Yeast grow on sugar and produce carbon dioxide.
Yeast can attack sugary foods such as fruit and jam and cause spoilage.
Yeast is used usefully to make alcohol. Bacteria Bacteria are single celled organisms which are widely found in soil and air. Some bacteria are useful but others cause food spoilage and are poisonous. Useful bacteria are used to make yoghurt and cheese. Moulds Moulds are a form of fungi.