Definition of Saccharomyces boulardii

Saccharomyces boulardii is a probiotic yeast that has been consumed for decades and whose benefits have been the subject of hundreds of studies and scientific publications.

This yeast belongs to the Saccharomyces family (genus), the same as one of the best known yeasts: brewer's or baker's yeast, Saccharomyces cerevisiae.

Scientists have long debated whether S. boulardii is really distinct from S. cerevisae.

The latest data show that there are differences, but that S. boulardii should be considered a specific strain of S. cerevisiae.

It would therefore be more accurate to speak of Saccharomyces cerevisiae boulardii.

This means that S. boulardii is a "daughter" of S. cerevisiae that has evolved to give it special properties.

One important difference is that, unlike its relative, S. boulardii is unable to form hyphae, growths that allow it to invade living tissue.

On the other hand, it is more resistant to the acidity and temperature of the human body, an interesting property that increases its chances of survival in the intestine.

As a yeast, S. boulardii is a eukaryotic organism, much larger and more advanced than a bacterium (prokaryote).

Unlike bacteria, yeasts have a nucleus containing their DNA and mitochondria, which produce energy, like all cells in our body.

S. boulardii develops by budding, from which a new yeast grows and then detaches (more or less) from the mother cell.

This is how it can be observed under the microscope.

S. boulardii is the only yeast that is considered to be probiotic, i.e. able to interact positively with the human organism.

All other probiotics are bacteria, most commonly starter cultures.

What are the benefits of Saccharomyces boulardii?

S. boulardii is one of the most studied probiotic strains in the world, with nearly a hundred published clinical trials evaluating the effects of this yeast on more than 10,000 people.

To be effective, probiotics need to reach their site of action (the gut) alive and survive long enough.

S. boulardii is particularly suitable because it develops optimally at our body temperature (37°C) and in a low-acid environment (such as the small intestine).

S. boulardii is also resistant to stomach acid.

Scientific studies have shown that S. boulardii is unable to colonise the intestine because the natural microbiota prevent it from doing so.

This property explains its excellent tolerability.

However, yeast can have a protective effect on the microbiota in the case of dysbiosis (e.g. due to diarrhoea or the use of antibiotics).

Indeed, dysbiosis is characterised by a reduction in the density and diversity of bacteria and yeasts, which can no longer exert a barrier function.

The protection provided by S. boulardii is temporary and disappears when the balance of the microbiota is restored, usually after 1 or 2 months, allowing it to resume its barrier function.

The mechanism of action of S. boulardii is complex because the yeast can interact at different levels in the gut:

• By adhering to pathogenic micro-organisms and restricting their movement
• By producing antimicrobial substances that limit the proliferation of unwanted microorganisms
• By modulating the immune system, which is responsible for fighting pathogens
• Strengthening the integrity of the intestinal mucosa (trophic effect)

The benefits of S. boulardii have mainly been studied in adults and children suffering from antibiotic-induced diarrhoea or Clostridium difficile infection.

These situations are indeed associated with more severe dysbiosis (imbalance of the microbiota).

Studies have shown the benefits of taking them throughout the period of antibiotic use and in the weeks that follow.

Adverse effects of Saccharomyces boulardii

Consumption of S. boulardii is safe for children and healthy adults.

Pregnant and breastfeeding women can also consume S. boulardii.

As S. boulardii is a live micro-organism, the main theoretical risk would be an infection caused by this yeast (phongemia).

No cases of fungemia were observed in the 90 human studies.

The rare adverse effects that were observed consisted of very occasional constipation.

The safety of using S. boulardii is linked in part to its inability to colonise the gut and enter the body.

However, as a precaution, S. boulardii should not be taken in certain special cases: if the consumer has a severe immune system failure (related to a disease or drug treatment), if intestinal surgery has just been performed, or if a central venous catheter is present.