Bacteria

Bacteria are single cells with no nucleus (prokaryotes) and a cell wall around them. They come in three main shapes - round, rod-shaped and spiral - and live almost everywhere, even in hot springs, salt lakes and polar ice, where most living things can't survive. Most are harmless and many are useful: the bacteria in our gut help digest food and keep harmful ones from settling, and others turn milk into yoghurt, clean up oil spills and make medicines. A few cause disease, and some make poisons, such as the one that can grow in badly canned food; boiling destroys it, which is why such food is boiled before eating.