There are more than one hundred species and approximately one hundred thousand billions of bacteria, including the health beneficial probiotics and the harmful pathogens of Escherichia coli, reside in human gastrointestinal tracts. The balance of the microbiota in GI tracts will be changed with age, diet and stress. To keep your body healthy, you have to keep the microbiota in GI tracts balanced. It is probiotics or good bacteria in your GI tract.