I will try to speak intelligently about this<img src="i/expressions/face-icon-small-smile.gif" border="0"> I had one strain of PA for many many years. It was at the time I started using Tobi that I started culturing 2 strains of PA. From what I understand, the continuous use of inhaled Tobi and maybe other abx causes "mutated strains". I don't know that that is necessarily a bad thing because I think we can all agree that Tobi is a good thing. I doubt that is the only reason for having more than one strain; but, that is a contributing factor. What is interesting is that the new inhaled drug, Aztreanam, that is due to come on the market soon has shown to be effective in working against some of those mutated strains and also does not contribute to or cause mutated strains of PA. This is my understanding of things.