I wouldn't worry about it---I mean if we worried about every single thing that could hurt us, we would be living in a bubble.
I would bet the majority of CF patients are regular eaters of onions....yet only a small percentage have cepacia (myself included). And of those of us who culture cepacia, the majority of us know how we got it. Some have cultured it after being in the hospital. I got it from my cousin with cf (before we knew it was anything to worry about), etc.