There are 10 times MORE bacterial cells in your body than human cells! (Thank goodness, prokaryote cells like bacteria are much smaller than eukaryote cells like our human cells!)
All the bacterial cells in your body would fill a HALF GALLON JUG!
Source: Scientific American, 2007
Your large intestine is not just made up of cells. It is also an ecosystem, home to trillions of bacteria known as the "gut flora" (Figure below). But don't worry, most of these bacteria are helpful. Friendly bacteria live mostly in the large intestine and part of the small intestine. The acidic environment of the stomach does not allow bacterial growth.
Gut bacteria have several roles in the body. For example, intestinal bacteria:
Your intestines are home to trillions of bacteria.
A wide range of friendly bacteria live in the gut. Bacteria begin to populate the human digestive system right after birth. Gut bacteria include Lactobacillus, the bacteria commonly used in probiotic foods such as yogurt, and E. coli bacteria. About a third of all bacteria in the gut are members of the Bacteroides species. Bacteroides are key in helping us digest plant food.
It is estimated that 100 trillion bacteria live in the gut. This is more than the human cells that make up you. It has also been estimated that there are more bacteria in your mouth than people on the planet. There are over 7 billion people on the planet.
The bacteria in your digestive system are from anywhere between 300 and 1000 species. As these bacteria are helpful, your body does not attack them. They actually appear to the body's immune system as cells of the digestive system, not foreign invaders. The bacteria actually cover themselves with sugar molecules removed from the actual cells of the digestive system. This disguises the bacteria and protects them from the immune system.
As the bacteria that live in the human gut are beneficial to us, and as the bacteria enjoy a safe environment to live, the relationship that we have with these tiny organisms is described as mutualism, a type of symbiotic relationship.
Lastly, keep in mind the small size of bacteria. Together, all the bacteria in your gut may weight just about 2 pounds.
Source: ck12.org - reading and some of questions