We should start, then, by defining what probiotics and prebiotics are. Probiotics are microorganisms which live as part of the commensal microbiota of our body and are considered to be beneficial to us. Well known examples of those are gut bacteria such as Lactobacilli and Bifidobacteria. These microorganisms dwell inside our intestines fermenting nutrients that in most of the cases the human body is not able to digest. While doing so, they produce additional nutrients beneficial for our health, protect us against pathogenic microbes and regulate our natural defenses, i.e. the immune system.
We also have prebiotics, which are intrinsically linked to probiotics since they are nutrients, non-digestible by humans, which allow the beneficial bacteria that populate our organism to thrive while not enhancing the growth of pathogenic microbes. In a simple analogy, probiotics will be the machine while prebiotics may be considered the ghost in the machine. Natural compounds present in our diet, such as α-Oligoglucans, Inulin type fructans and β-Galacto-oligosaccharides have demonstrated specific prebiotic effects on beneficial bacterial groups in the gut such as Bifidobacteria, Lactobacilli and Bacteroides. The specificity of these compounds is due to the ability of these groups of bacteria to digest sugars and obtain energy from them.
Enzymes codified in their genomes break down the oligosaccharides allowing the microbes to feed on this energy source, which confer an advantage against other gut dwellers that do not contain such enzymes. Microorganisms that are able to digest Inulin and other fructans possess enzymes called β-fructofuranosidases (or fructanases), to digest β-Galacto-oligosaccharides they will have β-Galactosidases and to feed on α-Oligoglucans; α-glucosidases (α-amylases) that are able to break down the α1🡪6 bond between glucans. All this occurs in the very specific environment of the bowel, an environment, which although variable and extensive has at least some well-defined characteristics: its pH tends to neutrality, it is rich in nutrients, it has high water content and it is mainly an anaerobic niche.