Lactobacillus
A bacterium normally found in the mouth, intestinal tract, and vagina.
Lactobacillus can also live in fermenting products, such as yogurt.
Humans appear to have a symbiotic relationship with this bacteria.
Lactobacillus has been with us so long that some types have become an important part of food digestion, although Lactobacillus can also contribute to cavities in the teeth if allowed to remain too long within the mouth