Isn’t E. Coli what makes humans sick?
How do they make a strain of it that’s beneficial?
I suspect that it is a naturally occurring strain of E.Coli. Different populations of bacteria, even of the same species, can have radically different characteristics.
Only some types of E. Coli are pathogenic.
How do they make a strain of it that’s beneficial?
It wasn't made. It was discovered by Alfred Nissle in 1917 during a Shigellosis outbreak. If you check the first link in comment# 10, then you'll find:
Listen up - this supplement contains E. coli Nissle 1917, a probiotic with a long and interesting history. Back during WW1, when Shigellosis was make life in the trenches a living hell, one soldier was unscathed. Shigellosis is a particularly nasty type of diarrhea with cramp, vommitting and bloody faeces.This interested microbiologist Alfred Nissle who studied the stools of more soldiers than any one man should have to and declared that the reason this one soldier was immune to the bloody thunderbolt tearing through the bowels of his comrades was because a useful bacteria named E. coli Nissle 1917 had set up home in his colon.