Sechenov School
Home › Microbiology › Protozoa Morphology and Classification

Protozoa Morphology and Classification

Protozoa

For medical students2 min readUpdated 2026-10-10

Protozoa are eukaryotic, single-celled microorganisms equipped with a complete set of cellular organelles. They are capable of active locomotion, nutrient engulfment, and forming protective stages to survive harsh environmental conditions.

SizeVaries from 2 to 100 µm; their structure is studied under a microscope.
StainingWith Romanowsky-Giemsa stain, the nucleus turns red and the cytoplasm blue.
SurvivalForm cysts that are resistant to temperature and humidity fluctuations.
Cell coveringCovered by a pellicle, which is a dense membrane acting as a cell wall analogue.

Cytology and Physiological Features

Protozoa belong to the domain Eukarya. Previously, all such microorganisms were grouped into the kingdom Protozoa, but modern taxonomic data have established a separate kingdom, Chromista. This kingdom includes organisms previously considered protozoa or fungi, such as Blastocystis, oomycetes, and Rhinosporidium seeberi.

The protozoan cell contains a distinct nucleus with a nuclear envelope and a nucleolus. The cytoplasm contains the endoplasmic reticulum, mitochondria, lysosomes, and numerous ribosomes. The cell is covered externally by a pellicle.

Nutrition occurs via phagocytosis—the microorganism engulfs solid particles to form digestive vacuoles. Metabolic waste products are eliminated through contractile vacuoles. Protozoa exhibit two main forms of existence:

Locomotion Organelles

The morphology of locomotion organelles is a key feature used to divide protozoa into classes:

  1. Pseudopodia (false feet)—cytoplasmic projections that provide amoeboid movement and phagocytosis.
  2. Flagella—originate from a basal body (blepharoplast). A flagellum consists of a membrane and an internal microtubule skeleton (nine peripheral doublets and a central pair). In some parasites, the flagellum connects to the cell body, forming an undulating membrane. Special fibrils may provide structural support.
  3. Cilia—structurally similar to flagella, but significantly shorter, thinner, and present in large numbers across the cell surface.

Classification of Protozoa

Medical microbiology divides the kingdom Protozoa into four main groups based on their mode of locomotion and lifestyle.

1. Amoebae (Sarcodina) Move using pseudopodia. Reproduce asexually (by binary fission). Include both free-living species and harmless commensals (Entamoeba coli), as well as dangerous pathogens. The main representative is Entamoeba histolytica, which causes amoebic dysentery.

2. Flagellates (Mastigophora) Possess one or more flagella. Classified by their site of parasitism:

3. Sporozoans (Sporozoa) Exclusively obligate intracellular parasites. Adult forms lack locomotion organelles. They use an apical complex to invade host cells. They feature a complex life cycle with alternation of hosts and reproductive types (asexual schizogony and sexual sporogony).

LocalizationRepresentativesDisease
BloodPlasmodium vivax, Babesia bovisMalaria, piroplasmosis
Tissues and intestinesToxoplasma gondii, Cryptosporidium parvumToxoplasmosis, cryptosporidiosis

This group also includes the causative agents of sarcocystosis (Sarcocystis), isosporiasis (Isospora), and cyclosporidiasis (Cyclospora).

4. Ciliates (Ciliata) Locomotion organelles are numerous cilia. A typical human pathogen is Balantidium coli, which infects the large intestine and causes balantidial dysentery.

Mnemonic

Protozoan classes are easy to remember by their organelles: Amoebae "flow" (pseudopodia), Flagellates "beat" (flagella), Ciliates "shiver" (cilia), and Sporozoans "sleep" inside cells (no locomotion organelles).

Frequently asked questions

What specific stages does the life cycle of the malaria parasite include in the human and mosquito hosts?

The life cycle of the malaria parasite includes sexual development in the mosquito and asexual reproduction in the human.

In the mosquito:

  • Sporogony—fusion of male and female gametocytes, zygote formation, and development of sporozoites that migrate to the salivary glands.

In the human, the cycle includes:

  • Tissue schizogony—sporozoites inside hepatocytes progress through trophozoite and schizont stages, producing tissue merozoites.
  • Erythrocytic schizogony—merozoites in erythrocytes transform into trophozoites (ring, young, semi-mature, mature), then into schizonts, which form erythrocytic merozoites.
  • Gametogony—some merozoites differentiate into immature sex cells (microgametocytes and macrogametocytes).
Which Plasmodium species cause malaria in humans?

Human malaria is caused by four main species of Plasmodium.

  • Plasmodium falciparum—causative agent of falciparum malaria, causing the most severe clinical course.
  • Plasmodium vivax—causative agent of vivax (tertian) malaria.
  • Plasmodium ovale—causative agent of ovale malaria (a form of tertian malaria).
  • Plasmodium malariae—causative agent of quartan malaria.
Which amoebic species are harmless commensals of the human intestine?

Non-pathogenic commensals of the human large intestine include:

  • Entamoeba coli
  • Entamoeba hartmanni
  • Iodamoeba buetschlii

Overall, 7 amoebic species parasitize the human large intestine; most are non-pathogenic. The exception is Entamoeba histolytica (dysenteric amoeba). Occasionally, non-pathogenic amoebae may provoke disease.

What is the difference between a trophozoite and a cyst?

A trophozoite is the active, vegetative stage of a protozoan that feeds and moves, but is easily destroyed in the external environment. A cyst is a resting stage covered by a protective wall that allows the microorganism to survive unfavorable conditions.

What is a blepharoplast?

A blepharoplast is a basal body—a structure in the protozoan cytoplasm from which a flagellum originates. It serves as the microtubule-organizing center for the motor apparatus.

What is the hallmark of the sporozoan life cycle using Plasmodium as an example?

The sporozoan developmental cycle involves alternating hosts. For instance, the malaria parasite reproduces sexually in the mosquito (definitive host), whereas in human cells (intermediate host) it undergoes multiple asexual division—schizogony.

Go deeper

More topics in Microbiology

Odontogenic InfectionsImmunological ToleranceDeuteromycetes (Fungi Imperfecti)Viral Morphogenesis and AssemblyDental CariesHypersensitivity ReactionsAbortive InfectionPeriodontal Diseases: Pathogenesis and MicrobiologyType I HypersensitivityMorphology and Classification of VirusesIntegrative Infection (Virogeny)Type II HypersensitivityMicrobiology →