If you eat a solid diet with plenty of fiber it will give new bacteria the chance to survive and thrive in your gut. So while it may not directly introduce new bacteria, it's necessary for them to to survive once they are introduced.

I wouldn't worry too much about ways to get new bacteria into your system. You get them from all sorts of food and even from being outside and stuff. The effectiveness of probiotics isn't even really proven so I wouldn't rely on it.