Protozoa: Features and Structure

Protozoa

• Single-celled eukaryotic microorganisms that perform reproduction, digestion, excretion and respiration
• Named from the Greek words "protos" (first) and "zoon" (animal)
• Parasitic protozoa adapt to different host species

General Features

• Most protozoa are non-pathogenic, but some can cause major diseases like malaria
• Opportunistic protozoa can cause severe disease in immunocompromised individuals
• Can range in size from 1-150 μm with a wide variety of shapes and structures

Structure of Protozoa

• The protozoan cell is covered with a cell membrane and is formed of cytoplasm and a nucleus

Cytoplasm

• Composed of ectoplasm and endoplasm
• Ectoplasm is the outer homogeneous part of cytoplasm, serving as an organ for locomotion and engulfment of food via pseudopodia
• Ectoplasm also aids in respiration, ingestion, discharging waste and provides a sensory and protective covering
• Endoplasm is the inner granular portion of cytoplasm, containing nucleus, golgi bodies, endoplasmic reticulum and food vacuoles
• Contractile vacuoles in the endoplasm regulate osmotic pressure and the function of the cytoplasm is nutrition (metabolism) and reproduction

Nucleus

• Usually single, but can be double or multiple; some species can have up to 100 nuclei in a single cell
• Contains one or more nucleoli or a central karyosome (DNA-containing body)
• Chromatin may be distributed along the periphery
• The nucleus regulates cell activity and reproduction

Movement

• Protozoa can move by pseudopodia (Amoeba), cilia (Balantidium), and flagella (Giardia and Leishmania)
• Trypanosoma uses an undulating membrane for movement
• Apicomplexan protozoa lack locomotive organs but use an apical complex to penetrate host cells (e.g., Plasmodium, Toxoplasma, Cryptosporidium)

Respiration and Nutrition

• Respiration occurs by directly taking oxygen or via a metabolic process
• Nutrition occurs via absorption of liquid food, or ingestion of solid food by pseudopodia or cytostome, surrounded by food vacuoles

Excretion and Secretion

• Excretion occurs through osmotic pressure, contractile vacuoles, diffusion, or cytopyge
• Protozoa can secrete cyst walls, pigments, digestive and proteolytic enzymes, and toxic/antigenic substances

Reproduction

• Can reproduce asexually or sexually

Asexual Reproduction

• Binary fission: parasite divides longitudinally or transversally into equal parts (e.g. Amoeba, ciliates)
• Multiple fission: the nucleus undergoes several successive divisions called schizogony or merogony (e.g. Malaria parasite (Plasmodium) in humans)
• Endodyogeny: parasite multiplies by internal budding, resulting in two daughter cells (e.g. Toxoplasma)

Sexual Reproduction

• Conjugation: two organisms join and exchange nuclear material (e.g. Balantidium)
• Gametogony: male and female gametocytes produce a zygote after fertilization, giving rise to sporozoites (e.g. Malaria parasite in mosquitoes)

Life Cycle

• Simple: protozoa need only one host to multiply asexually (e.g. Amoeba)
• Complex: protozoa multiply asexually in one host (human) and sexually in another (mosquito) (e.g. Malaria parasite)

Life Stages

• Trophozoite: active, feeding, moving and multiplying stage of the parasite
• Cyst: resting or resistant stage of protozoa, bounded by a tough cell wall

Modes of Infection

• Ingestion of the infective stage: Giardia intestinalis cyst, Entamoeba histolytica cyst, Cryptosporidium parvum oocyst, Toxoplasma gondii oocyst
• Eating improperly cooked meat: Toxoplasma gondii cyst
• Insect bite: Female Anopheles transmit malaria, female sand fly (Phlebotomus) transmit leishmaniasis and kissing bug (Reduviid or Triatomine) transmits Chagas disease
• Sexual transmission: Trichomonas vaginalis trophozoite
• Blood transfusion: Plasmodium trophozoites and merozoites
• Organ transplantation: Leishmania donovani amastigote
• Transplacental transmission: Toxoplasma gondii tachyzoites
• Autoinfection: Internal or external (fecal-oral transmission) e.g., Giardia lamblia, Entamoeba histolytica, and Cryptosporidium parvum

Prevention and Control

• Health education
• Environmental sanitation and personal hygiene
• Case treatment
• Pure water supply
• Safe and clean food
• Sanitary disposal of feces
• Vectors control

Classification (Phylum)

• Sarcomastigophora
  • Subphylum Sarcodina (Amoebae): Parasitic and free-living amoeba
  • Subphylum Mastigophora (Flagellates):
    • Intestinal (Giardia)
    • Urogenital (Trichomonas)
    • Haemo-somatic (Leishmania and Trypanosoma)
• Ciliophora (Balantidium)
• Apicomplexa (Plasmodium, Toxoplasma, and Cryptosporidium)