Learning path: After antibiotics · Chapter 2 of 5

Saccharomyces boulardii: the probiotic yeast

Why a yeast survives antibiotics, what it is proven to help with, and when it can become dangerous.

How solid is the research? Well supported

Well established: S. boulardii roughly halves the risk of antibiotic-associated diarrhoea and eases side effects of Helicobacter treatment. For C. difficile only as an add-on and with a weak recommendation. Results in IBS are conflicting. Yeast bloodstream infection is rare, but real in the severely ill.

Healthy state

Saccharomyces boulardii is a yeast, genetically a variant of baker's yeast. Unlike baker's yeast, it grows well at body temperature and survives stomach acid and bile. Because it is not a bacterium, antibiotics do not harm it. It does not settle permanently: within a few days a steady level is reached, and a few days after stopping it has been excreted again. Fungi altogether make up only a very small part of the gut inhabitants; more in the chapter on fungi and Candida.

How it works: it releases a protease, an enzyme that cuts up the toxins of Clostridioides difficile A bacterium that can spread after antibiotics and cause severe diarrhoea with its toxins. More in the chapter “After antibiotics: C. difficile and faecal transplants”. This was shown in rat gut and in human colonic mucosa. There are also hints from lab and animal experiments: sugar structures in the yeast's cell wall bind some pathogens like a decoy, and the yeast influences defences in the gut lining. How much of this reaches humans in daily life is less clear than the benefit seen in trials.

Why antibiotics cause diarrhoea in the first place: they thin out the Microbiome, gut microbiota The community of all microbes in the gut, mainly bacteria, plus viruses, fungi and archaea. Most live in the colon. More in the chapter “Building microbial diversity”. Fewer bacteria ferment fibre, so fewer Short-chain fatty acids Acids such as acetate, propionate and butyrate that gut bacteria make from fibre. They nourish the gut wall and act throughout the body. More in the chapter “Butyrate and other short-chain fatty acids” form, which help the colon absorb water. The conversion of Bile acids Substances made by the liver that make fat digestible. About 95 percent are reabsorbed in the last part of the small intestine; gut bacteria transform the rest. More in the chapter “Bile acids: when bile causes diarrhoea” also gets disrupted, so more of them irritate the gut. And a gap opens in which germs such as C. difficile can settle. The yeast does not fill this gap permanently; it acts more as a placeholder until the flora recovers.

Antibiotic-associated diarrhoea: in 21 Randomised trial A study in which chance decides who gets the treatment and who does not, for comparison. This makes results particularly reliable. with 4780 participants, the risk fell from 18.7 % to 8.5 %. Ten people need to take it for one to avoid diarrhoea. The effect was similar in children and adults.

Helicobacter pylori: given alongside treatment, it reduced diarrhoea as a side effect and slightly increased the cure rate, in adults by about six percentage points. The treatment consists of several antibiotics and an acid blocker, so it hits the gut flora twice.

The antibiotic hits the bacteria, the yeast stays. With an enzyme it cuts up the toxins of C. difficile.

Disrupted state: causes and effects

C. difficile: as prevention, S. boulardii clearly lowered the risk in the Meta-analysis An analysis that combines the results of several studies. It is only as good as the studies it includes. only in children, not in adults. The US gastroenterologists suggest it for people on antibiotics at high risk of C. difficile, with a weak recommendation. For treating an existing infection, they recommend Probiotics Live microorganisms that are meant to provide a benefit in sufficient amounts. The effect depends on the individual strain. More in the chapter “Using fermented foods well” only within trials. More on the infection in the chapter on antibiotics and C. difficile.

Irritable bowel syndrome Recurring abdominal pain linked to bowel movements, without tests finding another cause. Today it is understood as a disorder of gut–brain interaction. More in the chapter “The gut-brain axis”: results for strain CNCM I-745 are conflicting. Another yeast, Saccharomyces cerevisiae CNCM I-3856, did better in a meta-analysis. The label does not always name the strain, and results only apply to the tested one.

The risk: rarely, the yeast enters the blood. In an intensive care unit in 2003, three patients developed a yeast bloodstream infection; the only shared risk factor was S. boulardii. Those most at risk are severely ill people in intensive care, after abdominal surgery and with a central line. In a review of 60 cases, most patients were in intensive care, and 17 died. Cases have also been described from Finland. Two routes are discussed: the yeast crosses a damaged gut wall, or it reaches the bloodstream via contaminated hands and catheters, for example when capsules are opened in a patient's room. Neither affects healthy people much.

Antifungal medicines also act against S. boulardii. During such treatment it does little. It is unsuitable with a yeast allergy. Some people report bloating or constipation on it; if you stay on it longer, check whether it is worth it. In Crohn's disease, some people form antibodies against baker's yeast; whether this matters for taking it is unclear. Discuss it with your treatment team.

Differences between people: in children the evidence for antibiotic diarrhoea is best; in older people the risk of C. difficile is higher, but the data thinner. In pregnancy and breastfeeding good studies are lacking. With immune deficiency the risk usually outweighs the benefit.

In severely ill people with a central line, the yeast can rarely enter the blood.

What often helps

Lifestyle

Diet, exercise, sleep, social contact and dealing with stress.

With diarrhoea, fluids and salts come first The most important thing in acute diarrhoea is replacing water and salts, for example with an oral rehydration solution from the pharmacy. Probiotics come after that. Established How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.
Not a long-term product S. boulardii only works while you take it. It makes sense for a specific period; taking it daily without a reason has not been studied. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.
Feed the flora after the antibiotic Plant variety, fibre from pulses, oats and vegetables, and fermented foods support your own gut flora in recovering. A probiotic does not replace this. For individual foods this is not well proven; it is a plausible addition. Plausible How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.

Natural supplements

Supplements do not replace a medical check-up. Discuss dosage and interactions with your doctor or pharmacist.

During antibiotic treatment Start on the first day of the antibiotic and continue until about a week after the end. Studies mostly used 250 to 500 mg once or twice a day. As antibiotics do not attack the yeast, no time gap is needed. Not with a central venous catheter, serious illness or severe immune weakness: the yeast can then, rarely, enter the blood and cause a serious infection. Well supported How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [1][2][3]
During Helicobacter treatment Taken in addition, it reduces diarrhoea as a side effect and can help you complete the treatment. Here too, not with a central venous catheter, serious illness or severe immune weakness. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [4][5]
Not together with antifungals Antifungal medicines make the yeast ineffective. During such treatment, choose a bacterial probiotic with studies or do without. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.
Pay attention to the strain Most studies used strain CNCM I-745. A product that only says “Saccharomyces boulardii” is not automatically the same. A product with a named strain and germ count is easier to judge. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [2][6]

Medical treatments

Information, not a treatment recommendation. Decide together with a doctor. Approval and availability differ from country to country.

Not with severe illness, a central line or severe immune deficiency Here the yeast can enter the blood. In intensive care, after major abdominal surgery, with a central venous catheter or with severe immune deficiency, only after medical consideration. Well supported How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [7][8]
With C. difficile only as an add-on A C. difficile infection is treated with antibiotics or a faecal transplant. S. boulardii can be considered as prevention at high risk but does not replace treatment. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [9][1]
Acute diarrhoea in children: guidelines disagree European paediatric gastroenterologists name individual probiotic strains for certain situations, while US gastroenterologists advise against them in acute infections in children. Discuss it with your paediatrician. Promising How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open. [10][9]
Warning signs belong to the doctor, not a probiotic Bloody or black stool, fever, severe abdominal pain, diarrhoea lasting longer than about three days or signs of dehydration need medical assessment. A probiotic is then not the right answer. Well supported How we rate the evidence Established Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies. Well supported Several randomised studies in humans with largely consistent results. Promising Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies. Plausible Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms. Experimental Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.

From the community: what people report

Experiences and tips from people with the same problems that come up again and again, but for which there are no meaningful studies yet. They are explicitly not evidence.

Constipation and bloating on S. boulardii

In SIBO and microbiome communities the observation recurs that continuous use of S. boulardii triggers or worsens constipation, bloating and sometimes light-headedness or restlessness. It is said several times that constipation is to be expected when a methane overgrowth is already present, and that the yeast should then be stopped. Others report exactly the opposite: for them it was the only remedy that helped.

Evidence: no studies comes up again and again Reported in: r/SIBO (Reddit community),r/Microbiome (Reddit community),r/ibs (Reddit community)
After a Blastocystis finding

In r/ibs, patients again and again report that after a positive Blastocystis finding in stool they largely lost their diarrhea-predominant IBS complaints with S. boulardii; reports that it cured their IBS recur in many variations. Important: in seriously ill or immunocompromised people and with a central venous catheter in place, the yeast is linked to bloodstream infections – the long-term use common in the forum overlooks this limit.

Evidence: first hints comes up again and again Reported in: r/ibs (Reddit community),r/Microbiome (Reddit community)

Personal reports can mislead: what helps many people may be a placebo, and what helps one may harm another. Treat them as ideas to raise with your doctor or pharmacist – not as instructions.

In the mood for something completely different? Another one, at random

How we rate the evidence

Trust score 5 of 5:Established: Recommended in medical guidelines or supported by several consistent meta-analyses of randomised human studies.
Trust score 4 of 5:Well supported: Several randomised studies in humans with largely consistent results.
Trust score 3 of 5:Promising: Single randomised or controlled studies in humans, not yet independently confirmed — or many consistent observational studies.
Trust score 2 of 5:Plausible: Two cases: first observational or pilot studies with small samples — or a settled basis that is not in dispute, such as movement, sleep or meal pauses. But not tested in humans for symptoms.
Trust score 1 of 5:Experimental: Barely studied in humans. Mainly lab and animal studies or theoretical reasoning; whether it helps is open.

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