Protozoa: Features and Structure

General Features

• Protozoa are single-celled eukaryotic microorganisms classified under the kingdom Protista.
• A single protozoal cell performs all necessary functions
• The majority of protozoa are nonpathogenic, while some cause diseases like malaria, leishmaniasis, and sleeping sickness.
• Cryptosporidium parvum and Toxoplasma gondii are opportunistic pathogens in immunocompromised patients, such as those with HIV.
• Protozoa vary in size (1-150 µm) shape, and structure, but all possess essential common features.

Structure

• A typical protozoan cell is bounded by a trilaminar unit membrane.
• Contractile fibrils support the membrane, enabling the cell to move and change shape.
• The Cytoplasm has two portions, Ectoplasm and Endoplasm
• Ectoplasm, the outer homogeneous part, functions in locomotion, engulfment of food, respiration, waste discharge, and protection. Pseudopodia are produced in the ectoplasm.
• Endoplasm, the inner granular portion, contains the nucleus, Golgi bodies, endoplasmic reticulum, food vacuoles, and contractile vacuoles.
• Contractile vacuoles regulate osmotic pressure.
• The nucleus is usually single, but some species can have double or multiple nuclei, with some having as many as a hundred.
• The nucleus contains one or more nucleoli or a central karyosome.

Terminology

• Chromatoid body: Extranuclear chromatin material, as found in Entamoeba histolytica cyst.
• Karyosome: A DNA-containing body within the nucleus, found in intestinal amoeba (e.g., E. histolytica, E. coli).
• Kinetoplast: Non-nuclear DNA, found in trypanosomes and located near the point of origin of the flagellum. The point of origin is called the basal body.
• Cilia: Fine, needle-like filaments covering the body surface of ciliates (e.g., Balantidium coli).
• Trophozoite: The active feeding and growing stage of protozoa, deriving nutrition from the environment.
• Chromatin is distributed along the periphery (peripheral chromatin) or as a condensed mass around the karyosome.

Reproduction

• Asexual reproduction is common in protozoans.
• Sexual reproduction occurs in ciliates and sporozoas.

Asexual Reproduction

• Binary Fission: A single parasite divides longitudinally or transversally into two or more equal parts.
• The nucleus divides mitotically, followed by the cytoplasm.
• Division occurs along any plane in amoebae, longitudinally in flagellates, and transversely in ciliates.
• Multiple Fission (Schizogony): The nucleus undergoes several divisions within the schizont, producing merozoites (e.g., Plasmodium).
• Endodyogeny: Some protozoa, like Toxoplasma, multiply by internal budding to form two daughter cells.

Sexual Reproduction

• Conjugation: Ciliates join and exchange nuclear material (e.g., Balantidium coli).

Life Cycle

• Single Host: Intestinal flagellates and ciliates require only one host, in which they multiply asexually and transfer via the cystic form.
• Second Host: Some protozoa (e.g., Plasmodium) reproduce asexually in one host (man) and sexually in another (mosquito).

Classification of Protozoa

• Protozoan parasites are classified under the kingdom Protista, subkingdom Protozoa, which is further divided into four phyla: Sarcomastigophora.
• Phylum Sarcomastigophora is divided into two subphyla based on locomotion:
• Sarcodina: Protozoa without permanent locomotory organs, moving via pseudopodia (e.g., Amoebae).
• Mastigophora: Protozoa with whip-like flagella (e.g., Trypanosoma and Trichomonas).
• Amoebae can assume any shape and crawl via pseudopodia (false feet).
• Structurally simple, they are believed to have evolved from flagellates by losing flagella.
• Gametogony or syngamy: In sporozoa, male and female gametocytes are produced which after fertilization form the zygote, which gives rise to numerous sporozoites by sporogony (e.g. Plasmodium).
• Apicomplexa.
• Ciliophora.