The bacteria
The large intestine holds most of the body's bacteria, about 38 trillion of several hundred species, roughly as many as the body's own cells. Together they weigh about 200 grams. Most of them live without oxygen.
Escherichia coli, one of the bacteria of the human colon, electron micrograph magnified 10,000 timesWikimedia Commons, Eric Erbe and Christopher Pooley, USDA Agricultural Research Service, Public domain (E coli at 10000x, original.jpg)
They ferment the fibre that human enzymes cannot split and turn it into short fatty acids, which feed the cells of the colon lining. They also make vitamin K, some B vitamins and gas. Some 7 to 10 litres of gas enter the colon each day, most of it is absorbed, and about 0.6 litres is passed.
Faeces are about three quarters water. About a third of the solid part is dead bacteria, and the rest is fibre, fat, salts and shed cells. The brown colour comes from bile pigments that the bacteria change.
Sources
- Ron Sender, Shai Fuchs and Ron Milo, Revised estimates for the number of human and bacteria cells in the body, PLoS Biology 14 (2016): e1002533.
- Arthur Guyton and John Hall, Textbook of Medical Physiology, 13th ed. (2016), ch. 64 and 66.
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