Congrats on those precious babies, mama!!
These are your babies, and you have EVERY right to know what is going on with them. DEMAND they tell and explain to you the bacteria and how it made them sick. Freeze your milk for now. Consider donor milk like PP said. And don't stop looking for answers. Doctors are not always right, so maybe even demand a second opinion and get someone new in there...get them transferred to a diff hospital, even...that is if they are stable enough to be transferred.
My heart goes out to you and your babies!