Prebiotics, Probiotics, Synbiotics, and the Immune System

Abstract and Introduction

Abstract

Purpose of review The intestinal immune system is constantly exposed to foreign antigens, which for the most part should be tolerated. Certain probiotics, prebiotics, and synbiotics are able to influence immune responses. In this review, we highlight the recent publications (within the last 2 years) that have substantially progressed this field.

Recent findings The immunological mechanisms underpinning probiotics, prebiotics, and synbiotics effects continue to be better defined with novel mechanisms being described for dendritic cells, epithelial cells, T regulatory cells, effector lymphocytes, natural killer T cells, and B cells. Many of the mechanisms being described are bacterial strain or metabolite specific, and should not be extrapolated to other probiotics or prebiotics. In addition, the timing of intervention seems to be important, with potentially the greatest effects being observed early in life.

Summary In this review, we discuss the recent findings relating to probiotics, prebiotics, and synbiotics, specifically their effects on immunological functions.

Introduction

The mammalian gastrointestinal tract is a highly evolved system specialized to perform the essential functions of nutrient digestion, absorption, and waste disposal. The intestinal immune system has the unenviable task of maintaining intestinal integrity in the presence of vast quantities of external or foreign antigens. Sophisticated cellular and molecular networks need to be constantly coordinated in order to tolerate nonpathogenic antigens, while also protective immune responses to potential pathogens must be maintained and can be induced effectively on demand. Inappropriate immune response to bacterial or dietary antigens is a significant component in several intestinal diseases, including inflammatory bowel disease, irritable bowel syndrome, and food allergies.^[1,2]

The balance between immune tolerance and inflammation is regulated in part by the crosstalk between innate and adaptive immune cells and the intestinal microbiota. Disrupted communication between the microbiome and the host because of altered composition and metabolism is thought to negatively influence the intestinal immune homeostatic networks. The deliberate modification of microbial species and their metabolism has led to the probiotic and prebiotic concepts.^[3]

Probiotics can be defined as live micro-organisms which, when administered in adequate amounts, confer a health benefit on the host. Notably, the definition of a probiotic does not differentiate between the wide range of potential health benefits and it is clear that not all probiotics will influence the immune system in the same way. In this review, we have focused on the probiotic studies with immunological endpoints, but these findings cannot be extrapolated to other probiotic strains. In addition, we have included microbial species not currently used in the industry as probiotics, even though these microbes fulfill the definition provided above.

Prebiotics can be defined as selectively fermented ingredients that allow specific changes, both in the composition and in the activity of the gastrointestinal microflora, which confer benefits upon host well being and health. Prebiotics typically are fibers that cannot be digested by the host, but are metabolized by the colonic microbiome resulting in expansion of certain bacterial species and the release of metabolites, such as short-chain fatty acids (SCFAs). As with the probiotic definition, not all prebiotics will have the same effect on immunological functions.

The combination of probiotics and prebiotics is termed 'synbiotics'.