Microscopic unicellular eukaryotes known as protozoa have a very complicated internal structure and engage in intricate metabolic processes. Propulsion structures and various modes of mobility are present in several protozoa.

General Concepts


One-celled organisms known as protozoa are present in most ecosystems on earth. Although the majority of species are free-living, all higher animals are infected with one or more protozoan species. Depending on the type and strain of the parasite and the host’s resistance, infections can range from asymptomatic to fatal.


Microscopic unicellular eukaryotes known as protozoa have a very complicated internal structure and engage in intricate metabolic processes. Propulsion structures and various modes of mobility are present in several protozoa.

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Currently, the protozoa are divided into six phyla based on their morphology at the light and electron microscopic levels. The majority of organisms that infect humans belong to the phyla Sacromastigophora and Apicomplexa.

Life Cycle Stages

Trophozoites are a common name for the feeding and reproducing stages of parasitic protozoa, while alternative names may be used for similar stages in other protozoa. Cysts are developmental stages with a thicker or protective wall. Cysts produced by protozoa that must live outside of the host often have walls that are more robust than cysts that develop in tissues.


The most frequent mode of reproduction, binary fission, is asexual; repeated asexual division happens in certain species. The Apicomplexa may reproduce both sexually and asexually.


All parasitic protozoa require prepared organic materials, meaning that they eat like higher animals do.


Despite being assigned to the kingdom Animalia in the classical classification system, the Protozoa are now regarded as a subkingdom of the kingdom Protista. Protozoa have been classified into more than 50,000 species, the majority of which are free-living creatures. They may be found practically anywhere. Protozoa existed in the Pre-Cambrian epoch, as evidenced by the fossil record of shells in sedimentary rocks. Using microscopes he made from basic lenses, Anton van Leeuwenhoek was the first to see protozoa. In addition to free-living protozoa, he also described various parasitic species from animals and Giardia lamblia from his own faeces between the years 1674 and 1716. Protozoa live in or on almost every human at some point, and many people are infected with

The severity of protozoan illnesses can range from extremely minor to fatal. People whose immune systems can only contain but not completely eradicate a parasitic infection become carriers and serve as a source of infection for others. In regions where parasite incidence is high, well-tolerated infections are frequently not treated to eliminate the parasite since doing so would reduce the person’s immunity to the parasite and increase the probability of reinfection.

Immunosuppressed patients, especially those with acquired immune deficiency syndrome (AIDS), are more susceptible to several protozoan infections that are undetectable or mild in healthy people. There is evidence that many healthy individuals have small amounts of Pneumocystis carinii in their lungs. However, in immunosuppressed patients, such as those with AIDS, this parasite causes a pneumonia that is frequently fatal. Common protozoan parasite Toxoplasma gondii often causes a very minor first sickness followed by a latent infection that lasts for a very long time. Toxoplasmic encephalitis, however, can be lethal in AIDS patients. Although Cryptosporidium was first identified in the 19th century, it has only recently been realised that humans are infected on a large scale. Another protozoan that can cause significant difficulties in AIDS patients is Cryptosporidium. Only a few cases of microsporidiosis in humans had been documented before

Amebas that are free-living and found in soil and water are called anthamoeba species. Stages of a cyst may be airborne. People who wear contact lenses have been reported to experience serious, potentially blinding corneal ulcers caused by Acanthamoeba species. It’s likely that tainted lens-cleaning solution spreads the parasites. Naegleria-genus amebas, which live in freshwater bodies of water, are virtually always to blame for instances of primary amebic meningoencephalitis, a typically deadly condition. According to theory, the amebas enter the body when water splashes against the upper nasal passages when swimming or diving. Based on laboratory research on Acanthamoeba infections in cell cultures and in animals, human infections of this sort were anticipated before they were recognised and reported.


The majority of human parasitic protozoa are smaller than 50 m in size. The smallest (mostly intracellular types) range in size from 1 to 10 m, whereas Balantidium coli may grow to a length of 150 m. Protozoa are eukaryotes with only one cell. The nucleus is surrounded by a membrane, as it is in all eukaryotes. Other than ciliates, all protozoa have vesicular nuclei, which are characterised by dispersed chromatin that gives the nucleus a diffuse appearance. One variety of vesicular nucleus has an endosome or karyosome, which is a more or less central body. In trypanosomes and parasitic amebas, the endosome is devoid of DNA. On the other hand, the vesicular nucleus of the phylum Apicomplexa has one or more nucleoli that carry DNA. The ciliates possess a micronucleus as well as

Protozoa’s organelles serve comparable purposes as higher species’ organs do. Along with covering the cytoplasm, the plasma membrane also covers the protruding locomotory elements including flagella, cilia, and pseudopodia. Some protozoa have an outer surface layer called a pellicle that is sufficiently rigid to maintain a unique shape, such as trypanosomes and Giardia. However, when moving through their environment, these organisms can easily bend and twist. The cytoplasm of most protozoa is divided into two layers: the ectoplasm, which is the exterior, transparent layer, and the endoplasm, which is the inner layer containing organelles. Species with protruding pseudopodia, such amebas, make it easiest to observe the cytoplasm’s structure. Some protozoa have a cytosome or cell “mouth” for ingesting liquids or solid particles. Osmoregulation is accomplished via contractile vacuoles.


The Protozoa were divided into six phyla according to a taxonomy system released in 1985 by the Society of Protozoologists. The two phyla Sarcomastigophora and Apicomplexa include the majority of the species responsible for human illness. This plan is based on morphology as seen using scanning, electron, and light microscopy. For instance, Dientamoeba fragilis was classified as a member of the Entamoebidae family since it was believed to be an ameba. However, internal features visible through electron microscopy demonstrated that it belongs in the flagellate protozoan order Trichomonadida. On the basis of factors like geographic distribution and clinical signs, certain organisms that look the same under a microscope have been given multiple species names. A prominent example of this is the genus Leishmania, for which subspecies names are frequently employed.

The kinetoplast, a special mitochondrion found in hemoflagellates and other members of the order Kinetoplastida, has been the subject of much research. The DNA connected to this organelle is really intriguing. Cloning is frequently employed in taxonomic research, for instance, to examine variations in disease virulence or symptoms in isolates of a single species collected from various hosts or geographical locations. Unknown isolates are identified using antibodies, in particular monoclonal antibodies, to recognised species or to certain antigens from a species. The microscope is still the most useful instrument for detecting a protozoan parasite, even if molecular taxonomy may eventually show to be a more trustworthy basis than morphology for protozoan taxonomy. The protozoa with medicinal importance are listed in Table 77-1.

Life Cycle Stages

A protozoan often goes through multiple phases during its life cycle, each of which has a different structure and activity. The name “trophozoite,” which means “animal that feeds” in Greek, refers to the active, feeding, and replicating stage of the majority of protozoa. This is the stage that is typically linked to pathogenesis in parasitic organisms. The phrases amastigote, promastigote, epimastigote, and trypomastigote refer to trophozoite stages in hemoflagellates that vary depending on whether a flagellum is present or absent as well as the location of the kinetoplast connected to the flagellum. For example, Toxoplasma gondii’s phases are referred to as tachyzoite and bradyzoite in different contexts. The merozoite, which is the form produced by the fission of a multinucleate, is one of the additional phases in the intricate asexual and sexual life cycles present in this phylum.

Cysts produced by certain protozoa may include one or more infectious types. In other species, the cysts multiply, releasing several organisms during excystation. For instance, the Entamoeba histolytica trophozoite has a single nucleus when it first develops into a cyst. Four nuclei are produced by nuclear division as the cyst ages, and four uninucleate metacystic amebas develop during excystation. The trophozoite and a recently encysted Giardia lamblia both have the same number of internal structures (organelles). The cyst’s organelles do, however, double in size as it develops, and two trophozoites are produced. Depending on the species and environmental factors, cysts discharged in faeces contain a protective wall that allows the parasite to live outdoors for a time period ranging from days to a year. Cysts


The human-infecting amebas and flagellates reproduce asexually, while the important medicinal Apicomplexa reproduce both asexually and sexually. Binary fission is the most prevalent form of asexual reproduction. In this process, the protozoan duplicates its organelles before splitting into two distinct species. Flagellates and ciliates divide longitudinally, but amebas appear to lack an evident anterior-posterior axis. Toxoplasma and certain similar species exhibit endodyogeny, a type of asexual division. Within the parent cell, two daughter cells develop. The parent cell then bursts, releasing the smaller offspring, who then grow to maturity before repeating the cycle. The nucleus splits a number of cells during schizogony, a frequent kind of asexual division in the Apicomplexa.

Some protozoa have complicated life cycles that need two separate host species, whereas others may complete their life cycles with just one host. An enormous population might be created by just one infectious protozoan invading a weak host. The host’s defence mechanisms or other occurrences, such as the host’s death, can either remove the parasite or balance parasite reproduction such that a chronic infection results. For instance, malaria can develop when a feeding Anopheles mosquito introduces just a few Plasmodium falciparum sporozoites—possibly ten or less in exceptional cases—into a host without immunity. Schizogony cycles repeated in the circulation can infect 10% or more of the erythrocytes, or around 400 million parasites.


All protozoa require organic resources, whether they are particulate or in solution; this is known as holozoic nutrition. Amebas use a kind of temporary mouth to take in food particles or liquid droplets, execute digestion and absorption in a food vacuole, and then expel the waste. The cytosome or micropore, which is a permanent mouth for many protozoa, allows food to be eaten and then pass through to be encased in food vacuoles. Pinocytosis is an ingestive process in which fluid is drawn via minute, transient gaps in the body wall. A food vacuole is formed by the substance that has been swallowed being encased by a membrane.

Protozoa require the same kinds of organic and inorganic chemicals and have metabolic pathways that are comparable to those of higher animals. Significant progress has been made in recent years in the development of chemically defined media for the in vitro cultivation of parasitic protozoa. The resultant organisms are devoid of a number of compounds that might obstruct immunologic or biochemical research when present in organisms separated from a host or cultured in complicated conditions. The metabolic processes used by parasites but not by their hosts are of urgent relevance for research because they might serve as targets for antiprotozoal drugs that would block those metabolic processes while being safe for humans. Long before their mechanism of action was understood, many antiprotozoal medications were used empirically.

Many parasites multiply quickly, which raises the possibility of mutation and the possibility of changes in virulence, medicine sensitivity, and other traits. Examples include the resistance to chloroquine in Plasmodium falciparum and the resistance to arsenic in Trypanosoma rhodesiense.

Due to the relatively tiny quantities that parasitic protozoa need, competition for nutrients is typically not a significant component in disease. The nutritional health of the host can be greatly impacted by some parasites that live in the small intestine, such as Giardia and Cryptosporidium. The host’s nutritional requirements rise as a result of the parasites’ metabolic activity destroying the host’s cells and tissues. This might have a significant impact on how an illness affects a person who is undernourished. Furthermore, extracellular


Protozoa are a class of single-celled eukaryotes that can be free-living or parasitic and feed on organic materials such as other microbes, organic tissues, and detritus. The plural form of protozoa is protozoans. Protozoans were often thought of as “one-celled animals” because they frequently exhibit animal-like traits including predation and motility and lack a cell wall, which is present in plants and many types of algae.

The taxon Protozoa was established as a class within the Animalia when Georg Goldfuss (originally spelt Goldfuß) first presented it in 1818; the name “protozoa” means “first animals. It was promoted to various higher levels in later categorization systems, such as phylum, subkingdom, and kingdom, and occasionally it was placed inside Protoctista or Protista. In the late 19th and early 20th centuries, the classification of Protozoa within the Animalia was commonplace but not universal. By the 1970s, it had become customary to demand that all taxa be both holophyletic (including all known offspring of that same ancestor) and monophyletic (derived from a common ancestor that would also be considered a protozoan). These requirements are not met by the taxon ‘Protozoa’, and the methods used by

Despite being aware that the term “Protozoa” did not meet modern taxonomic standards, some authors have persisted in using it while using it to refer to various scopes of organisms. The taxon Protozoa has been used to classify a narrow range of creatures and has been classified as a kingdom by Thomas Cavalier-Smith and associates since 1981. Eight phyla that are not closely related to one another are included in the Kingdom Protozoa in a plan put up by Ruggiero et al. in 2015. These phyla include Euglenozoa, Amoebozoa, Metamonada, Choanozoa sensu Cavalier-Smith, Loukozoa, Percolozoa, Microsporidia, and Sulcozoa. The parasitic apicomplexans, dinoflagellates, ciliates, and foraminifera, which were classified in other groups like Alveolata and Stramenopiles under the polyphyletic approach, are notable exclusions from the protozoa under this method.

Frederick Chapman's The foraminifera: an introduction to the study of the protozoa
Clockwise from top left: Blepharisma japonicum, a ciliate; Giardia muris, a parasitic flagellate; Centropyxis aculeata, a testate (shelled) amoeba; Peridinium willei, a dinoflagellate; Chaos carolinense, a naked amoebozoan; Desmarella moniliformis, a choanoflagellate


In fresh, brackish, and salt water as well as other moist settings, such as soils and mosses, free-living protozoa are common and frequently prolific. Inhospitable settings, such hot springs and hypersaline lakes and lagoons, are ideal for some species. All protozoa need a moist environment to live in, but some can endure dry conditions for a long time by developing resting cysts that allow them to hibernate until the situation changes.

Protozoa that are parasitic or symbiotic exist on or within other species, such as vertebrates, invertebrates, plants, and other single-celled animals. Others, like babesia, malaria, and toxoplasmosis, may be important causes of illnesses. Some are benign or helpful to their host organisms.

It can be advantageous for both parties when protozoan symbionts and their host organisms associate. Termite stomachs are home to flagellated protozoa like Trichonympha and Pyrsonympha, which aid in the breakdown of complicated carbohydrates into more manageable components, allowing their insect host to digest wood.[49] In the rumens of ruminant animals like cattle and sheep, a broad variety of protozoa exist in commensal communities. These Aciliated protozoa like Isotricha and Entodinium as well as flagellates like Trichomonas. All of the mouthless symbionts in the ciliate subclass Astomatia have evolved for life in annelid worms’ guts.


All protozoa are heterotrophic, which means they obtain their nutrition from other living things either by phagocytosing them whole or by osmotrophy, which is the assimilation of dissolved organic materials or microscopic particles. Phagocytosis may entail ingesting food through a specialised mouth-like aperture known as a cytostome, employing stiffened ingestion organelles, or swallowing organic particles with pseudopodia (as do amoebae).

There are many distinct feeding tactics used by parasitic protozoa, and some of them might alter as they progress through different stages of their life cycle. For instance, the malaria parasite Plasmodium feeds by pinocytosis during its juvenile trophozoite stage of life (ring phase), but when it matures inside a host’s red blood cell, it forms a specific feeding organelle (cytostome).

Protozoa can also exist as mixotrophs, which combine an autotrophic diet with a heterotrophic one. The symbiotic photosynthetic algae (zoochlorellae), which live and grow inside the membranes of the bigger cell and supply nutrients to the host, create intimate connections with certain protozoa. Instead of being digested, the algae proliferate and are dispersed throughout the division products. Sometimes, the organism may gain from getting part of its nutrients from the algal endosymbionts or from being able to survive anoxic circumstances thanks to the oxygen created by algal photosynthesis. Some protozoans engage in kleptoplasty, where they steal the chloroplasts from their prey and keep them within their own cell bodies to continue photosynthesis and produce nutrients. The ciliate Mesodinium rubrum uses the cryptophyte algae it consumes to keep functional plastids.

Protozoa can also exist as mixotrophs, which combine an autotrophic diet with a heterotrophic one. The symbiotic photosynthetic algae (zoochlorellae), which live and grow inside the membranes of the bigger cell and supply nutrients to the host, create intimate connections with certain protozoa. Instead of being digested, the algae proliferate and are dispersed throughout the division products. Sometimes, the organism may gain from getting part of its nutrients from the algal endosymbionts or from being able to survive anoxic circumstances thanks to the oxygen created by algal photosynthesis. Some protozoans engage in kleptoplasty, where they steal the chloroplasts from their prey and keep them within their own cell bodies to continue photosynthesis and produce nutrients. The ciliate Mesodinium rubrum uses the cryptophyte algae it consumes to keep functional plastids.

Life cycle

Some protozoa have two-phase life cycles that alternate between resting cysts and proliferative stages (such as trophozoites). Some protozoa can endure difficult circumstances as cysts, including prolonged deprivation of food, water, or oxygen, exposure to harmful chemicals or extreme temperatures. Encysting permits parasitic organisms to spread from one host to another and to exist without a host. Protozoa actively feed when they are in the form of trophozoites (the Greek word tropho means to nurture). Entitystation and excystment are terms used to describe the processes through which trophozoites go from trophozoite to cyst form.

Most protozoa reproduce asexually by binary or multiple fission. Numerous protozoa exchange genetic material through sexual reproduction, usually through conjugation, but this is rare.

Resting cyst of ciliated protozoan Dileptus viridis.
sotricha intestinalis, a ciliate present in the rumen of sheep

Most protozoa reproduce asexually by binary or multiple fission. Numerous protozoa also translocate genetic material sexually (typically through conjugation), but this process is typically unrelated to reproduction and does not immediately lead to an increase in population. Sexuality is thus a choice.

Although modern eukaryotes frequently engage in meiotic sex, it has been unknown up until recently whether or not eukaryotes were sexually active before that. In an increasing number of protozoa from lineages that separated early in eukaryotic history, evidence for some type of meiotic sex has been discovered thanks to recent developments in gene identification and other approaches. (See reproduction in eukaryotes.) These results imply that meiotic sex emerged early in the development of eukaryotes. Examples of meiotic sexuality in protozoa are given.

Life cycle of parasitic protozoan, Toxoplasma gondii

Almost all habitats with unrestricted water, at least occasionally, have free-living protozoa. In natural ecosystems, they play a crucial part in the mobilisation of nutrients. Their function is best understood in relation to the microbial food web, in which they are among the most significant bacterivores. They aid in the transmission of bacterial and algal output to succeeding trophic levels in part, but they also help to stimulate microbial growth by solubilizing the nutrients found in microbial biomass. As predators, protozoa feed on bacteria, microfungi, filamentous algae, and micro-carrion. Protozoa have been suggested as a possible food source for microinvertebrates in the context of previous ecological theories of the micro- and meiofauna.


In the past, Protozoa were categorized as “unicellular animals” as opposed to Protophyta, which were single-celled photosynthetic creatures (algae) and were thought to be the first forms of plants. Both groups were frequently assigned the phylum rank within the Protista kingdom.

Once a few extraneous members (like Stephanopogon or protociliates and opalinids) were eliminated, it became clear that of the main sub-groups of Protozoa, only the ciliates (Ciliophora) formed a natural group, or monophyletic clade. This was made possible by the development of molecular phylogenetics and tools that allowed researchers to directly compare the DNA of various organisms. The polyphyletic groupings Mastigophora, Sarcodina, and Sporozoa were. These groups’ members had separately developed through convergent evolution into similar in terms of look and lifestyle.

Most eukaryote classification schemes, including one presented by the International Society of Protistologists, distribute members of the former phylum Protozoa among a number of supergroups.


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