A Gastroenterologist's Guide to Probiotics

Matthew a. Ciorba, MD


Clin Gastroenterol Hepatol. 2012;10(9):960-968. 

In This Article

The Human Microbiome and Probiotic Mechanisms

To understand the role that probiotics may have in influencing health, it is important to have an appreciation of the roles of the normal intestinal microbiome (commensal microbiota). The human GI tract is host to over 500 bacterial species as well as a less well-described virome. These microbiota form a virtual bioreactor facilitating digestion, nutrient provision, and the shaping of our immune system.[2] Our intestinal bacteria weigh up to 1 kg and bacterial cells outnumber human cells by 10:1. The bacterial genome may outnumber the human genome by 100:1. Nutritional factors including several B vitamins, vitamin K, folate, and short-chain fatty acids are produced by these bacteria. Up to 10% of an individual's daily energy needs can be derived from the by-products of bacterial fermentation. Gastrointestinal microbiota are also critical for normal immune system development.[3] The physiologic impact mediated by our resident microbes is substantial enough to have earned the label of "other organ" from some.[4]

Beyond contributing to or modifying the metabolic and nutritional functions of the commensal microbiota, probiotic bacteria have several putative mechanisms by which they may confer specific beneficial effects. General categories include modulation of immune or sensory-motor function, enhancement of mucosal barrier function, and antipathogen effects (Figure 1).[5–7] Some of these mechanisms have been worked out in animal models and/or in vitro systems only.

Figure 1.

Mechanisms of action of probiotics in the gastrointestinal tract.

Soluble products secreted or shed by probiotics also mediate important physiologic benefits; thus, viable bacteria are not necessarily required for all benefits.[8,9] The mechanisms by which probiotics exert benefit varies by specific probiotic strain and likely depends on the clinical indication.[10,11] Therefore, as with antibiotic prescribing, clinical use of probiotics should focus on matching the probiotic strain and dosage to the condition for which it has shown benefit in clinical trials. In the future, greater understanding of probiotic-specific mechanisms could allow for precise selection of a particular probiotic strain to target a patient's specific pathogenic defect and clinical problem.