Plasmodium life cycle in sufficient detail that I am aware of all the various cell types and environments in which they might be found (i.e., I don't need to know their names, just what if anything special that they do and where they do it). Keep track of ploidy, fertilizations, mitotic divisions, and meiotic divisions. [PEEK]
- Name an example of a sporozoan. [PEEK]
- In what way does a biological vector differ from a mechanical vector? [PEEK]
- Name three diseases which are caused by a protozoan (protist) pathogen or name the pathogen which causes the disease (but not both for any one disease/pathogen). [PEEK]
- ii, Toxoplasma gondii
- ii, fungi
- iii, Plasmodium spp.; ii, Toxoplasma gondii is also a correct answer, though not as good an answer unless the human host is capable of passing the parasite to a new host.
- aqueous and well mixed, i.e., any environment in which such particles are likely to exist (probably all) but in which retention of exoenzymes in close proximity is difficult. Engulfment assures retention of analogous enzymes in close proximity to both the nutrient material and the individual organisms which produce the enzymes (the protozoa).
- the mobile feeding stage of a protozoa
- The following are relevant details which may be included in this essay:
- Plasmodium are haploid cells and not gametes during most of their life cycle.
- Haploid (not gametic) cells are found in the salivary glands of mosquitoes.
- They are injected by the mosquito into the human host when a blood meal is taken.
- These cells take up residence in host liver cells.
- There they replicate mitotically.
- Haploid progeny are released from liver cells upon liver cell lysis.
- These progeny have a red blood cell tropism.
- Within red blood cells they replicate mitotically.
- Haploid progeny are released from red blood cells upon red blood cell lysis.
- Some of the released progeny differentiate into male and female gametes.
- Non-gamete progeny retain a red blood cell tropism.
- Fertilization occurs within the gut of a mosquito which has taken a blood meal from an infected host.
- The diploid products of fertilization implant themselves in the stomach wall of the mosquito.
- There they undergo meiosis.
- Haploid meiotic products migrate to the mosquito salivary gland thus completing the cycle.
- Plasmodium spp.
- A parasite goes through part of its life cycle in/on a biological vector but does not in/on (typically on) a mechanical vector.
- Giardiasis, Toxoplasmosis, Chagas disease, malaria, amoebic dysentery, Trichomonas
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- Tortora, G.J., Funke, B.R., Case, C.L. (1995). Microbiology. An Introduction. Fifth Edition. The Benjamin/Cummings Publishing, Co., Inc., Redwood City, CA, pp. 313-317.
- Talaro, K., Talaro, A. (1996). Foundations in Microbiology. Second Edition. Wm. C. Brown Publishers. pp. 146-154, 709-710, 718-719.
- Vodopich, D., Moore, R. (1992). Biology Laboratory Manual. Wm. C. Brown Pub. pp. 217-226.