They are microscopic organisms which are found everywhere and are invisible to the human eye, although they can be seen through a microscope.
Not all bacteria are harmful – in fact the majority are harmless, many are beneficial, for example those naturally present in milk which are responsible for the production of yogurt and cheese. We also require bacteria in our bodies to help us digest food.
Then there are some harmful bacteria which cause food spoilage and then the pathogens which cause illness like food poisoning.
Bacteria need certain conditions to multiply and grow – these are
WARMTH
FOOD
MOISTURE
TIME