bacteria are fun and useful! we play with them in molecular genetics all the time. it's because of the e coli in our intestines that lactose intolerant people have issues? Okay, so not always useful. Good stuff, man, good stuff.
Lactose intolerance is the inability to digest significant amounts of lactose, the major sugar found in milk. Lactose intolerance is caused by a shortage of the enzyme lactase, which is produced by the cells that line the small intestine.
Yes...I know this. But the specific issue is that the gene that creates the protein to digest lactose is turned off in lactose intolerant people, so all of the milk you put into your body goes straight through to your intestines where the e.coli lives, so the e.coli, whose lactose gene is perfectly intact, gets a lot of nutrients and becomes more active and you get all the lovely symptoms of lactose intolerance.