Microbiology chapters
Entamoeba histolytica and Giardia
The two intestinal protozoa in the syllabus: Entamoeba histolytica (cysts and trophozoites, amoebic dysentery with flask-shaped ulcers, amoebic liver abscess with anchovy-sauce pus, diagnosis and metronidazole plus a luminal agent) and Giardia lamblia (pear-shaped trophozoites, malabsorption and fatty diarrhoea, diagnosis and treatment), compared with bacillary dysentery.
- Entamoeba histolytica spreads by faecal-oral ingestion of quadrinucleate cysts (the infective form; resist chlorine) in contaminated water and food; trophozoites (invasive, ingest red blood cells) form in the colon, attach with the Gal/GalNAc lectin, and make flask-shaped ulcers (narrow neck, broad base), mainly in the caecum and ascending colon, causing amoebic dysentery (gradual, bloody mucoid stools, little fever); spread via the portal vein gives an amoebic liver abscess (right lobe, single, "anchovy sauce" pus, raised right hemidiaphragm, often no dysentery at the time); many infected people are asymptomatic cyst passers (carriers); the look-alike E. dispar is harmless.
- Diagnosis: stool microscopy (trophozoites with ingested RBCs in fresh warm stool; cysts), stool antigen or PCR (separates E. histolytica from E. dispar), serology and ultrasound for liver abscess; treat invasive disease with metronidazole (or tinidazole) followed by a luminal agent (diloxanide furoate or paromomycin) to clear cysts.
- Giardia lamblia is a flagellate with pear-shaped trophozoites (two nuclei like "eyes", ventral sucking disc, falling-leaf motility) that attach to the duodenum and jejunum and cause non-bloody, foul-smelling, fatty diarrhoea, bloating, malabsorption and lactose intolerance; it spreads via cysts in water (resistant to chlorine), is worse in IgA deficiency, and is diagnosed by stool microscopy or antigen tests and treated with metronidazole or tinidazole.
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Entamoeba histolytica
Forms and life cycle
| Form | Features | Role |
|---|---|---|
| Cyst | Round, 10–15 µm; four nuclei when mature (quadrinucleate); chromatoid bars | Infective form; survives outside the body for weeks; resists chlorination (killed by boiling and filtration) |
| Trophozoite | 20–40 µm; one nucleus with central karyosome; directional motility with pseudopodia; ingested red blood cells (diagnostic of E. histolytica) | Invasive and disease-causing; dies quickly outside the body (not infective) |
Life cycle: humans swallow cysts in faecally contaminated water or food (also flies, food handlers, sexual oral-anal contact) → excystation in the small intestine → trophozoites multiply in the colon → some invade the wall; others encyst and are passed in stool (asymptomatic carriers pass cysts and keep transmission going). Humans are the only important host.
Pathogenesis
- Trophozoites attach to colonic epithelium using the Gal/GalNAc lectin, kill cells (contact-dependent; amoebapores, cysteine proteases) and invade.
- Flask-shaped ulcers: narrow opening in the mucosa, broad base in the submucosa; mucosa between ulcers looks normal; mainly caecum, ascending colon, then sigmoid and rectum.
- Spread through the portal vein to the liver (and rarely lungs, brain, skin).
Clinical forms
- Asymptomatic infection (cyst passer): most infections.
- Amoebic dysentery (amoebic colitis): gradual onset, diarrhoea with blood and mucus, abdominal pain, tenesmus; often little or no fever; can become chronic.
- Complications: toxic megacolon, perforation, haemorrhage, amoeboma (inflammatory mass mimicking carcinoma, often caecal), strictures.
- Amoebic liver abscess: the commonest extra-intestinal form; more in adult men; right lobe (most portal blood), usually single; fever, right upper quadrant pain, tender hepatomegaly, pain referred to the right shoulder, raised right hemidiaphragm, right pleural effusion; only a minority have current dysentery. Pus is "anchovy sauce" (chocolate-brown, odourless, sterile liquefied liver); trophozoites are found in the abscess wall, not the pus. Can rupture into the pleura, lung, pericardium or peritoneum.
Amoebic vs bacillary dysentery
| Feature | Amoebic (E. histolytica) | Bacillary (Shigella) |
|---|---|---|
| Onset | Gradual | Acute |
| Fever and toxaemia | Usually absent or mild | Present |
| Stools | Fewer, larger, offensive, blood mixed with faeces and mucus | Frequent, small, mostly blood and mucus, odourless |
| Microscopy | Few pus cells, RBCs in clumps, trophozoites with ingested RBCs, Charcot-Leyden crystals | Many pus cells (neutrophils), RBCs separate, macrophages |
| Ulcers | Flask-shaped, caecum | Superficial, irregular, rectosigmoid |
| Reaction | Acidic | Alkaline |
Diagnosis
- Stool microscopy: fresh, warm saline mount for motile trophozoites with ingested RBCs; iodine mount for quadrinucleate cysts; three samples on different days.
- Stool antigen (ELISA) or PCR: distinguishes E. histolytica from the harmless E. dispar (identical cysts).
- Serology (antibodies): positive in most patients with liver abscess and invasive disease (may stay positive for years).
- Ultrasound / CT: liver abscess; aspiration only for large abscesses, risk of rupture, or no response.
- Colonoscopy: ulcers; biopsy.
Treatment and prevention
- Invasive disease (colitis, liver abscess): metronidazole (or tinidazole), a tissue amoebicide, followed by a luminal amoebicide (diloxanide furoate or paromomycin) to eradicate cysts in the gut lumen and prevent relapse and spread. Metronidazole alone leaves many luminal cysts.
- Asymptomatic cyst passers: luminal agent alone.
- Prevention: safe water (boiling, filtration; chlorine at normal levels does not kill cysts), sanitation, handwashing, food hygiene, treating food handlers who carry cysts.
Giardia lamblia (G. intestinalis, G. duodenalis)
Forms
| Form | Features |
|---|---|
| Trophozoite | Pear-shaped (tear-drop), bilaterally symmetrical; two nuclei (look like eyes: "old man with spectacles" or monkey face); four pairs of flagella; ventral sucking (adhesive) disc; "falling leaf" motility |
| Cyst | Oval, four nuclei; infective form; resists chlorine; survives in cold water |
Transmission and pathogenesis
- Faecal-oral: contaminated drinking water (outbreaks; streams), food, person to person (day-care centres, institutions), sexual (oral-anal); animal reservoirs (beavers: "beaver fever").
- Low infective dose (10–25 cysts).
- Trophozoites attach to the duodenal and jejunal epithelium with the sucking disc (not invasive), causing villous blunting, loss of brush-border enzymes (lactase) and malabsorption.
- Risk factors: IgA deficiency, hypogammaglobulinaemia (common variable immunodeficiency), achlorhydria, malnutrition, travel.
Clinical features
- Many infections are asymptomatic.
- Acute: watery, foul-smelling, greasy (fatty) diarrhoea (steatorrhoea: pale, floating stools) without blood or mucus, abdominal bloating and cramps, flatulence, sulphurous belching, nausea, anorexia; no fever.
- Chronic: malabsorption (fat, fat-soluble vitamins, B12), weight loss, failure to thrive in children, lactose intolerance that can last weeks after cure.
Diagnosis
- Stool microscopy for cysts and trophozoites (repeated samples; excretion is intermittent).
- Stool antigen tests (ELISA, immunofluorescence) and PCR: more sensitive.
- Duodenal aspirate, biopsy, or the string test (Entero-test): a gelatin capsule on a string is swallowed and pulled up after some hours to look for trophozoites.
Treatment and prevention
- Metronidazole (5–7 days) or tinidazole (single dose); albendazole, nitazoxanide; in pregnancy, paromomycin.
- Boil or filter water (cysts survive normal chlorination); hand hygiene; sanitation.
Giardia = upper small bowel, fatty, non-bloody diarrhoea, malabsorption. Entamoeba histolytica = large bowel, bloody dysentery, liver abscess.
Cryptosporidium (brief)
Cryptosporidium parvum/hominis: water-borne (oocysts resist chlorine), watery diarrhoea; self-limiting in healthy people but chronic, severe diarrhoea in AIDS (CD4 < 200); oocysts are acid-fast on modified Ziehl-Neelsen stain; treatment: nitazoxanide and immune restoration (ART).
Expect: E. histolytica cyst (infective, quadrinucleate) vs trophozoite (invasive, ingested RBCs), faecal-oral spread and carriers, flask-shaped caecal ulcers, amoebic vs bacillary dysentery, amoeboma, liver abscess (right lobe, anchovy sauce, trophozoites in the wall), E. dispar and how to separate it, diagnostic tests, metronidazole plus luminal agent; Giardia morphology (sucking disc, falling-leaf), site (duodenum/jejunum), steatorrhoea and lactose intolerance, IgA deficiency, string test, treatment; Cryptosporidium modified acid-fast in AIDS.
Quick revision
- E. histolytica: quadrinucleate cyst (infective) → trophozoite with ingested RBCs (invasive).
- Flask-shaped ulcers in caecum; amoebic dysentery gradual, little fever.
- Liver abscess: right lobe, anchovy sauce pus, raised right hemidiaphragm.
- E. dispar looks the same but is harmless (antigen/PCR).
- Treatment: metronidazole then diloxanide furoate or paromomycin.
- Giardia: pear-shaped, sucking disc, falling-leaf; duodenum; fatty non-bloody diarrhoea; IgA deficiency.
- Giardia diagnosis: stool cysts, antigen, string test; metronidazole or tinidazole.
- Cysts of both resist chlorine: boil or filter water.
Written by The House Job from standard microbiology and parasitology references (Levinson's Review of Medical Microbiology and Immunology).
FAQ
What is the infective form of Entamoeba histolytica?
The mature quadrinucleate cyst, swallowed in faecally contaminated food or water.
What shape are the colonic ulcers in amoebiasis?
Flask-shaped, with a narrow opening and a broad base in the submucosa, mainly in the caecum and ascending colon.
Why is a luminal agent given after metronidazole?
Metronidazole kills invading trophozoites in tissues but leaves cysts in the bowel lumen; diloxanide furoate or paromomycin clears them to prevent relapse and spread.
Which part of the gut does Giardia infect and what diarrhoea does it cause?
The duodenum and jejunum; it causes foul-smelling, fatty, non-bloody diarrhoea with bloating and malabsorption.