Microsporidia: the pathway in the body

This page shows the biochemical pathway behind the laboratory value Microsporidia (Enterocytozoon, Encephalitozoon): which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

Microsporidia are very small, spore-forming organisms from among the relatives of the fungi. They deliver their contents into a gut cell through a tube that shoots out, and they multiply only inside it.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

1. Spores → Signal in the gut · Wasser, Salze In the gut the spore absorbs water, and the pressure inside it rises. Acid, salts and charged particles from the surroundings give the signal for this. Source 3↑ supplies The rising pressure is the drive for the next step: it forces the coiled tube outwards in fractions of a second. The signals come together only in the gut, not in the surroundings. observed in studies Source 3, 2
   ⚖ When the balance tips

   too much — Where the signals come together fully, many spores germinate at once.

   too little — Where they are absent, the spore stays closed and passes through the gut unchanged.

   observed in studies · Source 3

   Field of research — Germination of the spores is studied in reviews on the biology of microsporidia. Source 3

2. Signal in the gut → Polar tube Auskeimen The tube shoots out of the spore and pierces the membrane of a gut cell. It thereby forms a short bridge from the spore into the interior of the cell. Source 2, 4↓ depletes The tube is tool and route in one: it opens the cell membrane and in the same movement leads the contents of the spore through it, without the cell having to engulf the organism. observed in studies Source 2, 4
   ⚖ When the balance tips

   too much — Where several tubes meet one cell, its membrane is opened at several places.

   too little — Where the tube meets no cell, the contents of the spore stay in the gut contents and are carried away.

   observed in studies · Source 2, 4

3. Polar tube → Contents in the cell Through the tube the contents of the spore pass into the gut cell: the nucleus and the cell matter of the organism. It now lies within the cell, not upon it. Source 1, 4↓ depletes Inside the cell, antibodies do not reach the organism. Over the course of their evolution microsporidia have largely given up making energy themselves and draw ATP from the host cell. observed in studies Source 1, 7
   ⚖ When the balance tips

   too much — Where the contents of many spores reach cells, energy is drawn off in many cells.

   too little — Where they reach few, the consumption stays small, and the cells carry on working.

   observed in studies · Source 1, 7

4. Contents in the cell → Multiplying in the cell Inside the cell the organism divides and forms new spores. Enterocytozoon bieneusi stays mainly in the cells of the small-bowel villi and the bile ducts; Encephalitozoon intestinalis reaches further. Source 1, 5, 6↓ depletes Multiplication fills the cell with spores and draws on its energy; in the end the cell perishes, and the spores are set free. With each cell, a piece of the absorbing area of the villus is lost. observed in studies Source 1, 5, 6
   ⚖ When the balance tips

   too much — Where the organism multiplies in many cells, many cells of the villi perish, and the villi grow shorter.

   too little — Where it multiplies in few cells, fresh supply from the villus base replaces the lost cells as it goes.

   observed in studies · Source 1, 6

5. Multiplying in the cell → New spores in stool Neue Sporen The new spores leave the body with the stool. In the laboratory their genetic material is sought in it by PCR; under the microscope the spores measure only a few thousandths of a millimetre and are hard to make out. Source 5, 6↑ supplies The spores carry the organism onwards: they reach water and the surroundings and from there another gut. What arrives in the stool also settles what the test finds. established physiology Source 1, 5
   ⚖ When the balance tips

   too much — Where many spores are shed, PCR finds more genetic material.

   too little — Where they are shed unevenly, a light colonisation may go undetected in a single sample.

   established physiology · Source 5, 6

6. Multiplying in the cell → Route to other organs Encephalitozoon intestinalis enters phagocytes and travels with them out of the gut wall. Where immune defence is markedly weakened, spores have also been found in bile ducts, kidney and airways. Source 5, 6↓ depletes The phagocyte carries the organism: it engulfs it, and the organism goes on multiplying within it. In this way it leaves the gut without being exposed to the blood directly. observed in studies Source 1, 6
   ⚖ When the balance tips

   too much — Where immune defence is markedly weakened, the organism spreads further by this route.

   too little — Where the T cell response is intact, Enterocytozoon bieneusi stays confined to gut and bile ducts.

   observed in studies · Source 6, 8

   Field of research — Occurrence in further organs is studied in reviews on people with weakened immune defence. Source 8

7. Infected gut cell → Cytotoxic T cells Cytotoxic T cells bind the infected cell and bring it to an orderly death. Multiplication of the organism in that cell thereby ends. Source 1, 5↕ both, depending on amount This route clears the organism together with its host cell: it denies it the place to multiply and costs the lining a cell at the same time. Antibodies alone do not reach the organism inside the cell. observed in studies Source 1, 5
   ⚖ When the balance tips

   too much — Where many cytotoxic T cells are at work, infected cells are cleared quickly.

   too little — Where they are absent, for instance with markedly weakened immune defence, the organism goes on multiplying unhindered.

   observed in studies · Source 1, 8

   Field of research — The part played by T cells is studied in reviews on people with weakened immune defence. Source 8

8. Cytotoxic T cells → Interferon gamma T helper cells release interferon gamma. This signalling molecule puts phagocytes and gut cells on alert and has them make substances that bear on the organism inside the cell. Source 1, 5↑ supplies Interferon gamma aims the immune answer at organisms within cells: it makes phagocytes better at breaking down what they have engulfed and slows multiplication inside the cell. observed in studies Source 1, 5
   ⚖ When the balance tips

   too much — Where much interferon gamma is released, multiplication inside the cells is slowed more strongly.

   too little — Where little is released, the organism goes on multiplying inside the cells.

   observed in studies · Source 1, 5

9. Villi and area → Fat and sugar Fats are absorbed at the villus tip with the help of bile salts, sugars through transporters of the same cells. Both routes hang on a complete surface. Source 5, 6↓ depletes Fats left in the gut lumen carry fat-soluble substances along with them, and the stool grows richer in fat. Sugars left behind are fermented by bacteria in the large intestine into gases and acids. observed in studies Source 5, 6
   ⚖ When the balance tips

   too much — Where the surface is complete, fats and sugars are absorbed, and little stays behind in the gut lumen.

   too little — Where it is reduced, more stays behind in the gut lumen and leaves the body with the stool.

   observed in studies · Source 5, 6

Further stations

• Spores — Dormant form with a tough coat Microsporidia leave the body as a spore: a very small body with a tough coat, holding a coiled tube and the genetic material. Spores reach the mouth through water, food or contact. Source 1, 5↑ supplies The coat carries the spore through stomach and small intestine. Microsporidia belong among the relatives of the fungi, yet they live only inside cells; outside, the spore is the single form in which they persist. established physiology Source 1, 5
  ⚖ When the balance tips

  too much — Where many spores reach the gut, more cells are entered at once.

  too little — Where few arrive, the T cells of the gut wall settle whether the organism spreads.

  established physiology · Source 1, 5

• Infected gut cell — displays fragments The infected cell displays fragments of the organism on its surface. T cells in the gut wall recognise these marks and turn against the cell in a targeted manner. Source 1, 5↑ supplies Displaying is the single mark that reaches the outside from an organism inside the cell. By that route alone does the immune system find the infected cell. observed in studies Source 1, 5
  ⚖ When the balance tips

  too much — Where many cells display fragments, more T cells are prompted.

  too little — Where few display them, the organism stays unnoticed inside the cell and goes on multiplying.

  observed in studies · Source 1, 5

• Villi and area — Area for nutrients As cells of the villus tip perish, the villi grow shorter. The area through which water, salts, fats and sugars pass into the blood thereby shrinks. Source 5, 6↓ depletes What is not absorbed stays in the gut lumen and binds water to itself. At the same time the transporters and enzymes that sat on the lost surface are missing. observed in studies Source 5, 6
  ⚖ When the balance tips

  too much — Where the area is complete, water, fats and sugars are absorbed, and the stool stays formed.

  too little — Where it is reduced, fats and sugars remain in the gut lumen, bind water and are carried onwards.

  observed in studies · Source 5, 6

Cofactors in this pathway

• Bile acids — Needed at the villus tip so that fats from food pass into the cell Source 6In the ORY catalogue as a laboratory value: Gallensäuren (Stuhl)
• Glutamine — Fuel of the gut cells; from it they draw energy for the renewal of the villi Source 12In the ORY catalogue as a laboratory value: Glutamin
• Zinc — Building block of enzymes with which the villi form new cells and renew their junctions Source 9In the ORY catalogue as a laboratory value: Zink
• ATP (adenosine triphosphate) — Energy carrier of the host cell; the organism draws it in because it barely makes any itself Source 7
• Vitamin A — Retinoic acid arises from it and switches B cells in the gut to the IgA build Source 10
• Vitamin D — Its active form has maturing B cells form the homing receptor with which they find the gut wall Source 11In the ORY catalogue as a laboratory value: Vitamin D

Sources

1. Reinke AW. Microsporidia: evolution, infection mechanisms and host impact. Nat Rev Microbiol 2026 · PubMed 42768185
2. Fayet M et al. New insights into Microsporidia polar tube function and invasion mechanism. J Eukaryot Microbiol 2024 · PubMed 38973152
3. Huang Q et al. Germination of Microsporidian Spores: The Known and Unknown. J Fungi (Basel) 2023 · PubMed 37504762
4. Tamim El Jarkass H et al. The ins and outs of host-microsporidia interactions during invasion, proliferation and exit. Cell Microbiol 2020 · PubMed 32748538
5. Angitha KP et al. Microsporidiosis: An emerging opportunistic parasitic infection. Trop Parasitol 2026 · PubMed 42199679
6. Anane S et al. Microsporidiosis: epidemiology, clinical data and therapy. Gastroenterol Clin Biol 2010 · PubMed 20702053
7. Keeling PJ et al. Shrink it or lose it: balancing loss of function with shrinking genomes in the microsporidia. Virulence 2011 · PubMed 21217203
8. Asghari A et al. A global overview of microsporidia infection in HIV/AIDS patients: an updated systematic review and meta-analysis. Int Health 2026 · PubMed 41601338
9. DiGuilio KM et al. Micronutrient Improvement of Epithelial Barrier Function in Various Disease States: A Case for Adjuvant Therapy. Int J Mol Sci 2022 · PubMed 35328419
10. Bos A et al. The role of retinoic acid in the production of immunoglobulin A. Mucosal Immunol 2022 · PubMed 35418672
11. Fakhoury HMA et al. Vitamin D and intestinal homeostasis: Barrier, microbiota, and immune modulation. J Steroid Biochem Mol Biol 2020 · PubMed 32194242
12. Blachier F et al. Metabolism and functions of L-glutamate in the epithelial cells of the small and large intestines. Am J Clin Nutr 2009 · PubMed 19571215

Related pathways

• Clostridioides difficile — spores
• Viral enteric organisms — Gallensäuren (Stuhl), Glutamin
• Bacterial enteric organisms — Glutamin, Zink
• Worms in the gut — Glutamin, Zink
• Borrelia — Zink, Vitamin D

As of 2026-10-05. Draft, written by Claude to schema v2; sources checked in PubMed; expert approval pending
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