A library is potentially quite large - essentially, you need a phage that's for that particular organism. Potentially a large number of each species, given that

A library is potentially quite large - essentially, you need a phage that's for that particular organism. Potentially a large number of each species, given that phage and bacteria are in an evolutionary arms race. The trickier part is you have to keep them active and functional over long periods. That's not something most hospital labs are equipped to deal with.