Your digestive tract is home to trillions of individual bacteria and more than 500 different species of microflora. The two most prevalent probiotics are Lactobacilli, which make up the majority of the probiotics living in your small intestine, and Bifidobacteria, the most prevalent beneficial probiotic living in your large intestine. Most healthy people have 100 times more Bifidobacteria than Lactobacilli.