Probiotic WhitePaper

A review of the yeast probiotic Saccharomyces boulardii

By Robert P. Franklin, DVM Dip.ACVIM - Keith M. Latson, DVM Dip.ACVS

Take home message: Saccharomyces boulardii is a novel yeast probiotic that is safely administered to horses and has the potential to treat and prevent antibiotic associated diarrhea, Clostridium difficile diarrhea, non-specific diarrhea and inflammatory bowel disease. Elucidation of mechanisms of action and many randomized clinical trials with meta-analysis support its use in humans and animals. This probiotic has the most evidence for veterinary use based on the current literature.


Probiotics are defined as live microorganisms that provide a positive effect on the treatment and prevention of gastrointestinal disturbances.1 Humans and animals that ingest fermented food products have been utilizing crude forms of probiotics for centuries. Probiotic microorganisms must have certain qualities to withstand the natural defense factors of the host. This includes resistance to gastrointestinal transit: enzymes, bile salts, organic acids and pH fluxes; and the ability to thrive at animal core temperatures. The majority of the probiotic organisms are bacteria. Lactobacilli sp. have the longest history of proclaimed probiotic effects.2 The human food industry and veterinary feed manufacturers have primarily marketed the benefits of live food ingredients but the mechanisms of health promotion have been largely ill defined. The preponderance of claimed health benefits with the poor quality of data supporting such claims has led to skepticism in the medical community.

Scientific criticisms are cornered on the following four criteria.

1) Isolates marketed as probiotics are poorly characterized and combined with other poorly characterized isolates to leave a milieu of dubious microorganisms without indication of the properties of any particular microbe.

2) The lack of consistent, reproducible mechanism of actions for probiotic isolates. A relative paucity of medical studies.

3) There is a lack of compliance with standards of quality, namely: dosage, viability, stability and formulation.3

4) The veterinary community shares this distrust and skepticism that our human counterparts have maintained. Although basic scientists are actively striving to characterize the hundreds of proclaimed probiotic isolates, benefits and mechanisms have only been elucidated in a few: Lactobacillus sp., Bifidobacterium sp., and Saccharomyces boulardii (S. boulardii).4

Controlled studies in humans have been performed with enough regularity now to lend themselves to meta-analysis.4 Veterinary studies, on the other hand, are frustratingly sparse. Still, commercial preparations continue to manufacture probiotics with inconsistent standards of quality.

In fact, Weese illustrated that out of 44 human or veterinary probiotics, organisms were improperly identified in 43% of human or 35% of veterinary samples and 25% of human or 18% of veterinary products had isolates that were even misspelled.5

Further, Weese examined 13 veterinary or human probiotics in the laboratory and found label description and concentrations were accurately stated in only 2 products, neither was a veterinary product.

All of the eight veterinary products examined contained <2% of the listed concentration of probiotic microorganisms.6

A similar finding occurred when 5 of 19 human products were examined and found to not have the number of live microorganisms stated on the label.7

The purpose of this review is to identify the body of knowledge related to a very well studied probiotic, S. boulardii, and make recommendations for its use.

S. boulardii (Saccharomyces cerevisiae var. boulardii) is a non-pathogenic, facultative anaerobic yeast first identified in 1920 by French microbiologist Henri Boulard whilst looking for a heat tolerant yeast for wine fermentation in Indochina.2,8

There have been almost 250 peer-reviewed articles studying S. boulardii since the first paper in 1982 making this one of the most studied probiotics.

Randomized, placebo controlled, double-blinded studies have been reproduced to claim support from meta-analysis reviews that S. boulardii is an effective biotherapeutic agent for the treatment of antibiotic associated diarrhea, C. difficile associated disease, and other causes of acute diarrhea in laboratory, human and veterinary settings.8,9,10

The organism, S. boulardii, maintains distinct taxonomic and physiological differences from Saccharomyces cerevisiae or ‘Brewer’s yeast’, which apparently has no probiotic effect.8, 11

Overwhelming Benefits of S. boulardii

  • S. boulardii is an ideal probiotic as it is able to withstand the stresses of gastrointestinal transit and is resistant to all known antibacterial antibiotics, therefore making it effective when given concurrently with antibiotic treatments.12
  • S. boulardii has a preferred vegetation temperature of 37°C which is ideal for most host species.13
  • After oral administration in humans it reaches a steady state within 3 days and is completely eliminated 2-5 days after discontinuing administration.14
  • S. boulardii is not found naturally in the animal.15
  • The yeast is able to be recovered within 5 days of administration and is no longer present within 10 days of discontinuing therapy.10
  • This rapid accumulation of yeast within the bowel is important, as there is a concentration dependent effect noted on interactions with C. difficile.16
  • Yeasts are model probiotics and superior to bacteria because they are not able to transmit genetic material that is keyed for antibiotic resistance.2
  • Genes conferring resistance to tetracycline, erythromycin and vancomycin have been identified in Lactobacillus sp. used as probiotics and the genes could transfer to pathogens by horizontal transmission.17
  • S. boulardii has several mechanisms of action that provide protection from enteric pathogens such as Clostridium difficile and Escherichia coli, destruction and blockade of C. difficile toxins and also immunostimmulatory and anti-inflammatory effects on the host intestinal tract. Accordingly, these characteristics provide S. boulardii a distinct advantage as a biotherpeutic agent when compared to yogurts and ill-defined commercial preparations of bacteria.

Mechanisms of Action

Toxin neutralization

C. difficile pathology is stimulated by the virulent effects of toxin A and B on the host. A novel protease is release by S. boulardii in vivo which inhibits the entertoxic and cytotoxic effects of these toxins. Experimentally, this protease is able to block binding of toxin A and B to mucosal membranes, lyse toxin A specifically, reduce toxin-induced secretions and decrease intestinal permeability.18, 19, 20

Gram-negative organisms such as Escherichia coli liberate large amounts of potent endotoxin capable of a systemic inflammatory response. S. boulardii produces a novel protein phosphatase that is capable of dephosphorylating endotoxin in an experimental model and inactivating cytotoxic effects.21

Cell Signaling

S. boulardii is capable of modifying cell signaling away from a proinflammatory state. Several studies have localized the actions of the yeast to inhibiting E. coli induced NF-kappa B DNA binding and activation of MAP kinases. Blocking these two pathways is likely why a reduction in synthesis of TNF-alpha is seen with S. boulardii.22, 23

Immune modulation

Treatment in rats with S. boulardii leads to a large increase in secretory IgA production. A more specific effect was seen in mice previously given a toxoid of C. difficile toxin A. Secretion of a specific toxin A IgA and IgM antitoxin was noted after S. boulardii administration.24, 25

Trophic effects

S. boulardii enhances brush border enzyme (sucrase-isomaltase, lactase, maltase-glucoamylase) activity in rats and man. These enzymes are important in nutrient digestion and are often altered in cases of gastrointestinal disturbances.26-29

Even more important when dealing with colonic microflora is the effects of altered short-chain fatty acid (SCFA) concentrations. S. boulardii treated human patients receiving total enteral nutrition had a higher fecal butyrate concentration, a SCFA known to have anti-inflammatory properties, without modifying the fecal microbiota.30

Antisecretory effects

As previously mentioned, S. boulardii reduces C. difficile toxin A derived secretions.24,25

When given as prior to castor oil in rats, S. boulardii is capable of significantly minimizing secretory diarrhea, possibly by modulating expression of iNOS.31

Anti-inflammatory effects

A small anti-inflammatory molecule called Saccharomyces anti-inflammatory factor has been identified.32 An increase in the anti-inflammatory SCFA butyrate was illustrated in a study regarding total enteral nutrition.33

Pathogen blockade

Yeasts are 10x the size of most bacteria and therefore provide a level of stearic hindrance to mucosal invasion of pathogens.2

Additionally; the cell wall of yeasts is a combination of chitin, mannose and glucan. This composition has been suggested to be very attractive to both Salmonella and E. coli, whereby S. boulardii acts as a ‘decoy’ for the host.34

In vitro evidence exists to demonstrate that S. boulardii are capable of inhibiting cell adherence of C. difficile.35 Pathogen Adherent Microflora (PAM) are probiotics that bind pathogens and eliminate them with probiotic transit. S. boulardii is an ideal PAM because attracts many pathogens to the mannose wall layer and it does not colonize the host intestinal tract and therefore is able to eliminate any yeast bound pathogens prior to them attaching to the host.36

Clinical studies involving S. boulardii

Antibiotic Associated Diarrhea Antibiotic associated diarrhea (AAD) is a common problem in both man and veterinary species. Prevention of diarrhea has been examined in several randomized controlled trials. In a meta-analysis of 5 randomized controlled trials including 1076 human adults and children, S. boulardii was found to be effective in reducing diarrhea from 17.2% to 6.7% (RR: 0.43; 95% CI:0.23-0.78; NNT: 10; 95% CI 7-16). The majority of patients were receiving antibiotics for respiratory infections.37

In a study examining several probiotic, placebo controlled randomized clinical trials in children positive effects were again noted. Six trials comprising 766 children were included in the analysis. Probiotics collectively reduced the risk of diarrhea from 28.5% to 11.9% (RR: 0.44, 95% CI 0.25-0.77). Effective probiotic strains included the bacteria Lactobacillus GG, Bifidobacterium lactis and Streptococcus thermophilus and the yeast S. boulardii. Even better results were noted when S. boulardii was examined alone: a trial of 246 children treated with S. boulardii had a RR:0.2, 95% CI 0.07-0.6. The overall NNT was 7 in this meta-analysis.38

A meta-analysis examining 19 trials using a mixture of probiotics to prevent AAD showed a RR 52% (95% CI 0.35-0.65). Probiotics included in the study were Lactobacillus GG, L. acidophilus and S. boulardii. The greatest protection was noted when probiotics were started within the first 3 days of antibiotic administration.39

A larger meta-analysis was performed assessing 25 randomized controlled trials with 2810 patients given various probiotics to prevent or treat AAD. A beneficial effect was seen from the use of probiotics in preventing diarrhea, RR 0.43, 95% CI 0.31-0.58. Probiotics found to be useful included Lactobacillus GG and S. boulardii. Higher doses were found to be significantly more effective.40

Although Lactobacillus GG has been shown to colonize the intestinal tract of foals, it does not colonize in adult horses and its clinical utility in horses is unknown.41

C. difficile diarrhea

C. difficile diarrhea (CDD) is a disease noted in both man and horse. Often the disease is associated with the use of antibiotics and relapse is problematic. McFarland’s meta-analysis noted above also examined the use of probiotics to prevent CDD. Six randomized controlled trials were included. The RR 0.59, 95% CI 0.41-0.85 was significant but only S. boulardii was noted to be effective for CDD prevention.40

A randomized controlled trial was performed to evaluate the effects of S. boulardii as adjunctive treatments to metronidazole or vancomycin in 124 patients. The administration of the probiotic S. boulardii resulted in a 50% reduction of CDD relapses in patients that had previously experienced relapse.41

Acute diarrhea in youth and adults

Diarrhea due to various causes (viral, bacterial, ‘traveler’s diarrhea’, unknown causes) is commonly treated with fluid therapy. A very large Cochrane review of 1917 patients within 23 studies examined the benefits of administering probiotics with rehydration therapy and determined probiotics reduced the risk of diarrhea (RR: 0.66, 95% CI 0.55-0.77) and duration of diarrhea by 30.48 hours (95% CI 18.51-42.46 hours).42

A meta-analysis on five randomized controlled trials of 619 children treated with S. boulardii for acute diarrhea showed a significant reduction of diarrhea from control groups: -1.1 days 95% CI -1.3 to -0.8). Also, the risk of prolonged diarrhea >7 days was reduced significantly in the treatment group: RR 0.25, 95% CI 0.08-0.83; NNT=5, 95% CI 3-20.43

Kurugol studied the effects of S. boulardii on 200 children with acute diarrhea in a double-blind randomized controlled trial. S. boulardii groups had a significantly shorter duration of diarrhea as well as number of days of hospitalization.44

Diarrhea in horses

A small randomized, blind, placebo controlled trial involving 14 horses was performed at the University of Pennsylvania. Horses were administered S. boulardii along with standard care of acute diarrhea. The authors noted a significant reduction in severity and duration of diarrhea during hospitalization in the horses treated with S. boulardii. A proof of concept study was simultaneously performed. This pilot project did prove that administration of 10-20x109

S. boulardii are able to survive gastrointestinal transit in the horse and, similar to humans, the yeast is cleared from the body within days of discontinuation.10

Inflammatory bowel disease (IBD)

Crohn’s disease is a form of IBD in humans. Several forms of IBD probably exist in the horse. A double blind trial of 20 patients with Crohn’s disease found that adding S. boulardii to anti-inflammatory therapy significantly reduced bowel movements.45

A pilot study of 25 patients with ulcerative colitis showed that by adding S. boulardii to anti-inflammatory treatments 68% of patients were able to claim therapeutic success in otherwise uncontrolled disease.46

Tube feeding diarrhea

Horses and humans both experience diarrhea from total enteral nutrition diets. Three studies involving patients in intensive care units have been performed. Patients receiving total enteral nutrition and S. boulardii experienced a modest reduction in incidence and duration of diarrhea.47-49 A significant improvement in tolerance to enteral formulas was also noted in one study.49


Dr. Clancy’s four reasons for skepticism regarding the use of probiotics are carefully being answered with the intensive study of S. boulardii. Many of the mechanisms of action have now been determined: protease and phosphates production in vivo that specifically degrade toxins and their receptors thereby preventing toxin effects, proinflammatory cell signaling blockade, immune modulation, trophic effects, antisecretory effects, anti-inflammatory properties and direct pathogen blockade.

Medical trials have been completed both in the human and veterinary fields. Digestion of these trials through the use of meta-analysis has provided strong evidence for the efficacy of S. boulardii when used to prevent or treat AAD, CDD, non-specific diarrhea of adults and children. Although the veterinary literature remains sparse, evidence does exist for the use of S. boulardii in horses. In fact, this small body of literature GREATLY OUTWEIGHS any probiotic study done prior. Further medical evidence also supports the use of S. boulardii for inflammatory bowel disease and as an adjunct to total enteral nutrition.

While horses are not humans, there is obviously a wealth of data being mined both on the basic science level and in randomized clinical trials that warrants veterinary scrutiny. Indeed, a proof of concept has clearly been demonstrated in the laboratory, human patients and in a small equine study. S. boulardii, the only yeast probiotic with biotherapeutic benefits, offers a novel therapy to veterinarians in the care of horses.

With the limitations that have been identified with currently available veterinary probiotics and the small number of clinical studies available, S. boulardii at 20-50 x 109 PO q 12-24 h in a documented formulation, is the only evidence-based probiotic available to equine veterinarians for treatment and prevention of gastrointestinal disease.

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