Intestinal bacteria--passengers or partners?

The gut of all warm-blooded animals contains a large population of bacteria. The metabolic activities of this diverse collection of bacteria can affect the host animal in a variety of ways, some of which are harmful and some of which are beneficial. The way in which an organism reacts with its host can be affected by diet or by other bacteria present in the gut. The complex inter-relationship between the different bacteria and between the bacteria and the host are discussed.