What is Lactic acid bacteria?
[[File:Lattobacilli.jpg|thumb|A rod-shaped species of lactobacillus, under the microscope. Magnification is about 1000 times]] '''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 (biology)|order]] ''Lactobacillales.'' Most of them are [[Anaerobic organism|anaerobic]]: they do not need [[oxygen]]. Usually, they can live well in [[Acid|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 [[Genus|genera]]. Some species occur in the [[gut]] of [[Animal|animals]], including [[Human|humans]]. Several lactic acid bacteria are [[Pathogen|pathogens]]: they cause infectious [[Disease|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 intolerance|lactose intolerant]] (as, for example, [[Cat|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''.<ref>Swallow, Dallas M. 2003. Genetics of lactase persistence and lactose intolerance. ''Annual Review of Genetics''. '''37''' (1): 197–219. [https://www.annualreviews.org/doi/10.1146/annurev.genet.37.110801.143820] {{Webarchive|url=https://web.archive.org/web/20201006233720/https://www.annualreviews.org/doi/10.1146/annurev.genet.37.110801.143820 |date=2020-10-06 }}</ref> They cannot digest lactose products after [[weaning]]