>>"Alot of kefir contains the bacteria B. subtilis.
How is this possible?"<<
A better question may be, how is it possible to keep it out? After all, it is known as the grass or hay bacillus... and is found most everywhere, having resilience against both heat, disinfectant sterilization methods, and survive pH values that other microorganisms do not.
If you have pastured livestock, or live around or near it, have a yard with soil and a lawn, have pets, you likely get B. subtilis (and many other microorganisms) on a daily basis whether you like it or not.
>>"I mean B. subtilis is supposed to be an Antibiotic and kefir contains alot of healthy lactobaccilus bacteria."<<
good bacteria, especially bacillus species produce huge amounts of enzymes, vitamins, antifungal peptides, and * Antibiotics * against *competing* species but not against those species that it coexists with peacefully or lives symbiotically with.
A strain of B. Subtilis you may be familiar with is B. Natto used to produce natto... produces the enzyme natokinase, the vitamin K, etc.
These *products* are actually the wastes of the organisms metabolic functions... and it is *big business* in the pharmaceutical (patented drugs), chemical (laundry detergents, etc.), supplement (vitamins and enzymes) and food industries (food additives).
One way or the other you are getting it, and its byproducts (waste) whether you try to avoid it or not.