Interesting theory. I think there is bit more to it than it just being about a destructive bacteria. I think the individuals bodies response plays a huge role as well. I mean sure you could take a person who has this bacteria that is invading their gut and yet they wont have any symptoms or adverse effects. But them some people will.
They have been studying this as well in women who have chronic UTI's. They have been finding evidence that some ecoli bacterias have developed a biofilm that protects it from being killed off by antibiotics and also protects it from even being detected( like a cloaking device). So it can go about doing it's damage and nothing can kill it let alone even detect it.. If ecoli bacteria is doing this, I am sure other pathogenic bacteria are too..
But again, now whether a person develops crohns, I think it depends on quite a few things( individual immune system response, genetics, blood type, persons health etc..).
Thanks for sharing the info, very interesting.