Here is a case where the research is lagging the knowledge base. Every holistic MD has, for decades, preached eating foods enriched with healthy bacteria. What we need now is research that looks at outcomes.
The problem with this is that they are frequently wrong. Many people respond poorly to these type of probiotic diets, and holistic medicine has observed outcomes in a small portion of the population and thus encouraged us to generalize them. All we really know about the gut is that it is extremely complicated and that in terms of gut conditions' ("IBS") relationship with gut microbiota they run the gamut. Many of them are either not directly affected by microbiota population (ie: nerve damage, circadian rhythm ), or gut microbiota are part of a multi-causal problem.
We need more science like this, that reaffirms what we "know," not more holistic bullshit.
Source: IBS sufferer who has spent years talking to GI specialists and reading research papers about this stuff. It's hard, and as soon as you think you have it figured out it smacks you in the face with another factor you haven't considered.
"Turnbaugh’s team found that switching mice to a high-sugar, high-fat diet reshaped the abundance of the community of microbes in the gut to a new, stable makeup within three days, in a reproducible manner that was largely independent of genetic differences among individual mice."
When babies are born, their gut is sterile before it gets colonized from outside. The diet definitely can affect which strain will outgrow others, but all gut bacteria is exogenous.
[0] http://www.whitemountainfoods.com/YogurtProductPage.html
For that case, you could find your favourite local milk and make yogurt who's ingredients you are certain of, because you put them in.
[0] http://www.lifeway.net/Products/OrganicKefir/WholeMilkKefir/...