The human body has about 10^13 cells and 10 times more microorganisms, mostly bacteria, in the large intestine. The bacterial flora helps with digestion, prevents diseases, and plays a crucial role in the immune system. About 60% of stool mass is made up of bacteria, some of which can be pathogenic.
The average adult human body has about 1013 cells, about 1600 times more than the number of humans on Earth. It is estimated that for every cell in the human body there are about ten microorganisms, mostly bacteria, in the large intestine. This is possible because the volume of a typical bacterial cell can be more than 1000 times smaller than that of one of the body’s cells.
Along with the bacterial flora of the body, all are also colonized by fungi (mainly yeasts), protists, and archaea (mainly methanogens), although less is known about these due to their scarcity compared to bacteria. In the gross body flora domain, when found within the body, these are usually harmful parasites such as tapeworms. However, some humans may have thousands of gross body flora in their skin or hair, often in the form of mites. Scientists believe that there may be up to a million species of mites in the world, adapted to every imaginable environment, including the human body.
99% of the bacteria in the intestines come from 30-40 species. Commonly sighted genera include Bacteroides, Clostridium, Fusobacterium, Eubacterium, Ruminococcus, Peptococcus, Peptostreptococcus and Bifidobacterium. The bacterial flora of the body tends to have a symbiotic relationship with its host. Bacteria help digest complex carbohydrates that would otherwise be indigestible, promote intestinal cell growth, repress pathogenic microbes, prevent allergies, inflammatory bowel disease, and play a crucial role in the immune system. The body flora and the body it occupies have been co-evolving for tens of millions of years.
About 60% of the stool mass is made up of bacteria. Some bacteria found in feces can be pathogenic to the person they come from and to the people around them. This is one reason for modern hygiene. Bacteria shed in feces include E. coli and Vibrio cholerae, the bacterium that causes cholera.
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