All dogs can benefit from probiotics, which aid digestion and modulate the immune system. Probiotics are beneficial bacteria that live in the digestive tract. There are a variety of different species belonging to genera that include Lactobacillus, Bifidobacterium, Streptococcus, and Enterococcus (often abbreviated by first initial only in names). Some species, such as Lactobacillus, live primarily in the small intestine, while others, such as Bifidobactera, reside in the large intestine (colon).