The subject of this film is bacteria. They are constantly there. We cannot survive without them, but they can also be a danger for us. And there are lots of them: ten to the power 14 bacteria live on and in our bodies, making up about 2 kg of our bodyweight. Normally we don't notice much of our busy cohabitants' goings-on. There are 500 different types of bacteria in our mouths alone. But if bacteria like streptococcus mutans reproduce there unhindered, we can get tooth decay. Bacteria are single-cell organisms. But unlike other cells, they do not have a nucleus. Their DNA is contained in the cell plasma. They reproduce through division.