Why are not all bacteria harmful to humans?

The average person gets the impression of bacteria as extremely harmful creatures, the cause of disease and spoilage of food, but this idea is incomplete. Thanks to the bacteria that inhabit the intestines and supply enzymes, a person is able to digest more varied food. The same bacteria play a role in providing a person with vitamins and training immunity. Those bacteria that do not live inside us are also useful: many are used in biotechnological enterprises to obtain substances, including medicinal ones, including pigments, proteins, lipids.