There are so many different bacteria that will ferment milk.
But the true Tibetan kefir is made by the special bacterial colonies that form 'kefir grains'. The kefir grains need to be removed from kefir and used to made a fresh kefir batch. That's all I can say on the topic.
There are many other bacterias, both mesophilic and thermophilic, that ferment milk - this is how different yogurts are made. For these, it is enough to take a few spoonfuls of the yogurt and mix it with fresh milk for the next batch.