Our bodies are full of bacteria. Believe it or not, that's not necessarily a bad thing. In fact, there are more bacteria in our intestines than there are cells in the entire body! Thankfully, some bacteria is actually good for you.
Some of the microbes in our gut are there for a valid reason. For example, there's a bacterium called thetaiotamicron in the intestines that lives off the food we eat. In return, it helps us digest certain foods by breaking them down into sugars and vitamins. This particular microbe may also regulate certain intestinal genes and help create new blood vessels. Finally, it helps produce a chemical that kills disease-causing bacteria.
Consider this. Scientists created mutant mice without any germs in their intestines. These mice ended up getting sick a lot. In order to gain weight, they needed to eat much more than normal mice with intestinal bacteria. The truth is, we probably couldn't live without some bacteria in our system.