Saccharomyces boulardii

Saccharomyces boulardii is a tropical yeast first isolated from lychee and mangosteen fruit peel in 1923 by French scientist Henri Boulard. Although early reports claimed distinct taxonomic, metabolic, and genetic properties, S. boulardii is genetically a grouping of S. cerevisiae strains, sharing >99% genomic relatedness, giving the synonym S. cerevisiae var. boulardii. S. boulardii is sometimes used as a probiotic with the purpose of introducing beneficial microbes into the large and small intestines and conferring protection against pathogens. It grows at 37 °C (98.6 °F). In addition, the popular genome-editing tool CRISPR-Cas9 was proven to be effective in S. boulardii. Boulard first isolated this yeast after he observed natives of Southeast Asia chewing on the skin of lychee and mangosteen in an attempt to control the symptoms of cholera. In healthy patients, S. boulardii has been shown to be nonpathogenic and nonsystemic (it remains in the gastrointestinal tract rather than spreading elsewhere in the body). S. boulardii was characterized as a species separate from S. cerevisiae because it does not digest galactose and does not undergo sporulation. It also tolerates human body temperature, gastric acid, and digestive enzymes better. Despite all these phenotypic differences, its genomic sequence defines it as a clade under S. cerevisiae, closest to those found in wine. Like ordinary S. cerevisiae, it has 16 chromosomes, a 2-micron circle plasmid, and is diploid with genes for both mating types, MATa and MATα. However, the MATa locus contains some likely disabling mutations relative to spore-forming S. cerevisiae. Both S. boulardii and ordinary S. cerevisiae produce proteins that inhibit pathogenic bacteria and their toxins, specifically 63-kDa phosphatase pho8 (inhibiting E. coli endotoxin) and 54-kDa serine protease ysp3 (hydrolyzing C. difficile toxins A and B). A yet-unidentified 120 kDa protein also inhibits changes in cAMP levels induced by cholera toxin. S.