Lactic acid bacteria

Lactic acid bacteria are bacteria which make lactic acid. This is an end product of carbohydrate fermentation. The bacteria change carbohydrates to lactic acid and energy. 
The bacteria are the order "Lactobacillales." Most of them are anaerobic: they do not need oxygen. Usually, they can live well in acidic environments. They are either rod-shaped, or spherical. They are found in decomposing plants and milk products. 
Lactic acid bacteria are important, and they are common. There are about 40 genera. Some species occur in the gut of animals, including humans. Several lactic acid bacteria are pathogens: they cause infectious diseases.
Lactic acid fermentation has been used since the Stone Age. It makes food last longer, and be easier to digest. Lactic acid fermentation is used in the production of cheese and yogurt.
Adult humans originally could not digest milk products. Then, all humans were lactose intolerant (as, for example, cats are after weaning). "Lactase persistence" is the phenotype where genes prolonging the activity of lactase beyond infancy. Humans are unusual in having lactase persistence. We know it evolved fairly recently (in the last 10,000 years) among some populations. Still, "most people worldwide are lactase nonpersistent". They cannot digest lactose products after weaning. The evidence is that lactose persistence grew in human groups who herded cattle. To them it was a great advantage: they could digest milk throughout life.