The purpose of this study is to compare and contrast the effects of a prebiotic (Trametes Versicolor), a probiotic (Saccharomyces boulardii) and an antibiotic (amoxicillin) on the gut microbiota of healthy volunteers. It is expected that treatment will result in the rapid and reproducible alterations in fecal microbiota that will spontaneously reverse in the weeks after treatment is discontinued.

Inclusion Criteria:

• Age 18 to 39 years (male or female)
• Good general health
• Able to comply with study requirements and to provide informed consent
• For women of childbearing potential
• A negative urine pregnancy test immediately prior to starting the study treatment
• Agreement to comply with approved methods of contraception during the period of active study treatment (not required during follow-up)

Exclusion Criteria:

• Any diagnosed autoimmune disease
• History of organ transplantation
• Known chronic or recurrent systemic disorder associated with immunocompromise
• A history of allergy or hypersensitivity to mushroom or mushroom extract, brewer's or baker's yeast or to penicillins or cephalosporins (including amoxicillin).
• History of severe allergic reaction (requiring hospital admission and/or the administration of parenteral medication or associated with dyspnoea, wheezing, hypotension, loss of consciousness).
• Systemic antibacterial therapy during the 3 months prior to study enrollment
• New prescription medications during the 4 weeks prior to study enrollment
• Prescription or OTC medications or supplements that are known to alter gut function or microflora (i.e. acid antisecretory drugs, probiotics) during the 4 weeks prior to study enrollment
• Active gastrointestinal or metabolic disease
• Prior gastrointestinal surgery (apart from appendectomy or cholecystectomy)
• History of chronic constipation with passage of fewer than 3 spontaneous bowel movements per week on average or chronic or recurrent diarrhea with spontaneous bowel movements more often than 3 times daily
• Any condition or personal circumstance that, in the opinion of the investigator, renders the subject unlikely or unable to comply with the full study protocol.