Table 1 Definitions of common terminology and classifications in probiotic research

Concept

Definition

 Probiotics

Live microorganisms which, when administered in adequate amounts, confer a health benefit on the host [1].

 Prebiotic

A substrate that is selectively utilized by host microorganisms conferring a health benefit on the host [2].

 Synbiotics

A synbiotic product beneficially affects the host in improving the survival and implantation of live microbial dietary supplements in the gastrointestinal tract by selectively stimulating the growth and/or activating the metabolism of one or a limited number of health-promoting bacteria [3].

 Postbiotics

Postbiotics are bioactive components produced by beneficial bacteria (through a natural fermentation process) which have biological activity in the gut (e.g. short-chain fatty acids) [4].

 Immunobiotics

Inactivated probiotics (e.g. heat-killed), in which the dead cells maintain their immune benefit.

 Gut

The gastrointestinal tract is a long tube that starts in the mouth and ends at the anus. Its main function is to process food. Approximately 70% of antibody producing cells are is located in the digestive system.

 Microbiota vs. Microbiome

The gut microbiota is a diverse ecosystem consisting of bacteria, archaea, viruses, protists and fungal communities (mycobiome) living in the human gut. Microbiome refers to the collection of genomes from all microorganisms in a particular environment

 Transient vs. Resident Strain

Supplementary probiotics are transient strains. There is currently no evidence that supplementary probiotics can permanently colonize in the gut as resident strains resist colonization by transient strains. Transient probiotics strains may have numerous beneficial health effects by positively interacting with the immune system or stimulating growth of beneficial resident strains.

 Alpha-Diversity

Represents the number of species and the proportion in which each species is represented in the microbiota. A high alpha diversity is present when there is a high number of species and their quantities are alike.

 Beta-Diversity

Beta-diversity broadly reflects the species composition diversity between regional and local sites. The beta diversity measures the turnover of species between two regions in terms of gain or loss of species

Classes of probiotics

Definition

 Lactic acid bacteria (LAB)

Nonpathogenic, nontoxigenic, Gram-positive, fermentative bacteria that are associated with the production of lactic acid from carbohydrates. LAB grow anaerobically, but unlike other anaerobes, most can grow in the presence of oxygen. Examples include Lactobacillus (ssp. acidophilus, fermentum, plantarum, rhamnosus, casei, reuteri, gasseri), Streptococcus (e.g. salivarius, thermophilus) and Lactococcus.

 Bifidobacteria

Bifidobacteria are among the first microbes to colonize the human gastrointestinal tract. Examples include Bifidobacterium bifidum, longum, animalis, and breve. Bifidobacteria are not LAB. They are, however lactic acid producing bacteria (but through a very different metabolic pathway).

 Spore-forming bacteria

Soil-based probiotics, also referred to endospores, are the dormant form of bacteria that are highly resistant to physical and chemical influences. Upon ingestion, these spores have a high survival rate through the stomach and germinate in the small intestine. Examples include Bacillus (e.g. coagulans, subtilis). Spore forming bacteria are not necessarily of soil origin. They can also be found in fermented foods.

 Yeast

Examples include Saccharomyces boulardii.