The human gastrointestinal tract is home to an estimated 100 trillion microorganisms. Collectively referred to as the intestinal microbiota, they are thought to outnumber the cells of their host 10 fold and represent by far the largest microbial community associated with the human body . The intestinal bacteria in humans comprise 500-1000 bacterial species and complex metabolic relationships exist between bacterial communities and their host . Figure 8.1 illustrates the different compartments of the human gastrointestinal tract, physiological processes and conditions, and the major bacteria genera found therein . Very little is known about these interbacterial relationships and how they relate to host health, but it is known that increasing diversity of the large bowel bacteria promotes metabolic homeostasis and the ability to resist invading pathogens .