Ecoli in the blood....how do you treat it??

My father has been fighting ecoli in his blood stream for the past year and it is getting worse. He went into septic shock a few weeks ago. He is in ICU and they continue to treat him with the stage four antibiotics. It works after about 10 days, then he is sent home and sick again a week later and has to go back to ICU. They had put him on PCN to take everyday at home and it worked for about 6 weeks then the ecoli mutated and he is now sick again with a temp of 103.7. The antibiotics are not going to work much longer and they cant seem to find the place the ecoli is "living". Has anyone had this issue or have any suggestions as to how to deal with the problem. Please help!!! My dad is really sick!!!! In case it helps he is 59 Years old.