Saccharomyces boulardii
Saccharomyces boulardii is a tropical strain of yeast first isolated from lychee and mangosteen fruit in 1923 by French scientist Henri Boulard.  It is related to, but distinct from, Saccharomyces cerevisiae in several taxonomic, metabolic, and genetic properties.
S. boulardii has been shown to maintain and restore the natural flora in the large and small intestine; it is classified as a probiotic.  Boulard first isolated the 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.  S. boulardii has been shown to be non-pathogenic, non-systemic (remains in the gastrointestinal tract), and grows at the unusually high temperature of 37C.

# Medical Uses
There are numerous randomized, double-blind placebo-controlled studies showing the efficacy of S. boulardii in the treatment and prevention of gastrointestinal disorders.  Many gastroenterologists, pediatricians, and family practitioners suggest the use of S. boulardii for use in the following indications.

## Acute Diarrhea
Two studies each showed a significant reduction in the symptoms of acute gastroenteritis in children, versus placebo, by measuring frequency of bowel movements and other criteria.  Children over three months are recommended to take two doses of 250mg a day (BID) for five days to treat acute diarrhea.  Children under three months are recommended to take half a 250mg capsule or sachet twice daily for five days.
A prospective placebo-controlled study found a significant reduction in symptoms of diarrhea in adults as well taking 250mg of S. boulardii twice a day for five days or until symptoms are relieved.

## Recurrent Clostridium difficile Infection
Administration of two 500mg doses per day of S. boulardii when given with one of two antibiotics (vancomycin or metronidazole) was found to significantly reduce the rate of recurrent Clostridium difficile (pseudomembranous colitis) infection.  No significant benefit was found for prevention of an initial episode of Clostridium difficile-associated disease.

## Irritable Bowel Syndrome
A prospective placebo-controlled study found patients with diarrhea predominant irritable bowel syndrome (IBS) had a significant reduction on the number and consistency of bowel movements.

## Inflammatory Bowel Disease
Further benefits to inflammatory bowel disease (IBD) patients have been suggested in the prevention of relapse in Crohn's disease patients currently in remission and benefits to Ulcerative Colitis patients currently presenting with moderate symptoms.  The recommended dosage is three 250mg capsules a day (TID).

## Travelers' Diarrhea
Austrian vacationers taking S. boulardii traveling around the world were found to have significantly fewer occurrences of travelers' diarrhea than those taking placebo.  The more S. boulardii taken in prevention, starting five days before leaving, the higher the reduction in diarrhea reported.  The reduction was also found to be dependent upon where the vacationer travelled.  The recommended dosage is one 250mg capsule or sachet per day (QD).

## Antibiotic-Associated Diarrhea
There is evidence for its use in the prophylactic (preventative) treatment of antibiotic-associated diarrhea (AAD) in adults.    There is further evidence for its use to prevent AAD in children.

## HIV/AIDS-Associated Diarrhea
S. boulardii has been shown to significantly increase the recovery rate of stage IV AIDS patients suffering from diarrhea versus placebo.  On average, patients receiving S. boulardii gained weight while the placebo group lost weight over the 18 month trial.  There were no reported adverse reaction observed in these immunocompromised patients.

# Mechanisms of Action

## Antitoxin Effects
S. boulardii secretes a 54 kDa protease, in vivo.  This protease has been shown to both degrade toxins A and B, secreted from Clostridium difficile, and inhibit their binding to receptors along the brush border.  This leads to a reduction in the enterotoxinic and cytotoxic effects of C. difficile infection.

## Antimicrobial Effects
Escherichia coli and Salmonella typhimurium, two pathogenic bacteria often associated with acute infectious diarrhea, were shown to strongly adhere to mannose on the surface of S. boulardii via lectin receptors (adhesins).  Once the invading microbe is bound to S. boulardii, it is prevented from attaching to the brush border; it is then eliminated from the body during the next bowel movement.

## Trophic Effects on Enterocytes
The hypersecretion of water and electrolytes (including chloride ions), caused by cholera toxin during a Vibrio cholerae infection, can be reduced significantly with the introduction of S. boulardii.  A 120 kDa protease secreted by S. boulardii has been observed to have an effect on enterocytes lining the large and small intestinal tractinhibiting the stuimulation of adenylate cyclase, which led to the reduction in interocytic cyclic adenosine monophosphate (cAMP) production and chloride secretion.
During an E. coli infection, myosin light chain (MLC) is phosphorylated leading to the degredation of the tight junctions between intestinal mucosa enterocytes.  S. boulardii has been shown to prevent this phosphorylation, leading to a reduction in mucosal permeability and thus a decrease in the translocation of the pathogenic bacteria.
Polyamines (spermidine and spermine) have been observed to be released from S. boulardii in the rat ileum.  Polyamines have been theorized to stimulate the maturation and turnover of small intestine enterocytes.  This could aid in the increased recovery rate of a patient from diarrhea.

## Anti-Inflammatory Effects
Interleukin 8 (IL-8) is a proinflammatory cytokine secreted during an E. coli infection in the gut.  S. boulardii has been shown to decrease the secretion of IL-8 during an E. coli infection; S. boulardii could have a protective effect in inflammatory bowel disease.

## Increased Levels of Disaccharidases
The trophic effect on enterocytes has been shown to increase levels of disaccharidases such as lactase, sucrase, maltase, glucoamylase, and N-aminopeptidase in the intestinal mucosa of humans and rats.  This can lead to the increased breakdown of disaccharides into monosaccharides that can then be absorbed into the bloodstream via enterocytes.  This can help in the treatment of diarrhea, as the level of enzymatic activity has diminished and carbohydrate cannot be degraded and absorbed.

## Increased Immune Response
S. boulardii induces the secretion of Immunoglobulin A (IgA) in the small intestine of the rat.  Secretory IgA provides protection against invading microbes in the gastrointestinal and respiratory tracts.

# Human disease
Some cases of fungemia have been reported in patients with a central venous catheter.  Administration of an antimycotic (antifungal) leads to patient recovery from this systemic infection.  Patients with a yeast allergy are not encouraged to take S. boulardii. S. boulardii has been known to cause disease in immunocompromised humans.  In particular, it is marketed in a highly concentrated form as a probiotic supplement (Ultralevura) for the treatment of Clostridium difficile colitis.  Unfortunately, its use in this fashion can possibly cause fungemia in critically ill patients.

# Commercial Use
S. boulardii is often found in a lyophilized (patented dehydration process) form and is therefore often referred to as Saccharomyces boulardii lyo.  Biocodex is the largest manufacturer of Sb lyo which is marketed under a number of different labels in over 90 countries world-wide: Ultra-Levure, Florastor, Perenterol, Interflora, and more.  Sb lyo manufactured by Biocodex is the strain used in the vast majority of all studies performed on Saccharomyces boulardii. S.boulardii can also be bought in the UK under the name DiarSafe