My idea is that now I have a mycelium which is habituated to growing on coffee grounds and I can keep the culture going by providing it with my own kitchen-produced coffee grounds.
Oyster mushrooms will grow on a wide variety of substrates.
Trees, however, are the natural growth medium that oysters evolved to eat, and that is what they will live the longest on. With care you might keep this culture alive on coffee grounds for a few months, but it will eventually give out if you keep feeding it nothing but coffee. Once it is spent, you might try burying it beneath a pile of woodchips or in a mound of
compost. A drastic change like that can sometimes prod it back into growth.
Before your coffee oyster
bucket gives out, there are ways to keep your culture alive and begin expanding it. You can make clones and spore prints.
A low-tech cloning method somebody mentioned here already is to use wet corrugated cardboard - boil small pieces of corrugated cardboard for five minutes, cool, then layer bits of colonized coffee grounds or chunks of mushrooms (the stem buts work well and are a bit too chewy to eat) between the layers. Place this in a tupperware container in a cool location. Wait. Monitor every few days for signs of growth.
(This method is detailed in
Paul Stamets' book
Mycelium Running.)
You can also lay fresh oyster mushrooms on the damp cardboard, gills side down, for several hours, and allow the spores to drop onto the cardboard, then remove the mushroom (and eat it), and cover the cardboard. It takes a lot longer to see growth from spores.
Once you get good growth on the cardboard, use it to colonize more cardboard, which you use to colonize more cardboard, etc. and eventually you have enough to do more coffee grounds. (For long term storage place some colonized cardboard in a sealed container in the fridge.)
You can also make spore prints onto aluminum foil and store these prints in a dry place for months or even years, and later grow the spores out onto agar.