Without lab gear, your best bet is to slowly dry out the concentrate (don't heat, at least not above ~85ish F, use fan instead, thin layer, then pulverize powder). And then store powder in fridge. Freezer, unless you completely dessicate or freeze dry, will give you lower viability. Can also just dry out the bread and use after pulverizing. Lower concentration per gram then.
With lab gear, centrifuge, collect concentrate, purify if necessary, and store in hypotonic solution in fridge or lyophilize.