Decomposition - decomposition is something we humans benefit a lot from in agriculture.
Immunity - reffering to the help that bacteria gives us in the immune syste.
Pollution control - reffering to the fact that some bacteria decompose man - made pollutants such as DDT, petroleum, etc.
Symbiosis - reffering to the help some symbiotic bacteria pose to humans by decomposing vitamins B and K.
Hope it helped,
BioTeacher101