The internationally endorsed definition of probiotics is live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. Other definitions advanced through the years have been restrictive by specification of mechanisms, site of action, delivery format, method, or host. Probiotics have been shown to exert a wide range of effects. The mechanism of action of probiotics (e.g., having an impact on the intestinal microbiota or enhancing immune function) was dropped from the definition to encompass health effects due to novel mechanisms and to allow application of the term before the mechanism is confirmed. Physiologic benefits have been attributed to dead microorganisms [1]. Furthermore, certain mechanisms of action (such as delivery of certain enzymes to the intestine) may not require live cells. However, regardless of functionality, dead microbes are not probiotics.
The term “probiotic” is sometimes erroneously used as a synonym for putatively beneficial members of commensal microbiota. The context for this misuse is the assertion that certain dietary or environmental factors may “encourage your native probiotics.” Members of human commensal microbiota are often sources from which probiotics are isolated, but, until such strains are isolated and then adequately characterized for content, stability, and health effects, they are not probiotics.
The US Food and Drug Administration (FDA) uses other terms for live microbes for regulatory purposes; live microbes used in animal feeds are called “direct-fed microbials” [2], and, when intended for use as human drugs, they are classified as “live biotherapeutics” [3]. However, no legal definition of probiotics exists in the United States or in other countries, which allows the marketing of products labeled as “probiotics” that do not meet the fundamental criteria stipulated in the scientific definition. As public awareness of probiotics increases, the use of the term has implications both for the violation of the standard of truthful and not misleading labeling and for consumer confidence in this product category. In the absence of a legal definition of the term “probiotic,” it is incumbent on industry participants to adopt and adhere to the scientific definition of the term. If the products are not properly characterized and validated, the category will suffer.