The three important ways in which bacteria are useful to us are:
1) Formation of curd: When a small amount of curd (known as starter) is added to milk, the bacteria known as Lactobacillus, convert the milk into curd.
2) Nitrogen fixation: The bacteria known as Rhizobium live in the root nodules of leguminous plants. These bacteria fix the atmospheric nitrogen and converts it into nitrates that can be used by the plants.
3) Retting of jute: Bacteria help in separating jute fibres from the rest of the tissues of the jute plant. These fibres are used in making various articles.
Two ways in which bacteria are harmful to us are:
1) Diseases: Bacteria cause many diseases in humans like cholera and typhoid.
2) Food spoilage: Bacteria spoil food. Examples: putrefaction of meat and rotting of fruits and vegetables.