What exactly are human strain probiotics? Despite the name, human strain probiotics don’t actually contain human byproducts or ingredients. They are simply strains of beneficial bacteria that have been found to live in the human digestive tract. That means they are already adapted to thrive in the human gut. According to research in the International Journal of Medical Science, because they are native to the human intestinal tract, they are superior to probiotics from other sources. The two most prevalent types of native bacteria in your gut and in human strain probiotic dietary supplements are Lactobacilli, which are found in the small intestines, and Bifidobacteria, which reside in the large intestines.