Bacteria are single-celled microorganisms (meaning have one cell each and they are too small to see, it would take one million of them to cover a pinhead!). They live in water, soil, food, trees, and virtually every animal on earth, including us. They are usually dangerous because they cause infections inside the body.
By mitobacteria, do you maybe mean mycobacteria? If so, these are a special type of bacteria which is a pathogen – so it causes serious and deadly diseases like Tuberculosis. Luckily though, antibiotics are usually pretty good at fighting this bacteria off.
You can also get the “friendly” bacteria, which are called probiotic bacteria. These are the ones in yoghurt and those Yakult drinks. They are simliar to the bacteria in our gut, so eating them is supposed to be pretty healthy for us!