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  • 0:05 Early Classification Problems
  • 1:12 Protozoa
  • 3:48 Flagellates
  • 4:51 Amoebas
  • 6:13 Ciliates
  • 7:28 Sporozoans
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What are Protozoa? - Definition, Characteristics & Examples

Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college microbiology and anatomy & physiology, has a doctoral degree in microbiology, and has worked as a post-doctoral research scholar for Pittsburgh’s National Energy Technology Laboratory.

Taxonomy is the realm of science that attempts to group similar organisms together, which is not always an easy task. In this lesson, we will examine the protozoa, a diverse group of organisms that didn't really fit anywhere else. Updated: 10/15/2019

Early Classification Problems

Carl Linnaeus is thought of as the father of modern taxonomy. It was Linnaeus who came up with the two-word descriptions of organisms: the genus and species names that all scientists are familiar with. At first, organisms were placed in groups based solely on what could be observed: appearance, behavior, and ecological niche. But, what happens when you observe thousands of organisms, create hundreds of distinct groups, and end up with a bunch of single-celled oddballs that don't really fit anywhere else?

This was the problem facing noted plant ecologist Robert Whittaker in the 1950s when he proposed his plan for a five-kingdom taxonomic system. Without the ability to sequence and compare the genomes of these oddballs, all he could do was guess where to put them. In a decision borne more out of convenience than accuracy, he stuck them all in the Kingdom Protista and moved on. What resulted was a group of distantly related organisms forever lumped together and collectively called 'protozoa.'


The group of organisms known as 'protozoa' are defined by a few of their shared characteristics. Protozoa are non-phototrophic, unicellular, eukaryotic microorganisms with no cell walls. This diverse group of over 65,000 species generally share these basic attributes. Looking deeper, this group can be extremely complex and variable. In fact, the protozoa are often described as the pinnacle of unicellular complexity.

Unlike the relatively simple bacteria, protozoa can have many different intracellular organelles performing specific tasks. Some species of protozoa have structures that are analogous to mouths, GI tracts, and anuses. This probably goes against everything you've been taught about microbes being simply bags of proteins and enzymes.

Many protozoa cause diseases in animals and humans. Some, like Plasmodium, which causes malaria, can be devastating to people worldwide. Others, like Trichomonas, cause sexually transmitted diseases that are relatively benign and 100% curable. The vast majority of the species, though, are completely harmless. But, as is usually the case in microbiology, it's the dangerous ones that get the most attention.

The protozoa can have very diverse lifecycles with multiple morphological stages, depending on species. Most protozoa have a cyst stage, which is dormant and highly resistant to environmental stress. In the disease-causing species, these cysts are often the mode of infection, frequently acquired by fecal-oral contamination. The trophozoite stage is the active, reproductive, and feeding stage. The image below shows a purple trophozoite emerging from an oblong brownish-red cyst.

A trophozoite emerging from a cyst
purple trophozoite

The trophozoite is the stage that typically causes disease by pathogenic protozoa. This trophozoite can be very specific, infecting only one species, like humans. Or, it can be more general, infecting whole groups, like any mammal, for example.

Since the protozoa are so diverse and only distantly related, they have been separated into four major groups based on motility and the structures used to generate movement. We're going to look at all four groups individually, highlighting the key points and providing examples of important species within the group.


The first group are the Mastigophora, also known as the Flagellates. As the name indicates, this group swims by waving long, whip-like flagella. The protozoal flagellum is structurally different than the bacterial flagellum. Also, the protozoal flagellum waves while the bacterial flagellum spins.

There are several important disease-causing flagellates. Trypanosoma brucei causes African sleeping sickness, a disease that kills an estimated 65,000 people in Africa every year. The lifecycle of Trypanosoma is interesting because it uses two hosts. The disease manifests in humans but must be transmitted through the bite of an infected tsetse fly.

Giardia is a common pathogenic flagellate that causes diarrhea and is known informally as Beaver Fever. Another common protozoa is Trichomonas, a sexually transmitted flagellate that can cause urogenital symptoms in infected women.


The Sarcodina group are commonly known as the Amoebas. This is a huge group with members found in nearly every environment imaginable. These amoebas are characterized by having a trophozoite stage that is naked, meaning the cell has no structural components on its membrane that maintain a shape. What results is an amorphous blob that moves by pseudopod projections.

The cell extends forward a portion of its cell membrane, called a pseudopod, as it slowly withdraws the cell membrane on the opposite end. The result is slow movement in one direction by simply pushing forward and pulling up the rear. These are classic microbes examined in intro bio classes to demonstrate an interesting mode of motility.

There are many genera of Amoebas that live symbiotically with animals, typically in the oral cavity or gastrointestinal tract. Very few cause disease, but one species in particular, Entamoeba histolytica, can be quite deadly. The disease is acquired by drinking water contaminated with Entamoeba cysts, usually present in areas with poor sanitation. Entamoeba can cause amoebic dysentery characterized by painful ulcers in the large intestine and diarrhea. Every year about 100,000 people die worldwide from Entamoeba histolytica.

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