The human large intestine is a 5-foot long, dark, dank and twisting corridor whose repetitive contractions function to squeeze the last remaining drops of water and the final bits of nutrient from feces before expulsion from our bodies.
The average human body consists of about ten trillion(10,000,000,000,000) cells. In addition, each of us is host to one hundred trillion microbes. Thus our microbes outnumber our own cells ten to one. Some scientists joke that we are a minority in our own body. However, on average, human cells are about twenty times as large as a typical microbe. So, while we are a minority of cells, we are the majority in term of sheer bulk (phew!).
Those microbes help to break down and extract energy and nutrients from the food we eat. Especially, they help digest foods we would otherwise have to avoid. In this way the bugs contribute to our overall health.
Some of these tiny settlers are with us from birth, imparted from our mothers, while others gradually colonize our bodies as we grow. This microbial community is as diverse as any found in Earth's seas or soils, numbering up to 100 trillion individuals and representing more than 1,000 different species. This is the densest bacterial ecosystem known in nature.
Those bacteria make up most of the flora in the colon and 60% of the dry mass of feces. This fact makes feces an ideal source to test for gut flora for any tests and experiments.
Collectively, the microbes we host are known as our microflora. Nearly 80% of them - including all the ones important to the immune system - live in our intestines. The rest reside elsewhere in the digestive tract, on the skin, and in the lungs, as well as in the female reproductive tract. If a woman is a breastfeeding, the milk-producing areas of her breasts also contain microbes. Other parts of the body, such as the bones, heart, kidneys, and other external organs, are normally microbe free - unless they are infected.
