Protozoa: Overview | Zoology Optional Notes for UPSC PDF Download

Introduction

Protozoa are mostly single-celled, animal-like organisms. Although some are colonial or form loose aggregations, most live and function as separate cellular individuals. Most protozoa are chemoheterotrophs, i.e., organisms using preformed organic compounds for both energy and carbon. Most do not contain green pigments and are not capable of using light as an energy source (however there are exceptions).
They live in a variety of habitats including fresh and salt water and inside multicellular organisms including humans and other animals. Most protozoa are free-living organisms that obtain nutrients from decaying organic materials or feed on bacteria and smaller eukaryotic cells. Some are parasites and some are pathogens capable of causing disease in humans and other animals.
Since protozoa live in diverse habitats and function as individuals subject to a multitude of environmental challenges, it is not surprising that they (as a group) have evolved a variety of specializations.  Some of these are described below.

Locomotor Structures

Locomotor structures are specializations allowing cells to travel through their environment as a means of dispersing, locating food sources or escaping potential predators.

• Cilia: cilia (singular = cilium) are short, hair-like structures found on the surfaces of protozoa called ciliates (phylum Ciliophora).  As described earlier (Eukaryotic cell structure and function), each cilium is surrounded by the cell membrane and is supported by a cytoskeleton of microtubules arranged in a characteristic 9 plus 2 pattern.  They are capable of whip-like motion involving MAPs such as dynein and are coordinated by microtubules arranged just inside the cell membrane.  Cilia can be distributed more or less uniformly all over the cell surface, can occur in rows or patches, or be grouped together in tufts.  Most cilia are used for swimming and allow ciliates to move smoothly through their watery habitats, some provide a jumping motility, and cilia arranged in tufts called cirri allow cells to walk or jump along solid surfaces.
• Flagella: flagella (singular = flagellum) are long, whip-like structures found on the surfaces of many eukaryotic cells.  Like cilia they are surrounded by the cell membrane and contain microtubules arranged in a characteristic 9 plus 2 pattern.  Some cells such as those in the genus Trypanosoma have a single flagellum enclosed in a double layer of membrane running the length of the organism in a fin-like manner. Flagella are usually ness numerous than cilia and are often used to pull cells through their environments.
• Pseudopodia: Pseudopodia or false feet (singular = pseudopodium) are extensions of the protoplasm associated with amoeba-like organisms.  These vary considerably in size and shape but are like cilia and flagella are surrounded by the cell membrane and often supported by microtubules (not arranged in a 9 plus 2 pattern).  Pseudopodia typically form in a flowing fashion, and allow amoeba-like organisms to creep slowly along solid surfaces.  Some extend through holes in glass skeletons like a multitude of spokes radiating from spherical wheels.

Food gathering structures 

Structures involved in food gathering activities are often those used for locomotion, including cilia, flagella and pseudopodia. In addition to these are structures involved in ingestion and digestion of foods. Many protozoa are holozoic, i.e., organisms that take in whole organisms for food. They have no means of biting off small portions of their prey as would large, multicellular animals.

• Cytostome – The cytostome or cell mouth (cyto = cell, stoma = mouth) is a region on the surface of a cell where endocytosis can occur. Ciliated protozoa such as Paramecium can take in food only through their cytostome because the rest of the cell is covered by a tough pellicle. 
• Lysosomes – Lysosomes contain the digestive enzymes needed to break down food materials taken in through endocytosis. 
• Cilia and flagella can sweep food along the cell surface toward the cytostome and sometimes line a region of the cell called an oral funnel or oral groove. 
• Pseudopodia: Amoeba-like protozoa use their pseudopodia to capture food by extending them out and around the food and fusing them to form food vacuoles.

Osmoregulatory structures

The contractile vacuoles present in many types of freshwater protozoa are used primarily to pump excess water out of cells.  They are connected to the endoplasmic reticulum, so also have circulatory function and may be used to eliminate liquid wastes, i.e., also have excretory function.

Protective Structures 

Protozoa live in potentially dangerous environments, and have evolved a variety of protective structures that help them survive

• Pellicle: The pellicle is a resilient and flexible layer located external to the cell membrane in all types of ciliated protozoa. It serves to give the cell its distinctive shape and shields it from physical harm.
• Skeletons: Protozoa typically possess skeletons composed of either glass, such as silica dioxide, or calcium carbonate. Radiolaria exhibit glass skeletons with numerous perforations, allowing pseudopodia to extend through them. Foraminifera, on the other hand, possess calcium carbonate skeletons that resemble the shells of mollusks, like the chambered nautilus. These skeletons offer protection against predation and provide structural support for the protoplasm.
• Trichocysts: Trichocysts are dart-like structures that can be discharged from specific cells. Comprised of protein, they often have barbs and are tethered to the cell surface by microscopic threads. Trichocysts are released in response to chemical and/or physical stimuli and can be employed for defense or attachment purposes.

Life cycle stages

Protozoa often live in habitats subject to change due to climate and seasons, while some live parts of their lives inside different types of hosts.  To survive variations in living conditions, protozoa can switch between two different stages.

• Trophozoites: Trophozoites (troph = activity), are active protozoa sometimes called vegetative cells. While in their trophozoite form, protozoa are engaged in feeding, reproducing and moving about actively. Vernal pools, i.e., those filled with water during the spring, contain many trophozoites. As the weather warms up and pools dry out in the summer sun, the protozoa go into a resting state. 
• Cysts: Cysts are dormant structures produced by many types of protozoa under certain circumstances. They are metabolically inactive and much more resistant to heat, drying, radiation and chemicals than are trophozoites (active vegetative cells). Cysts allow protozoa to survive when their watery habitats dry out during summer months or freeze solid during the winter. They also allow gastrointestinal parasites to survive passage through the stomach without being damaged by stomach acids.

Protozoan Reproduction

Protozoa like fungi and algae can reproduce themselves both asexually and sexually.  There are many variations on these basic themes, but some of the most commonly encountered forms of reproduction are introduced below.

Asexual reproduction

Asexual reproductive processes allow individuals to reproduce without interacting with other cells.  In eukaryotic organisms, asexual reproduction requires mitosis (the separation of the chromosome) and cytokinesis (the separation of the cytoplasm forming new daughter cells).  Some specific examples include:

• Binary fission: Binary fission is a process involving the separation of the cytoplasm across the long axis of the cell. Most protozoa reproduce by binary fission. 
• Schizogony: Schizogony or multiple-fission is a process involving the splitting of one cell into many daughter cells. Sporozoans in the genus Plasmodium reproduce by means of schizogony while inside human RBCs. 
• Budding: Budding involves an unequal division of the protoplasm and results in the formation of a bud at the margin of a cell. If conditions are favorable, the bud will grow and eventually separate from the cell, but if conditions are poor, the bud may die with little consequence to the cell.

Sexual reproduction

In order to reproduce sexually, protozoa must interact with other, genetically different cells.  Sometimes this involves plasmogamy, karyogamy and meiosis, but not always, and these terms are rarely used in zoology texts.  Two examples of sexual reproduction include:

• Syngamy: Syngamy involves the fusion of two haploid cells to form a diploid zygote. Protozoa in the genus Plasmodium undergo syngamy while inside mosquitoes. 
• Conjugation: Conjugation requires that two cells with different genetic content meet and position themselves side-by-side. Portions of the cell membranes fuse allowing the formation of a cytoplasmic bridge, and then segments of genetic material (DNA) are exchanged between the two cells. Following conjugation, the cells separate again, but each one is now carrying a new combination of genetic material.

Medical Protozoology

Though most protozoa are free-living organisms, feeding on decaying organic materials, bacteria and other cells, some protozoa are parasites and some are pathogens.  Protozoa of medical significance are responsible for killing millions of humans, and remain a threat to human health throughout the world.  Some examples of medically significant protozoa include:

• Giardia lamblia: Protozoa identified as Giardia lamblia or G. intestinalis are flagellated organisms that infect the small intestine and occasionally the bile ducts of humans and other animals causing giardiasis. They enter the body in cyst form along with contaminated food or water, and each cyst contains two trophozoites. The trophozoites released within the intestine attach to the mucosa where they apparently feed on mucous and other epithelial secretions. If they are numerous (covering much of the intestinal mucosa) they interfere with digestion and absorption of nutrients, especially fats resulting in diarrhea often accompanied by large amounts of yellowish fatty mucous. Giardia are more commonly pathogenic in children than in adults.
• Trichomonas vaginalis: Various species of Trichomonas may live within the human host, but only T. vaginalis is pathogenic and causes trichomoniasis.  These organisms live primarily in the vagina, but may travel to the cervix or vulva, and can infect the urethra and prostate in males.  Infection causes inflammation accompanied by a creamy-white discharge, with severe itching and chaffing.  Transmission may be direct via sexual intercourse or may involve transfer from mother to infant during childbirth.  Some infections have been acquired in poorly maintained pools and hot tubs.  According to some texts, around 20-40% of women in some areas have Trichomonas infections.
• Entamoeba histolytica: Although most amoeba-like protozoa are free-living in fresh and salt water, many are parasites, and some are pathogenic. Entamoeba histolytica are amoebae that live in the large intestines of humans and other animals. They enter their human hosts in the cyst form usually with water or contaminated vegetable material. The cyst walls are digested away in the stomach and duodenum, allowing the trophozoites (four per cyst) to be released. The trophozoites live in the caecum and reproduce by binary fission. In most cases they cause no or little damage, living on food material that is passing through the host. However, the name histolytica (which means tissue lysing or splitting) indicates that these organisms can invade the tissues causing damage.

In about 10% of infections, the amoebae invade the intestinal mucosa causing tissue lysis and ulceration resulting in dysentery.  In severe infections they may penetrate the submucosa, muscularis and serous membrane to enter the peritoneal cavity (often resulting in secondary bacterial infection). Symptoms vary depending upon severity and location of the infection, but typically include nausea, cramps and diarrhea. More severe infections result in abdominal tenderness, dysentery, dehydration and general incapacitation. Symptoms may develop within days of exposure or as much as a year later depending on host condition. Rarely amoebae travel via the portal system to the liver causing amoebic hepatitis.  

• Acanthamoeba and other genera: There are a number of other intestinal amoebae, and there are also cases of amoebae infecting other areas of the body. Amoebae in the genera Acanthamoeba, Naegleria and Balamuthia are normally free-living in soil and water but have been found to cause eye infections (keratitis), skin infections and primary amoebic meningoencephalitis, a rapidly fatal infection of the brain and meninges. These amoebae can enter the nasal cavities and travel via the ethmoid bone into the brain where they cause extensive hemorrhage and tissue damage. Death can occur in less than one week. Contaminated outdoor pools and hot baths have been found to harbor amoebae.
• Plasmodium vivax, malariae, ovale, and falciparum: Protozoa belonging to the Plasmodium genus are sporozoans known as the causative agents of malaria. The transmission of malaria occurs through a mosquito of the Anopheles genus. These parasites enter their mammalian host when the mosquito bites, delivered alongside the mosquito's saliva, at which stage they are called sporozoites. 
  • These sporozoites enter tissue cells (referred to as the exo-erythrocytic stage), such as the liver, where they can persist indefinitely. Eventually, some of these parasites enter the bloodstream (in the erythrocytic stage), where they invade red blood cells (RBCs) and reproduce asexually through schizogony. As a result, the RBCs rupture, releasing a large number of parasites now called merozoites. 
  • The maturation of these parasites within the RBCs occurs at intervals of 48 hours in Plasmodium vivax, ovale, and falciparum, and every 72 hours in malariae, leading to the cyclic symptoms of the disease. The rupture of infected RBCs releases hemoglobin, causing fever. The symptoms typically begin with severe chills (including shivering and teeth chattering), followed by fever, headache, and nausea. The fever can reach as high as 106 degrees F. Subsequently, fever gives way to sweating and a drop in temperature, sometimes falling below the normal range, until the next cycle begins. Within the bloodstream, some merozoites undergo meiosis to develop into gametocytes. Mosquitoes can acquire these gametocytes from the blood of a mammalian host, and the parasites undergo sexual reproduction (syngamy) within the mosquito's gut. 
  • The term "Malaria" means "bad air" and was coined due to the earlier association of the disease with swampy areas, before the connection with mosquitoes was established. It was also referred to as "black water fever" due to the dark urine resulting from hemoglobin breakdown.
• Toxoplasma gondii: Sporozoans identified as Toxoplasma gondii are recognized as the causative agents of toxoplasmosis, and are known to infect all kinds of mammals and birds. In the 1970's it was estimated that between 17 and 35% of all Americans carred the organisms. Dogs and cats also serve as reservoirs and recent findings suggest the protozoa change the behavior of rodents to increase their susceptibility to predation by cats. 
  • Transmission can occur from contact with feces or flesh of infected animals (eating raw meat) and can also occur prenatally. Symptoms in adults include fever, rash, enlarged lymph nodes and eye disturbances. 
  • The parasites may be found free in the blood or within various tissue cells. In fetuses or infants born infected, nervous system damage is common, and fatal encephalitis is not unusual. Pregnant women and infants can be tested for the presence of antibodies against Toxoplasma.
• Cryptosporidium parvum: Protozoa identified as Cryptosporidium parvum are the most common cause of cryptosporidiosis in immunocompromised patients. The organisms develop within the microvilli of the intestinal mucosa and can be transmitted through animal feces and contaminated water. Some noscomial (hospital acquired) cases have also occurred. These parasites can also infect the lungs and the gal bladder causing potentially deadly disease symptoms.
• Balantidium coli: Although most ciliated protozoa are free-living, some are parasites, and some are pathogens. Balantidium coli are ciliates that are parasitic in the gut of man and other animals. They enter their human hosts in cyst form, along with contaminated food. 
  • Digestive enzymes in the gut dissolve the cyst walls, releasing the trophozoites within the colon. In most cases, these feed on bacteria and fecal debris without causing disease symptoms, but rarely they invade the mucosa and submucosa of the gut causing abscesses and ulcerations. Symptoms typically include chronic diarrhea alternating with constipation, but can result in severe dysentery. Fatal cases occur occasionally.
• Hemflagellates: Flagellated protozoa inhabiting the bloodstream are known as hemoflagellates and include organisms in two genera, Trypanosoma and Leishmania. Species of Trypanosoma cause African sleeping sickness and Chaga's disease, while Leishmania organisms cause a variety of diseases including Kala-azar and oriental sore. All hemoflagellates live within the circulatory system of their host and are transmitted by insect vectors.
  • Trypanosoma gambiense and T. rhodesiensis: Hemoflagellates known as Trypanosoma gambiense and T. rhodesiensis are the causative agents of African sleeping sickness and are transmitted by insect vectors, specifically Tse tse flies in the Glossina genus. These trypanosomes can also infect various alternate hosts, including antelope, pigs, monkeys, and dogs, creating a substantial reservoir in addition to humans. 
  • The Trypanosomes reside in the salivary glands of the flies and enter their hosts when the flies bite to feed. These bites lead to itching and the formation of lesions, followed by an infection characterized by intermittent fever and headaches. This stage can persist for weeks or months, rendering the affected individuals vulnerable to other diseases. If left untreated, the disease-causing organisms may eventually enter the cerebrospinal fluid (CSF), leading to the onset of true "sleeping sickness." 
  • During this phase, individuals experience excessive drowsiness and are unable to engage in physical activity. Muscular convulsions and tremors are common. In the absence of treatment, coma and death become inevitable. However, if the disease-causing organisms do not enter the CSF, the symptoms can spontaneously disappear, and hosts can recover. Preventing the disease necessitates controlling Tse-tse flies or eliminating the alternate hosts, a task that is highly improbable.
  • Trypanosoma cruzi: Hemoflagellates identified as Trypanosoma cruzi are the agents responsible for causing Chaga's disease and are transmitted by insect vectors, specifically "true bugs" in the Triatoma genus known as "kissing bugs," which feed on human blood. 
  • The trypanosomes enter the body when infected excrement from feeding bugs is inadvertently scratched into wounds. Subsequently, they infiltrate the bloodstream or lymphatic system and migrate to tissue cells. Within these tissue cells, the trypanosomes undergo replication and are eventually released when the cells rupture. 
  • Symptoms may include edema at the site of infection (at the bite or conjunctiva), followed by the development of acute headache, fever, and, in some cases, severe skin lesions, especially in children. Chronic infections can lead to enlargement of the liver, spleen, and lymph nodes, along with anemia and nervous system disorders. Treatment is often not highly effective, and chronic infections can result in fatalities. Prevention primarily involves the eradication of the vectors.

Currently, the protozoa responsible for causing malaria, sleeping sickness, Chaga's disease, dysentery and other diseases are more commonly encountered in tropical regions than here in the United States.  Microbiologists are concerned that changes in world climate will result in more widespread disease as vectors expand their ranges in response to global warming.