Bacteria in nature appear as diverse communities, which may comprise of hundreds of different species. The shared capacity of the bacterial communities to adapt in any imaginable set of conditions is remarkable. Many bacteria have growth requirements, as yet undiscovered, which are fulfilled by their natural habitats. Indeed, bacterial communities have members specialized on different functions and providing vital elements to other bacteria living in the same community. Therefore, most bacteria cannot easily be isolated from their habitats by the routine culturing methods used in most laboratories today. To overcome the difficulties in culturing of individual microbes, modern approaches analyse the structure of bacterial communities by determining the characteristic features of the microbial DNA extracted from the community samples. Using such techniques we have found that 90% of the bacteria in the chicken gastrointestinal tract represent previously unknown species. Furthermore, more than half of the 640 different species found represent previously unknown bacterial genera. Bacteria in the gastrointestinal tract derive most of their carbon and energy from dietary compounds which are either resistant to attack by digestive fluids or absorbed so slowly by the host that bacteria can successfully compete for them. The performance improvements on use of a growth promoting feed antibiotic is due to factors such as reduced competition for nutrients in the small intestine, reduced local inflammation due to control of pathogens, and reduced size of intestine. Since bacterial species differ in their substrate preferences and growth requirements, the chemical composition and structure of the digesta largely determines the species distribution of the bacterial community in the gastrointestinal tract. Consequently, it should be possible to shift the microbial community from harmful to non-harmful bacteria by changing the diet.