The concept that microbes might be beneficial, rather than simply detrimental, to health, enjoys a long history of anectodal evidence. Among the oldest observations is the Old Testament recording of Abraham ingesting sour milk (Genesis 18:8). The purported health benefits of microbial products are varied and often substantiated by testimonials rather than science. However, increasing, hypothesis driven research is generating evidence based examples of therapeutic benefits of biotherapy, indicating that credence might be reasonably given to selected products for selected indications.The term "probiotic" was originally coined in 1965 to refer to substances secreted by one microorganism which stimulates the growth of another. The term has been modified at least 5 times, and continues to vary with the author. A reasonable working definition, as proposed by the WHO definition works best: Live microorganisms which when administered in adequate amounts confer a health benefit on the host (WHO (World Health Organization), 2006) In contrast to probiotics, prebiotics refer to non-digestible food ingredients (including dietary fiber) that beneficially effect the bacterial population. They differ from other fermentable carbohydrates in that they interact with selective microorganisms.. As such, organisms presumably associated with health benefits will be stimulated, shifting the composition of the intestinal microbiota toward beneficial organisms. For example, fructo-oligosaccharides and inulin, transgalactosylated oligosaccharides and soybean oligosaccharides selectively promote the growth of bifidobacteria. Metabonomics is the scientific discipline that studies compounds formed from prebiotics. Synbiotics contain both pre and probiotics, with "syn" implying a synergistic effect of the prebiotic on the probiotic portion of the combination products. Note that the prebiotic portion should have demonstrated positive effects on the specific probiotic component of the combination product should "match". Understanding the impact of probiotics is incumbent upon a description of the normal gastrointestinal microflora. The total body microbiota (human) includes 10¹² to 10¹² bacteria, outnumber other cells by approximately 10 fold; 30% of dry fecal matter represents microbes. The number of species in the gastrointestinal tract (again in humans) is estimated to be between 400 and 500. Only a fraction are conducive to culture; based on 16S rRNA sequencing, some of these unculturable microbes are closely related to those already identified whereas others represent totally new species. Their role is critical, protecting against invasion of pathogenic strains of bacteria, facilitating normal bowel smooth muscle function, supporting digestion of certain foods, and contributing to nutrition through the production of vitamins (vitamin K; vitamin B in some species) and other nutrients (eg, short-chain fatty acids).. Microbes impact the maturation and maintence of the intestinal immune system, influence cell proliferation and facilitate energy salvage (eg, through conversion of nutrients to short chain fatty acids). The catalytic potential of the microbiota may contribute to (or detract from) health through production of beneficial (or detrimental) metabolites. Among the organisms most commonly cited as beneficial are Lactobacillus and Bifidobacterium. Beneficial metabolites include the short chain fatty acid carbohydrate product butyrate (energy source, anti-inflammatory, immunosuppressant). Benefits of microbial fermentation of plant products include the production of enterolignans associated with estrogenic and antioxidant effects. Oxalobacter formingenes transforms oxalates; its absence increases the risk of oxalate stones in humans. Bacterial byproducts have a number of adverse effects. Protein byproducts, produced primarily in the distal colon, include ammonia and amines resulting from deaminations; these products are associated with pro-carcinogenic effects (eg, nitrosamines). Cysteine and methionine degradation yields sulfides, which inhibit colonic use of butyrate. Anaerobic colonic fermentation of aromatic amino acids tyrosine to phenols and tryptophan to indoles, and their subsequent metabolism, yields several procarcinogenic compounds. Bacterial deconjugation and dehydroxylation of bile acids contribute to their enterohepatic circulation; other compounds conjugated by compounds such as taurine, glycine, sulfate may likewise be recycled. Many of these determinental compounds play a role in hepatic encephalopathy as well.