Figure 1.

From diversity to function. (a) Schematic outline of the human digestive tract with its characteristics and microbiota. The stomach is characterized by low pH conditions and a short retention time and colonization by bacteria is minimal. The small intestine (duodenum, jejunum and proximal ileum) has a short retention time (2–6 h) and high pH, the residing microflora is exposed to secreted bile salts and pancreatic juices at the proximal part. The lower digestive tract, comprising the terminal ileum and colon, is in contrast characterized by a longer retention time, neutral pH and is the most densely populated by microorganisms. (b) Overview of the major ‘omics' approaches using DNA, RNA, protein or culture-based techniques.