**3.1 Probiotics**

Probiotics are defined as "live microorganisms administered in adequate amounts that confer a beneficial health effect on the host" [32]. The scientific recognition of the health-promoting properties of live microbes began in the early 1900s when a Ukrainian scientist, Elias Metchnikoff, hypothesized that the beneficial microbes present in fermented milk could normalize bowel health and prolong life by inhibiting 'putrefactive' bacteria in the gut [32]. Then, lactic acid bacteria (LAB) strains isolated from fermented milk were commercialized with the intention to treat diarrhea in French children in 1906. The term probiotics has been used since 1962. In the past 2 decades, a significant research attention has been given to probiotics, and beneficial effects of probiotics for the mitigation of infections of oral cavity; respiratory, urogenital and gastrointestinal tract [33]; cancer [34] and obesity [35] have been identified.

Bacteria being used as probiotics today include LAB, non-pathogenic *Escherichia coli* and *Bacilli* [36]. The *Lactobacilli* and *Bifidobacteria* genera are the most commonly used as probiotics due to their specific health benefits, high safety profile, and stability [37, 38]. Probiotic bacterium possess unique genetic tools, special membrane structure and composition that allow them to survive under different environmental conditions after ingestion, adhere to the target niche, adapt to special nutrition conditions and integrate with the local microbiota [37].
