Q.Why is Entamoeba histolytica described as an obligate anaerobe?

Entamoeba histolytica is a shapeless, pseudopodia-forming parasitic amoeba of the human intestine, with a direct (single-host) life cycle alternating between an active, disease-causing trophozoite stage and a resistant, transmissible cyst stage.

Structure of Entamoeba histolytica:

• Shape: No definite/fixed shape — it constantly changes shape by extending temporary, finger-like projections called pseudopodia ('false feet'), which are also used for locomotion and for engulfing food (phagocytosis).
• Size: Microscopic, typically 15-30 µm.
• Nucleus: A single, small, spherical nucleus with a centrally placed karyosome and a thin, evenly beaded peripheral chromatin layer (visible on staining).
• Cytoplasm: Differentiated into an outer clear ectoplasm (used to form pseudopodia) and an inner granular endoplasm (containing the nucleus, food vacuoles, and — in the pathogenic trophozoite stage — often engulfed host red blood cells).
• No mitochondria and no contractile vacuole — an adaptation to its low-oxygen intestinal habitat and its isotonic internal environment (living in a host, it does not need to expel excess water like free-living freshwater protozoans).
• No cell wall/pellicle, only a delicate plasma membrane, allowing shape change.

Life cycle (a direct, single-host cycle):

1. Infection: A human ingests food or water contaminated with mature, resistant quadrinucleate cysts (cysts containing 4 nuclei) shed in the faeces of an infected person.
2. Excystation: In the small intestine, the cyst wall is digested and the cyst releases a single amoeba that divides to form 8 small trophozoites (metacystic trophozoites).
3. Trophozoite stage (active, pathogenic stage): These trophozoites migrate to and colonise the large intestine (colon), where they feed by phagocytosis, multiply by simple binary fission, and in pathogenic infections invade the intestinal mucosa, causing ulceration — this is the stage responsible for amoebic dysentery (bloody, mucus-containing diarrhoea) and, if it spreads via the bloodstream, amoebic liver abscess. …

Entamoeba histolytica is a monogenetic gut parasite of humans causing amoebic dysentery; it has a trophozoite (feeding) stage and a resistant cyst stage, and spreads by cysts through contaminated food and water.

(Diagrams to be drawn and labelled: (1) the TROPHOZOITE - showing ectoplasm, endoplasm, pseudopodia, a single nucleus with a central karyosome, and food vacuoles with ingested RBCs; (2) the mature CYST - a rounded, four-nucleate (tetranucleate) cyst with a cyst wall; and (3) the LIFE-CYCLE stages in sequence.)

Structure of Entamoeba histolytica:

• It is an endoparasitic protozoan living in the large intestine (colon) of humans.
• The trophozoite (active, feeding form) is amoeboid, with an outer clear ECTOPLASM and inner granular ENDOPLASM.
• It forms blunt PSEUDOPODIA for creeping movement and for engulfing food (including red blood cells and tissue).
• It has a single spherical NUCLEUS with a central karyosome (endosome) and peripheral chromatin.
• It has food vacuoles but NO contractile vacuole (being a parasite in an isotonic medium).

Life cycle (monogenetic - completed in a single host, man):

1. Trophozoite stage:

• The trophozoites live in the wall/lumen of the large intestine, feed on the mucosa and RBCs, cause ulcers, and multiply by binary fission. This produces amoebic dysentery (blood and mucus in stools).

2. Encystment (cyst formation):

• Some trophozoites become rounded, throw out food, and secrete a resistant CYST WALL around themselves, forming a precyst.

• Within the cyst the nucleus divides twice to form a mature TETRANUCLEATE (four-nucleate) cyst - the infective stage.

3. Transmission:

• The mature cysts pass out with the FAECES of the patient.

• Through contaminated food, water or hands (poor sanitation; house flies may act as carriers), the cysts are SWALLOWED by a healthy person.

4. Excystment and multiplication: …

Entamoeba histolytica is an amoeboid gut parasite that alternates between a feeding/pathogenic trophozoite stage in the host's colon and a resistant, quadrinucleate cyst stage that is passed in faeces and transmits the infection through contaminated food or water.

Structure:

• Trophozoite (feeding stage): Unicellular, with no definite/fixed shape (amoeboid), moves and feeds using blunt, finger-like pseudopodia (lobopodia). Cytoplasm is differentiated into a clear outer ectoplasm and a granular inner endoplasm (which often contains ingested/phagocytosed red blood cells in the pathogenic form). It has a single, small nucleus with a characteristic centrally placed karyosome and peripheral chromatin granules. No cyst wall is present at this stage; it thrives in the moist environment of the large intestine.
• Cyst (resting/infective stage): Spherical, surrounded by a thick, resistant wall that protects it from the external environment and the host's gastric acid. Young cysts are uninucleate, maturing to the fully infective quadrinucleate (4-nucleate) cyst; may also contain sausage-shaped chromatoid bodies (stored glycogen/RNA-protein food reserves) — more prominent in immature cysts.

Life cycle:

1. Infection: A human host ingests mature quadrinucleate cysts present in faecally contaminated food or water.
2. Excystation: In the small intestine, the cyst wall is digested, releasing a single amoeba which undergoes nuclear and cytoplasmic division to give rise to 8 small metacystic trophozoites. …