Vaginal flora: the pathway in the body · Biomarker & Swab

This page shows the biochemical pathway behind the laboratory value Vaginal flora composition: 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

The vaginal flora is the community of microorganisms in the vagina, often dominated by lactobacilli. These break down sugars from glycogen into lactic acid; other organisms such as Gardnerella or the protist Trichomonas can also occur. Lactic acid keeps the milieu acidic.

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. Oestrogen → Glycogen Glycogen is a branched chain of glucose. It is released when cells of the lining are shed. The flora lives on it. Source 1↑ supplies Glycogen is the store the flora lives on: only when cells are shed is it released, and enzymes break it down into sugars for the bacteria. established physiology Source 1, 2
   ⚖ When the balance tips

   too much — If plenty of glycogen is released, more sugar forms, lactobacilli produce more lactic acid, and the pH falls.

   too little — If little is released, the lactobacilli lack food; bacteria that can also use the sugars of the mucus then have an advantage.

   observed in studies · Source 1, 2, 4

2. Glycogen → Maltose and glucose α-amylase The body's own α-amylase in the mucus breaks glycogen down into smaller sugars. Only in this form can bacteria use it. Many lactobacilli cannot break down glycogen themselves. Source 2↑ supplies Maltose, maltotriose and glucose are the forms in which lactobacilli absorb the sugar; many of them cannot break down glycogen themselves and depend on α-amylase. observed in studies Source 2
   ⚖ When the balance tips

   too much — If α-amylase breaks down a lot of glycogen, more sugar is available, and lactobacilli grow better.

   too little — If there is little α-amylase in the mucus, more glycogen remains unused, and the lactobacilli lack usable sugar.

   observed in studies · Source 2

3. Sugar in the cell → Lactic acid Lactate dehydrogenase · NADH This breakdown produces lactic acid, in the L or the D form depending on the species. The bacteria release it into the mucus. It acidifies the mucus. Source 1↑ supplies Lactic acid acidifies the mucus. In its uncharged form it enters other bacteria and disturbs their metabolism; in cell experiments it also dampens inflammatory signals of the lining. observed in studies Source 1
   ⚖ When the balance tips

   too much — If a lot of lactic acid forms, the pH falls further; lactobacilli tolerate this, while many other bacteria then grow more slowly.

   too little — If little forms, the pH rises, and bacteria that tolerate an acidic milieu poorly grow better.

   established physiology · Source 1

4. Lactic acid → Acidic milieu Lactic acid keeps the mucus acidic. In this milieu many other bacterial species grow more slowly, while lactobacilli cope well with it. Blood and seminal fluid raise the pH. Source 1↓ depletes The acidic milieu inhibits many bacteria that do not belong to the lactobacillus flora; the acid acts mainly in its uncharged form. established physiology Source 1
   ⚖ When the balance tips

   too much — If the milieu is very acidic, mainly acid-tolerant species such as lactobacilli grow.

   too little — If it becomes less acidic, for example through blood during menstruation or through seminal fluid, other bacteria grow more easily.

   established physiology · Source 1

5. Mucins in the mucus → Free sialic acid Sialidase Gardnerella produces a sialidase. The enzyme detaches sialic acids from the sugar chains; the bacterium absorbs and uses them. Without them the mucus is broken down more easily. Source 4↓ depletes Free sialic acid is food for Gardnerella and other bacteria. Once it is missing from the chain end, the mucus becomes open to further enzymes. observed in studies Source 4
   ⚖ When the balance tips

   too much — If a lot of sialic acid is detached, Gardnerella and other bacteria find more food, and the mucus loses its covering layer.

   too little — If little is detached, the chains of the mucus remain intact.

   observed in studies · Source 4

6. Trichomonas vaginalis → Attachment to the cell Adhesins When the organism meets cells of the lining, it changes shape from oval to flat and attaches with surface proteins. Among other things, it feeds on bacteria of the flora. In cell experiments it damages the cells. Source 5↓ depletes Through attachment the organism alters the cell it sits on; in cell experiments it damages the cell through contact and through protein-splitting enzymes that it releases. observed in studies Source 5
   ⚖ When the balance tips

   too much — If many organisms attach, more cells of the lining are damaged, and an inflammatory reaction develops.

   too little — If few attach, they are more easily carried away with the mucus.

   observed in studies · Source 5

Further stations

• Oestrogen — hormone in the cycle Oestrogen makes the top cell layer of the vaginal lining grow and store glycogen. This is why the picture of the flora changes with hormone levels. More oestrogen means more glycogen. Source 1↑ supplies Oestrogen supplies the flora's raw material: it makes the lining thicker and store glycogen. When oestrogen rises, more sugar becomes available to the lactobacilli. established physiology Source 1
  ⚖ When the balance tips

  too much — If plenty of oestrogen is present, for example in pregnancy, the lining stores more glycogen, and lactobacilli find more food.

  too little — If little oestrogen is present, for example before puberty or after the menopause, the lining becomes thinner and stores little glycogen; the pH rises, and the flora becomes more varied.

  established physiology · Source 1

• Lactobacilli — L. crispatus, L. iners etc. In many samples a single species dominates the picture, such as Lactobacillus crispatus, L. iners, L. gasseri or L. jensenii. There are also communities made up of several species. They leave little room for other species. Source 3, 1↑ supplies Lactobacilli shape the milieu: they produce lactic acid, occupy the lining and leave little room and food for other species. How strongly they do this depends on the species. established physiology Source 1, 3
  ⚖ When the balance tips

  too much — If one lactobacillus species dominates the picture, the pH is low, and other bacteria remain sparse.

  too little — If few lactobacilli are present, the pH rises, and communities of many species fill their place; such communities are also found in women without complaints.

  observed in studies · Source 3, 1

• Sugar in the cell — absorbed by bacteria Lactobacilli absorb the sugars and break them down without oxygen. This gives them energy. Source 1↑ supplies Inside the cell the sugars supply energy: lactobacilli gain it by fermentation without oxygen, and lactic acid remains as the end product. established physiology Source 1
  ⚖ When the balance tips

  too much — If the bacteria absorb a lot of sugar, they produce more lactic acid and multiply more.

  too little — If they absorb little, they grow more slowly and produce less lactic acid.

  established physiology · Source 1

• Gardnerella vaginalis — bacterium of the flora Gardnerella vaginalis is one of the bacteria that can occur in the vaginal flora. It adheres to the lining and forms communities of several species there (biofilms). It feeds on mucus sugars. Source 4↓ depletes Gardnerella feeds on the mucus of the lining: with a sialidase it breaks down the mucus sugar chains and uses the detached sialic acid as food. observed in studies Source 4
  ⚖ When the balance tips

  too much — If a lot of Gardnerella grows, more sialic acids are detached, and the sugar chains of the mucus are broken down further.

  too little — If little grows, the mucus remains largely unchanged; Gardnerella also occurs in small numbers in samples with many lactobacilli.

  observed in studies · Source 4, 3

• Mucins in the mucus — proteins with sugar chains The mucus consists of proteins with long sugar chains. Sialic acids sit at the outer end of these chains. The sialic acids cover the sugars beneath. Source 4↑ supplies Mucins form the mucus layer that covers the lining and keeps bacteria at a distance. The sialic acids at the chain ends cover the sugars beneath. observed in studies Source 4
  ⚖ When the balance tips

  too much — If plenty of mucus with complete chains is present, the layer stays dense, and the sugars beneath remain covered.

  too little — If the chains are broken down, the sugars beneath are exposed, and bacteria can break them down further.

  observed in studies · Source 4

• Trichomonas vaginalis — single-celled parasite Trichomonas vaginalis is not a bacterium but a single-celled organism with flagella. It gains energy without oxygen in special organelles, the hydrogenosomes. This produces hydrogen. Source 5↓ depletes Trichomonas lives off the milieu of the lining: the organism engulfs bacteria, cell debris and iron and gains energy in its hydrogenosomes, producing hydrogen in the process. observed in studies Source 5, 7
  ⚖ When the balance tips

  too much — If Trichomonas multiplies strongly, it engulfs more bacteria of the flora, including lactobacilli, and attaches to more cells of the lining.

  too little — If it is absent or present only in small numbers, the rest of the flora remains largely unaffected by it.

  observed in studies · Source 5

Cofactors in this pathway

• NAD⁺ — Coenzyme of lactate dehydrogenase, which lactobacilli use to form lactic acid; it is regenerated from NADH Source 1
• Calcium — Bound ion of alpha-amylase, which breaks glycogen down into smaller sugars; it keeps the enzyme stable Source 6, 2In the ORY catalogue as a laboratory value: Calcium (intrazellulär)
• Chloride — Bound ion near the active site of alpha-amylase; it activates the enzyme Source 6
• Iron — Part of the iron-sulphur proteins in the hydrogenosomes of Trichomonas; where energy and hydrogen are produced Source 7

Sources

1. Amabebe E, Anumba DOC. The Vaginal Microenvironment: The Physiologic Role of Lactobacilli. Front Med (Lausanne) 2018 · PubMed 29951482
2. Spear GT, French AL, Gilbert D et al. Human α-amylase present in lower-genital-tract mucosal fluid processes glycogen to support vaginal colonization by Lactobacillus. J Infect Dis 2014 · PubMed 24737800
3. Ravel J, Gajer P, Abdo Z et al. Vaginal microbiome of reproductive-age women. Proc Natl Acad Sci U S A 2011 · PubMed 20534435
4. Lewis WG, Robinson LS, Gilbert NM et al. Degradation, foraging, and depletion of mucus sialoglycans by the vagina-adapted Actinobacterium Gardnerella vaginalis. J Biol Chem 2013 · PubMed 23479734
5. Leitsch D. Recent advances in the molecular biology of the protist parasite Trichomonas vaginalis. Fac Rev 2021 · PubMed 33718943
6. Brayer GD, Luo Y, Withers SG. The structure of human pancreatic alpha-amylase at 1.8 A resolution and comparisons with related enzymes. Protein Sci 1995 · PubMed 8528071
7. Tachezy J, Makki A, Hrdý I. The hydrogenosome of Trichomonas vaginalis. J Eukaryot Microbiol 2022 · PubMed 35567536

As of 2026-09-16. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending. The biological process is described; the aromatogram is intentionally not shown.
Legal notice Privacy policy All biomarkers