Well-studied live bacteria strains - Bifidobacterium longum, Lactobacillus acidophilus, Lactobacillus rhamnosus, Streptococus thermophilus. 
The word “probiotic” comes from the Greek “pro bios”, meaning “for life”. Various studies conducted over the past years show that probiotics help restore the balance of bacterial flora. 
The human microbiota offers many benefits to the host, through a range of physiological functions such as strengthening gut integrity, harvesting energy, protecting against pathogens and participating in host immunity. The important role of gut microbiota on metabolic disease opens new approaches in the treatment of obesity, insulin resistance and type 2 diabetes. 
Many clinical studies have shown that daily consumption of probiotics can contribute to restoration the natural balance of intestinal flora following a course of antibiotics, prevent or cure complaints such as diarrhoea (e.g. traveller's diarrhoea), maintain intestinal activity at an optimum level (supporting digestion, preventing constipation and flatulence), contribute to the normal immune function and overall wellbeing. Probiotic bacteria also help to produce vitamins, such as B2, B5, B9, B12 and K. [1, 2, 3]
The use of microbials started centuries ago when people first noted the beneficial health effects of eating fermented foods. The use of fermented products (e.g. milk and yogurt) are the part of human history. Hippocrates considered fermented milk both a food product and a medicine to cure intestinal disorders.
1. Markowiak P, Slizewska K. Effects of Probiotics, Prebiotics, and Synbiotics on Human Health. Nutrients. 2017;9(9):1021. doi: 10.3390/nu9091021.
2. Valdes AM, Walter J, Segal E, Spector TD. Role of the gut microbiota in nutrition and health. BMJ. 2018 Jun 13;361: k2179. doi: 10.1136/bmj.k2179.
3. Kovatcheva-Datchary P, Arora T. Nutrition, the gut microbiome and the metabolic syndrome. Best Pract Res Clin Gastroenterol. 2013 Feb;27(1):59-72. doi: 10.1016/j.bpg.2013.03.017.