PROTOZOA

 INTRODUCTION

Single cell eukaryote belongs to the kingdom Protista

 (Greek protos: first; zoon: animal)

GENERAL FEATURES

• The single protozoa cell performs all functions. 
• Most of the protozoa are completely non-pathogenic but few may cause major diseases such as malaria, leishmaniasis and sleeping sickness. 
• Protozoa like Cryptosporidium parvum and Toxoplasma gondii are being recognized as opportunistic pathogens (parasites (especially protozoa) that normally do not cause disease in healthy individuals but can produce serious infections when the host’s immune system is weakened)  in patients affected with human immunodeficiency virus (HIV) and in those undergoing immunosuppressive therapy.
• Protozoa exhibit wide range of size (1- 150 µm), shape and structure; yet all possess essential common features

STRUCTURE

A typical protozoan cell is bounded by a trilaminar unit membrane, supported by a sheet of contractile fibrils enabling the cell to move and change in shape. 

CYTOPLASM

Cytoplasm has 2 portions

1. Ectoplasm

• Outer clear part of the cell
• Helps the organism to:
  • Move using pseudopodia
  • Capture and engulf food
  • Breathe (respiration)
  • Remove waste materials
  • Protect the cell

2. Endoplasm

• Inner granular part of the cytoplasm
• Contains the nucleus and cell organelles such as:
  • Golgi bodies
  • Endoplasmic reticulum
  • Food vacuoles
  • Contractile vacuoles (Maintain osmotic balance by removing excess water from the cell)

NUCLEUS

Usually single nucleus, but some protozoa may have two or many nuclei.

The nucleus containsNucleolus or karyosome & Chromatin material

Chromatin arrangement may be distributed along periphery (peripheral chromatin) or as condensed mass around the karyosome.

IMPORTANT TERMINOLOGY IN PROTOZOOLOGY

1. Chromatoid Body: Extranuclear chromatin material (e.g., cyst of Entamoeba histolytica)
2. Karyosome: DNA containing body, situated peripherally or centrally within the nucleus (e.g., Entamoeba histolytica & Entamoeba coli).
3. Kinetoplast: Extra DNA present outside nucleus (e.g., Trypanosomes). Flagellum originates near the kinetoplast. Point of origin of flagellum is called as basal body.
4. Cilia: Fine hair-like structures covering the body which helps in movement (e.g., Balantidium coli)
5. Trophozoite: Active feeding and growing stage of protozoa. Obtains nutrition by diffusion, pinocytosis & phagocytosis.

REPRODUCTION

Reproduction usually occurs asexually in protozoans; however, sexual reproduction occurs in ciliates and sporozoans.

Asexual Reproduction

1. Binary fission: 

•  A single parasite divides either longitudinally or transversally into two or more equal number of parasites. 
• Mitotic division of nucleus is followed by division of the cytoplasm.
•  In amebae, division occurs along any plane, but in flagellates, division is along longitudinal axis and in ciliates, in the transverse plane

2. Multiple fission or schizogony: Plasmodium exhibits schizogony, in which nucleus undergoes several successive divisions within the schizont to produce large number of merozoites

3. Endodyogeny: Some protozoa like Toxoplasma, multiply by internal budding, resulting in the formation of two daughter cells.

Sexual Reproduction

1. Conjugation: In ciliates, the sexual process is conjugation, in which two organisms join together and reciprocally exchange nuclear material (e.g. Balantidium coli).

2. 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).

LIFE CYCLE

Single host: Protozoa like intestinal flagellates and ciliates require only one host, within which they multiply asexually in trophic stage and transfer from one host to another by the cystic form. 

Second host: In some protozoa like Plasmodium, asexual method of reproduction occurs in one host (man) and sexual method of reproduction in another host (mosquito).

CLASSIFICATION OF PROTOZOA

Protozoa of medical importance belong to:

• Kingdom: Protista
• Subkingdom: Protozoa

They are divided into 4 phyla:

1. Sarcomastigophora
2. Apicomplexa
3. Microspora
4. Ciliophora

I. Phylum Sarcomastigophora

Divided into 2 subphyla based on movement:

1. Sarcodina

• Move using pseudopodia (false feet)
• No permanent locomotory organs
• Example: Amoebae

2. Mastigophora

• Move using flagella
• Example:
  • Trypanosoma
  • Trichomonas

Amoebae

• Can change shape
• Move using pseudopodia
• Simple in structure

Types of Amoebae

1. Amoebae of alimentary canal

• Important example:
  • Entamoeba histolytica
• Causes:
  • Intestinal amoebiasis
  • Extraintestinal amoebiasis

2. Free-living amoebae

Found in soil and water.

Important types:

• Naegleria
• Acanthamoeba

Can cause:

• Eye infections
• Fatal meningoencephalitis

Flagellates

• Have whip-like flagella for movement
• Some possess undulating membrane

Types of Flagellates

1. Kinetoplastida

• Possess kinetoplast
• Single flagellum arises from kinetoplast
• Blood parasites transmitted by insects

Examples:

• Trypanosoma
• Leishmania

Cause:

• Systemic or local infections

2. Flagellates without kinetoplast

• Have multiple flagella
• Mostly intestinal parasites

Examples:

• Giardia
• Trichomonas

II. Phylum Apicomplexa

(Formerly called Sporozoa)

Characteristics

• Possess apical complex for attachment to host cells
• Tissue parasites
• Complex life cycle with:
  • Sexual stage
  • Asexual stage

Examples

• Malarial parasites (Plasmodium)
• Toxoplasma
• Sarcocystis
• Isospora
• Cryptosporidium
• Babesia
• Pneumocystis jirovecii

III. Phylum Ciliophora

Characteristics

• Move using cilia
• Cilia cover entire body

Important parasite

• Balantidium coli

Cause:

• Rarely causes dysentery

IV. Phylum Microspora

Characteristics

• Very small intracellular parasites
• Commonly infect immunodeficient people
• Rarely infect healthy individuals