My favorite cocktail for my very resistant PA is Imipenum and Tobra, which I am on at the moment. Colisten IV's along with Zyvoxid work very well for me if I have PA and MRSA.Vanco is just nasty! They usually give me just a heplock line because I have really good veins but the vancomycin eats away at it very quickly. RTforJCinAZ, to awnser your question, and I am no expert ( speak just out of experiance ) when the lab tests for resistance in antibiotics they usually only use a small sample of your mucus that they smeer on a petri-dish. They then place all antibiotics around the smeered mucus and see how much of the bacteria actually grows around the antibiotics. But because such a small sample of your mucus is used, it might not be that the sample used is the majority of the bacteria in the lungs. So the results might say that you are resistant to a specific antibiotic, but actually could work really well. And this has happened to me many times. So seeing most of the drugs are resistant on the results they try to give you more than one for a better chance of it actually working. I think there are more technical factors as well that make some cocktails better than others. Anyway that my 2 cents. don