Many people think of bacteria as harmful because they cause food poisoning and diseases such as tuberculosis and anthrax. Most bacteria are not harmful, however. Many are incredibly important. As decomposers, they break down dead and rotting plants and animals. They help make soil, and they help plants grow. The bacteria that live all over animals--including humans—help break down food and produce vitamins. Bacteria are also used to clean up pollution.
Bacteria are used a lot in the making of yoghurt and cheese. Yoghurt is just fermented milk, made by adding good bacteria that you can find in the ‘natural’ yoghurts in the supermarket, and heating for several hours. Cheese can be made from the milk of lots of animals, but they all need bacteria to turn the milk into cheese. The different types of bacteria give the different cheeses their own taste and texture (if they are runny or hard, for example). Both yoghurt and cheese come in a huge amount of flavors. Yeast is the bacteria that forms yoghurt and cheese.