Oh no!
Don't even think about it!
It's not the Real Thing at all!
Real kefir is a Living Entity. The whole *point* is to keep it and nurture it and eat the kefir it gives you.
The bought kefir has an inhibitor introduced to it apparently (so as the kefir doesn't continue to develope on the shelf of the supermarket fridge thereby causing the packaging to bulge uninvitingly).
The culture to which you refer is also a commercial ploy. I mean we can't have people making their own for free (or the price of the milk)! Also because of Dairy Indusry Standards they had to remove the beneficial yeasts from the traditional cultures. (Dairy products had to contain only lactic bacteria and kefir has these yeasts and moulds etc that aren't on the Approved Lists)

It transpires that my first yoghurt - given to me at the tender age of 16 under the name "Yoghurt Bug" - was actually kefir...

Ooohhhh ....I am sooooo excited