Foods rich in good probiotic bacteria, such as strains of lactobacillus and bifidobacterium are a good place to start. Anything cultured or fermented is usually a good bet. There are also probiotic supplements, if you want a particular strain.

My guess would be that certain bacteria or strains are better than others at outcompeting the undesirable colonies, but you'd have to look for specific research to say which ones. In lieu of that, though, it's not hard to experiment with probiotics and see which, if any, have a noticeable positive impact on you.