Bacteria are found everywhere - in air, water, soil, animals, people and food. They are very small living things. It takes one million to cover a pinhead, so you need to look at them through a microscope.

Some bacteria are useful to us.

• Bacteria can be used to make food like cheese and yoghurt
• Bacteria can be used to make medicines.
• Bacteria help us to digest our food.

Some bacteria are harmful and spoil our food. They make it go rotten and taste and smell bad. This food is not fit to eat. Sometimes these bacteria spoil the food and can make us ill when we eat it. When bacteria in food make us ill, we call it food poisoning.

Bacteria are so small that they can normally only be seen through a powerful microscope by experts working in a laboratory. Because bacteria are so small, the experts grow them in the laboratory until there are lots of them. These groups of bacteria are called colonies. Their shape and colour can help the experts decide what type of bacteria they are.

If experts know what type of bacteria they are, it helps them to find out where the food poisoning started. It also helps the doctors know how to treat the ill people and to choose the right medicine.

Bacteria which make us ill need to grow in the food so that there are enough of them to cause food poisoning. To help them grow they need:

food water warmth time.

Food Bacteria choose to grow in meat, chicken, fish, cooked rice and pasta, milk, cheese and eggs. They also like to grow in meat pies, sandwiches, gravy and salads because there is lots for them to eat. They can grow in both raw and cooked food.

Water Most food has enough water in it for the bacteria to grow. Bacteria cannot grow in dry foods such as milk powder and soup mixes. But once water is added to the dry food, the bacteria can start to grow again.

Warmth Some bacteria can grow in the cold and others in hot places, but most like warm temperatures between 5¼C and 63¼C. This temperature is called the growth or 'danger' zone because bacteria like it best and grow quickly when they are warm. But, cooking food at very hot temperatures kills most bacteria.

Time If they are warm, and have plenty of food and water, bacteria will grow by dividing into two every 20 minutes. After 6 hours, one bacteria could become 131,072 bacteria.