It has been estimated that there may be as many as 100 trillion microbes that call us home. In other words, there are many types of bacteria and microbes that live in harmony on and in our bodies. Some times a throat culture will return from the lab with results saying “normal flora.” Basically that means the bacteria there are normal residents of that neighborhood and no harmful bacteria are growing there.

Bacteria normally live on our skin, in our saliva, in the conjunctiva and in our gastrointestinal tracts without causing harm. These organisms may “perform tasks that are helpful to the human host,” and generally they do not cause disease but may be helpful in maintaining health and are therefore classified as “normal flora.”

There is a study called the Human Biome Project which has used DNA sequencing to identify microorganisms in humans. Disease may result if microbes grow in excessive numbers or in areas that they should not normally live. Generally these bacteria utilize available sugars or carbohydrates. Naturally the majority are found in our GI tract, primarily in the large intestine. Bacteria may account for 1 to 3 percent of our “body mass,” and there may be as many as 500 to 1,000 different species of bacteria in our healthy intestinal tracts.

Microbes help to break down sugars that we can not normally digest and also help to maintain a healthy immune system. It is believed that exposure to microbes may be “associated with protection from immune mediated diseases such as inflammatory bowel disease.” But a person’s healthy microbiome may change during certain illnesses or with exposure to antibiotics. A mother’s milk may contain up to 600 species of bacteria that help a child develop beneficial bacteria and therefore also a healthy immune system.

One form of diarrhea caused by prior use of antibiotics has in some cases only responded to the use of “donor stool” or bacterial fecal transplants to help restore the normal flora to the intestinal tract.

Where as there are many different human blood types, there are at least three different “enterotypes” or “intestinal bacterial communities,” and, in the future, we may consider this sort of identification as being helpful in the diagnosis and treatment of certain diseases.

A single bacterium is smaller than the average human cell but 100 trillion of them together seem to really “pull their weight.”

David Keisler is a gastroenterologist and internist in Aiken.