The sugar Lactose is unique, in nature it is almost found nowhere else but in milk. This also means, only a few microbes have the necessary enzymes to process milk, that is, digest lactose, and extract energy from it by converting it into lactic acid. This lactic acid is then released into the milk, where it accumulates and retards the growth of other microbes. There are two major groups of lactic acid bacteria. The smaller genus of Lactococcus, which is primarily found in plants. The other is the 50 or so members of Lactobacillus, which are found both in plants and in animals, including human mouth, digestive tract, and vagina. Prior to the start of industrial production of yogurt and buttermilk, humans traditionally left the milk overnight. This naturally fermented the milk, with bacteria like – Lactobacillus fermentum, L. acidophilus, L. brevis, L. plantarum and L. casei. These bacteria take residence in our intestine and shield the intestinal wall, secret antibacterial compounds, boost the body's immune response to particular disease microbes and reduce the production of potential carcinogens. Today the standard industrial yogurt and buttermilk used specialized bacteria that grow well in milk but can't survive inside the human body. Therefore, we do not receive the same benefit and protection the traditionally made yogurt provided. Even the intestines of an infant are colonized by lactic acid bacteria- the Bifidobacteria. These are fostered by breast milk and produce various antibacterial substances, providing protection to the infant.
On a totally unrelated note-, the famous holes of Swiss cheese are because of a bacterium called Propionibacteria. Propionibacter shermanii, which is used as a starter in Swiss cheese, consumes the cheese's lactic acid during ripening and converts it into propionic acid, acetic acid and carbon-dioxide gas. The carbon-dioxide forms bubbles, or the holes in the cheese. The cheese ripens at 24°c as it is the ideal temperature for Propionibacteria. This warm temperature also is a reflection of the fact that the bacteria originally lived in human and animal skin. Today at least three species of Propionibacteria inhabit our oily skin, the most famous P. acnes, which live in our plugged oil glands.