BTW what I mean by "banking" cultures is this: I have a certain type of yeast I like to use in my brewing. Purchasing the yeast (like probiotics) gets expensive at some point. So my aim is to grow the original culture to sufficient cell numbers that I can store (bank) the culture long-term, and supply myself with my own yeast.
There's a bit of a difference here with yeast and "probiotic" blends though. One strain of organism is easy to deal with. A multi-strain, multi-species culture, however, may require isolating each organism, culturing them individually, then recombining them into a "probiotic" formulation. The reason is if you simply take a blend of organisms and feed them something, there's no guarantee that all the organisms will survive or maintain the proper balance you want, because some organisms will out-compete each other or begin to dominate the culture.
So the beauty in certain "wild" cultures, like kombucha and kefir, is that you have a multi-strain culture that naturally maintains a certain balance without having to intervene with technical processes.
Also something for you to consider: One of the reasons probiotic manufacturers will tout cell counts, in millions or billions or whatever, is that storing microbes in this way will inevitably kill many of the cells and does not provide a buffering medium (like a fermented food) by which the microbes are consumed. In other words you have to have high cell counts in this method.