Petersen found that athletes had more diverse microbiomes than healthy nonathletes. In particular, many of the cyclists had an abundance of Prevotella bacteria, a genus that is often absent from typical American or European gut microbiomes. The cyclists also had a higher abundance and activity of Methanobrevibacter archaea, which consume by-products of bacterial fermentation of complex carbohydrates.

http://www.the-scientist.com/?articles.view/articleNo/49450/title/Athletes--Microbiomes-Differ-from-Nonathletes/