Re: Mutualistic bacteria within the body

Hi Charlotte -

A variety of bacterial species provide us with some benefit. When you say "within the body," I should caution that this mostly means bacteria living within the intestines, upper respiratory tract, and also on the skin or the vaginal epithelium. A bug, even a "good bug," found at any other location is not a good thing. Though we transiently have bacteria in our blood after brushing our teeth or after cutting the skin, blood and other bodily sites are intended to remain sterile.

Beneficial bacteria help us in a number of ways:

Vitamin K
Vitamin K helps our blood clot. The liver uses it to make modifications to certain clotting proteins it releases into the blood. We obtain vitamin K from bacterial species living in the intestine. Some of these species include members of the genus Bacteroides, a "Gram negative" anaerobe and Escherichia, parent genus of Escherichia coli, also a Gram negative organism, but one that is "facultative." Gram negative refers to a bacterial cell that has two cell membranes, one within the other - it's a common way to classify bacteria; anaerobes are bugs that cannot live in the present of oxygen; facultative anaerobes are those that can live in the presence or absence of oxygen.

The bacteria use vitmain K to transport electrons for energy processes within the cell. The liver uses it for a similar process, though the electrons are directed towards making the modifications in the clotting factors. We obtain the vitamin from bugs that lyse, or die within our intestine.

B Vitamins
Bacterial and some yeast species are also responsible for producing B vitamins (esp. vitamin B12). Though these organisms are not commonly found within the human intestine, many are found in the stomachs of ruminant animals (cows, goats, etc..).

Colonization resistance:
The bacteria living in our intestines also protect us from colonization with pathogens, or disease-causing bacteria. By having an intestine filled with "good bugs," it makes it more difficult for "bad bacteria" to gain a foothold in the body where they can cause disease. The species involved in this effect may number in the thousands! Some common genera include Lactobacillus, nonpathogenic members of the streptococci, Bacteroides, Bifidobacterium, and many others.

Help with digestion:
Though humans don't take full advantage of their intestinal flora, many animals do. The ruminants I mentioned previously have evolved a four-chambered stomach to help them get the most out of the grasses they eat. Bacteria living in the first two chambers help digest the long polysaccharides and otherwise "indigestible" substances in the food. The cow regurgitates the partially digested material, chews is some more ("chewing the cud"), then forcefully swallows it so it lands into the latter compartments of the stomach where it can complete its digestion. If you raise a ruminant in a germ-free environment, and feed it sterile grasses, the animal will starve because it can't digest its food on its own!

Rodents (mice & rats) take another approach by letting bacteria digest all the foodstuffs that make it from the small intestine into the large intestine. The first part of the large intestine, called the cecum, is greatly enlarged to facilitate the growth of bacteria which help digest the remaining foodstuffs. The bugs get something to eat, and the host gets some added nutrition from the degredation of complex carbohydrates, and also from bacteria that die within the intestine. Koalas are another animal with a greatly enlarged cecum filled with organisms that help break down complex carbohydrates in eucalyptus leaves.

For more information online, you might try The Digital Learning Center for Microbial Ecology.

The book, The Microbes, Our Unseen Friends, by Harold Rossmore (1976) is also a very good resource. Though out of print, many public libraries have it in circulation.

Hope this helps..

-L. Bry
Supporter of microbially made products..

Lynn Bry, MD/PhD
Department of Clinical Pathology
Brigham & Women's Hospital
Harvard Medical School
Boston, MA 02115