Seminar One
Biology, culture and psychology
Bacteria

Burchell's zebras from different family groups pause to drink from the Mara River in Kenya before daring to race across the river.

Photography by Greg and Mary Beth Dimijian

You must be wondering why I haven 't mentioned our gut mutualists, the bacteria which make our large intestine their comfortable home and whose cells outnumber the cells of our body. The drawings belows are diagrammatic layouts of the gut of a sheep on the left and a zebra on the right.

Diagrammatic layouts of the gut of a sheep and a zebra

The small intestine, very long in the sheep, has been drawn as if neatly folded on a table. It is longer than the small intestine of the zebra, because the sheep is a ruminant ,carrying out most of its digestion in the multi-chambered stomach and then passing the food through the elongated small intestine, where most of it is absorbed. Bacteria in the stomach break down the cellulose cell walls of plant food and manufacture vitamins and amino acids, needed by the host. The bacteria in turn get a constant supply of food and comfortable, warm housing.

The zebra is not a ruminant, but, like us, passes its food on to the hindgut (the large intestine). It is there that most of the bacteria are located, and there that the food is digested. Hindgut fermentation is not as efficient a strategy as foregut fermentation (rumination), so the zebra must eat a large amount of grass in order to get enough nourishment, whereas the sheep feeds on a much smaller amount of higher-quality vegetation with more protein, and recycles it again and again in the stomach to extract the most out of it.

A 2010 study has identified a stunning number of microbial species in the human large colon, with 150 times more genes than are found in the human body. This enormous population of mostly bacteria are partly shared among humans, but varies to a large degree from human to human and from child to adult. These microbes contibute to energy harvest from food and use mostly fermentation (at low oxygen levels) to convert sugars to organic chemicals useful to diverse cells in host organs. A number of essential amino acids are also synthesized by the bacteria, along with some vitamins. – Nature 464, 59-65 (4 March 2010)