Intestinal Amebae, Free-Living Ameba and Balantidium Coli Chapter Notes | Microbiology - NEET PG PDF Download

Intestinal Amebae: Entamoeba Histolytica

• Entamoeba histolytica resides in the large intestine of humans.
• It exists in three morphological stages: trophozoites, pre-cyst, and cyst.

Life Cycle

• Humans serve as the sole host for Entamoeba histolytica.
• The mature quadrinucleate cyst is the infective form.
• Transmission occurs primarily through the faeco-oral route, with rare cases (20–30%) via sexual transmission among homosexuals.
• Excystation happens in the small intestine, releasing eight small metacystic trophozoites.
• These trophozoites are carried to the large intestine, where they colonize the gastrointestinal mucosa.
• Depending on the host’s nutritional status and immunity, outcomes include:
  • Asymptomatic cyst passers.
  • Amebic dysentery.
  • Amebic liver abscess.
• Encystation occurs in the large intestine, starting with precyst formation, followed by immature cysts, and culminating in mature quadrinucleated cysts released in feces, which are the diagnostic form.
• For example, the life cycle illustrates how a person consuming contaminated water with mature cysts can develop amebic dysentery if trophozoites invade the intestinal mucosa, highlighting the faeco-oral transmission risk in poor sanitation settings.

Virulence Factors

• Amebic lectin antigen aids in adhesion to host tissues.
• Other factors include cysteine proteinase, amebapore, hydrolytic enzymes, neuraminidase, and metallocollagenase, which contribute to tissue invasion and damage.

Intestinal Amebiasis

• Affects males and females equally, with a 1:1 ratio.
• Amebic ulcers are flask-shaped, featuring a broad base and narrow neck.
• The ileocecal region is the most common site for these ulcers.
• Complications include:
  • Fulminant amebic colitis.
  • Amebic appendicitis.
  • Intestinal perforation and amebic peritonitis.
  • Toxic megacolon and intussusception.
  • Perianal skin ulcers.
  • Ameboma, a diffuse pseudotumor-like mass in the recto-sigmoid region, seen in chronic amebiasis.

Amebic Liver Abscess

• The posterior superior surface of the right lobe of the liver is the most commonly affected site.
• Male to female ratio is 9:1.
• Liver abscess pus, known as anchovy sauce pus, is thick and chocolate brown in color.
• Other organs that may be affected include the lung, brain, and skin.

Laboratory Diagnosis

• Collect at least three stool samples on consecutive days due to intermittent shedding of amebae.
• Stool microscopy identifies:
  • Trophozoites, indicating active infection.
  • Quadrinucleated cysts, indicating a carrier state.
• Differentiation is necessary between Entamoeba histolytica and Entamoeba coli, a commensal in stool.
• Stool culture is used for detection.
• Polyxenic culture includes bacterial supplements to nourish amebae, used for chronic or asymptomatic carriers with low cyst numbers.
• Polyxenic culture has 50–70% sensitivity and 100% specificity, considered the gold standard.
• Culture media include:
  • National Institute of Health (NIH) media.
  • Boeck and Drbohlav egg serum medium with Locke’s solution.
  • Balamuth’s medium, Nelson’s medium, and Robinson’s medium.
• Axenic culture, lacking bacterial supplements (e.g., Diamond’s medium), is useful for:
  • Studying ameba pathogenicity.
  • Testing antiamebic drug susceptibility.
  • Preparing amebic antigen for serological tests.
  • Harvesting parasites for zymodeme pattern analysis.
• Stool antigen detection (copro-antigen) uses ELISA or immunochromatographic tests (ICT) to detect lectin antigen.
• Amebic antigen in serum indicates recent or active infection, detected via ELISA for lectin antigen.
• Amebic antibodies appear in later stages of intestinal amebiasis, detected by ELISA, indirect fluorescent antibody (IFA), or indirect hemagglutination test (IHA).
• Zymodeme analysis detects isoenzyme markers such as malic enzyme, hexokinase, and isomerase.
• Nested multiplex PCR is used for diagnosis.
• Presence of Charcot-Leyden crystals in stool and moderate leucocytosis in blood are additional findings.

Laboratory Diagnosis of Amebic Liver Abscess

• Microscopy and histopathology of liver pus reveal trophozoites but never cysts, with low sensitivity (<25%) but high specificity.
• Stool microscopy and culture are not useful for liver abscess diagnosis.
• Lectin antigen is usually absent in stool but detectable in serum, liver pus, and saliva.
• Antibody levels are often elevated, detected by ELISA and IHA, but persist post-cure, making it difficult to distinguish recent from past infections.
• PCR is performed on amebic liver pus.
• Ultrasound (USG) of the liver identifies the abscess site and its extension.

Characteristics of Trophozoites and Cysts of Common Intestinal Entamoeba Species

E. Histolytica Comprises of Three Subspecies

• Subspecies include E. h. histolytica, E. h. dispar, and E. h. moshkovskii.
• Cysts and trophozoites of all three subspecies are morphologically identical.
• All colonize the large intestine, but only E. h. histolytica causes invasive disease.
• Differentiation is based on:
  • Lectin antigen, present only in E. h. histolytica.
  • Presence of RBCs inside trophozoites, indicating invasion, found only in E. h. histolytica.
  • PCR.
  • Distinct isoenzyme markers via zymodeme study.

Treatment

• Amebic dysentery or amebic liver abscess is treated with tinidazole and metronidazole.
• Luminal infection is treated with paromomycin and iodoquinol.

Free-living Amebae

Other Free-Living Ameba

• Balamuthia mandrillaris causes granulomatous amebic encephalitis (GAE) and skin lesions.
• Sappinia diploidea is another free-living ameba.

Balantidium Coli

• Only ciliate parasite that affects humans: The active form, called trophozoite, has tiny hair-like structures.
• Largest protozoan that can invade the human digestive system.
• Location affected: Primarily the large intestine.
• Infective stage: Cysts found in food or drinks that are contaminated.
• Both the trophozoite and cyst forms contain two nuclei: a macronucleus and a micronucleus.
• Trophozoites reproduce through binary fission and conjugation.
• Dysentery: Causes ulcers similar to those from amoebic infections but does not penetrate the muscle layer.
• Diagnosis: Identification of cysts is more common than finding trophozoites.
• Treatment: The main medication used is tetracycline.