Sure, gut bacteria help in digestion and no question that gut bacteria play a large role in our immune system health. But are our mental outlook and mood influenced by gut bacteria? Yes; studies are pointing that way. Your gut bacteria are a vast army, too, numbering approximately 100 trillion—10 times as many as human cells in the body. Likewise, two-thirds of the human gut microbiome—the technical name for this evolving collection of trillions of probiotic microbes living in and on the human body—is unique to each person.