Figure 1.

Impact of the bacterial flora on host physiology. (A) Germ-free mice were either ‘humanised' by inoculation of human-derived flora (left red arrow) or ‘re-conventionalised' by exposure to normal non-germ-free mice (right blue arrow). (B) Bile acids (orange nodes) are produced in the liver, processed by the gut microflora (green nodes) and are recycled to the liver via the enterohepatic circulation. Analysis of the correlation between gut flora composition and bile acid composition reveals that in ‘re-conventionalised' mice, the correlation network is much denser (blue edges) than in ‘humanised' mice (red edges). These changes may underlie the profound changes observed in host lipid metabolism and illustrate the close adaptation of the flora to its natural host.