Saccharomyces Boulardii

Saccharomyces boulardii is a strain of yeast belonging to the fungal kingdom, and it is the only yeast widely marketed as a probiotic. Although once regarded as its own species, genetic analysis has shown it to be a lineage of Saccharomyces cerevisiae, sharing almost all of its genome with that familiar baking and brewing yeast, so it is often written as S. cerevisiae var. boulardii ^[2]. Unlike many probiotics, which are bacteria, it is a fungus, a difference that gives it natural resistance to antibiotics since those drugs target bacteria ^[2].

The yeast was isolated in the 1920s by the French microbiologist Henri Boulard, who reportedly noticed people in Southeast Asia chewing the skins of lychee and mangosteen to control diarrhea during a cholera outbreak and traced the effect to a yeast on the fruit. The commercial strain that descends from this work is grown by a single manufacturer and has since been studied in a large number of clinical trials ^[2].

The best-supported use of Saccharomyces boulardii is the prevention of antibiotic-associated diarrhea, a common side effect of antibiotics; a meta-analysis of randomized trials found that it roughly halved the risk in both children and adults ^[1]. It has also been studied for acute infectious gastroenteritis, for reducing recurrence of Clostridioides difficile infection, as an add-on that improves eradication of the ulcer bacterium Helicobacter pylori while easing treatment side effects, and for diarrhea linked to travel and to HIV, among other gut conditions ^[2][3]. The strength of the evidence varies by condition, being strongest for antibiotic-associated diarrhea ^[4].

Saccharomyces boulardii is sold as capsules, sachets, and powders, often as freeze-dried live yeast, under names such as Florastor, and its regulatory status ranges from an over-the-counter supplement to a prescription medicine depending on the country ^[2]. It is generally considered safe in otherwise healthy people, but because it is a live organism there have been rare reports of it entering the bloodstream in seriously ill or immunocompromised patients and those with central venous catheters, so it is used cautiously in such groups ^[2].

Saccharomyces boulardii is a probiotic yeast that is generally well tolerated in healthy people, with mild gas or bloating the most common complaint. Its important risk is fungemia, a bloodstream infection: rare cases have occurred in immunocompromised, critically ill, or central-catheter patients, so it should be avoided in those groups. Handling of the powder near catheters has also been implicated in these infections.

Saccharomyces boulardii was isolated in the 1920s by the French microbiologist Henri Boulard, after whom it is named. While working in Indochina during a cholera outbreak, Boulard observed local people chewing the skins of tropical lychee and mangosteen fruit to control their diarrhea, and from these skins he cultured a heat-tolerant yeast. The organism was later characterized as a strain of Saccharomyces cerevisiae that thrives at human body temperature and behaves distinctly from ordinary baking and brewing strains. It was subsequently commercialized by the French pharmaceutical company Biocodex, which registered the reference strain CNCM I-745 and brought it to market as a lyophilized probiotic. Over the following decades it became the most extensively studied probiotic yeast, investigated in a broad range of diarrheal and gastrointestinal conditions.

Saccharomyces boulardii is widely regarded as the best-characterized probiotic yeast and one of the few probiotics with a coherent, mechanistically grounded evidence base. It is best known for reducing the incidence of antibiotic-associated diarrhea and for helping to prevent recurrence of Clostridioides difficile infection, and it has also been studied for traveler's diarrhea and acute pediatric gastroenteritis. Because it is a fungus, it survives exposure to antibacterial antibiotics, allowing it to be taken during a course of antibiotics when many bacterial probiotics would be suppressed. It is generally very well tolerated in healthy people. In fairness, it is not for everyone; rare cases of bloodstream infection have been reported in severely ill or immunocompromised patients and in those with central venous catheters, so it is avoided in those settings.