I think it is something like 1 in 4 of the population has staph in there nasal passages, and I would bet those with CF are closer to 100%. My guess is that you still had staph there, It is just the pseudo overpowers it in your cultures. There are some doctors who say having staph in your sputum is ok and won't treat it as if you treat it allows pseudo to take off and in general staph is less destructive than pseudo. I think it is somewhat of a general rule these days that if you take antibiotics for staph in your lungs that you should also treat the pseudo at the same time.