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Bacteria The commonest living things are bacteria. They are too small to see without a microscope. Bacteria are all around us in their billions. They float in air and live on icy mountaintops, in the scalding water of hot springs, and on the bottom of the sea. Vast numbers of bacteria live in the human body, especially on the skin and in the intestines. Most are made harmless by the immune system. In fact, we could not survive without bacteria, as we could not digest most foods or produce vitamins. However, a few kinds of bacteria can cause infectious diseases, such as tetanus and cholera.

Salmonella bacteria (red) invading human cells.
Rod-shaped bacteria, seen through a microscope

Kinds of bacteria

There are around 10,000 known kinds of bacteria, but probably many more yet to be identified. Most measure about one to five microns (0.001 to 0.005 millimetres) across. They vary in form but there are three main shapes. These are: spheres or balls known as cocci, cylinders or rods, called bacilli, and corkscrew-like spirilli.

Inside a bacterium

A quarter of a million bacteria can fit on to a single pinhead.

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