Figure 1.

Function, dominant bacteria, microbial density and oxygen pressure in the different segments of the human intestine. The pH of the intestine increases from the stomach (1.5–5) to the large intestine (5–7). Similarly, the bacterial density increases from 10² to 10¹¹ cells/mL. The small intestine consists of duodenum, jejunum and ileum. Each segment shows different functions, which are mainly responsible for nutrient digestion and absorption. The colon (large intestine) is responsible for absorption of water and fermentation products such as short chain fatty acids (SCFAs). A decrease in oxygen concentration and antimicrobial compounds along the intestine leads to an increasing diversity in the large intestine with several obligate anaerobic bacteria. In upper parts reside more facultative aerobic bacteria, which can tolerate oxygen. Abbreviations: FFA, free fatty acids.