Humans are covered in bacteria. In fact, it is estimated that we each have ten times more bacterial cells on our bodies than human cells which make up our bodies. A large portion of the masses of bacteria you drag around with you live in your gut. Some are helpful bacteria that break up foods that you otherwise cannot digest on your own. Some are pathological, such as C. dificile, a notorious and potentially deadly bacteria. Many of the bacteria in your gut are actually un-culturable. That is, we do not even know what they are because we cannot figure out how to grow them outside of the human body and have little idea of how they work. Your bacteria are different from my bacteria, are different from everyone else's bacteria, but it turns out that there are some patterns which might be of research interest.
Researchers reported in Nature on a discovery they are calling the enterotype. Bascially, they took a buch of fecal samples and measured the DNA in each of them. Even if you can not grow and culture all the bacteria, you can always cut the cells up and measure the DNA. Then you can measure parts of the DNA and guess about what you started with. It turns out that people fit into one of three basic patterns of DNA, the enterotype (entero = gut, type = category). Several aspects of this discovery are quite amazing. First, there does not appear to be a continuum of gut bacteria, but dinstinct "types". Second, enterotypes do not seem to follow geographic or cultural boundaries, and furthermore, are not associated with weight, gender, or age. It seems that regardless of your diet, upbringing, cultural and genetic heritage, your gut bacteria settle into one of three equilibira.
We do not know for sure if this phenomenon could explain health disparities, or why some people seem to respond to certain drugs while others do not. Perhaps it could shed light on why some people can easily lose weight while other struggle or never succeed. Alternatively, someone might try to make a lot of money off of it without ever publishing any meaningful research on the topic (Eat Right For Your Blood Type). Time will tell, but this is an exciting discovery.