My sense is that's right, but it makes it sounds a little too magical. It might be better to say that most of the strains you want to encourage are already pres

My sense is that's right, but it makes it sounds a little too magical. It might be better to say that most of the strains you want to encourage are already present, but a diet with lots of fiber (aka 'prebiotics') will encourage the good strains to outcompete the bad strains. It's a pretty complex balance, there are also phages, and some fungi all mixed up in there.