Bacteria are tiny, one-celled animals that can only be seen through a microscope.
They are a kind of microbe and were the first living things on earth. They live in our stomachs and help us digest our food. In nature, bacteria break down dead plant and animal matter into new soil for other plants to grow in. Because bacteria break down plant and animal material to provide nutrients for all living things, they are absolutely critical to life. Although we need bacteria to live, some bacteria can be very harmful and can even cause death.