Give one example in each case of the usefulness of bacteria in (a) a natural environment,
(b) an industrial process.
(a) In a natural environment bacteria bring about decay. In this way they stop accumulation of dead material and make available necessary compounds such as nitrates.
(b) Bacteria are used in industrial processes to make certain kinds of food (e.g. cheese, yoghurt), to make single-cell protein, to purify sewage and in the manufacture of certain chemicals (eg lactic acid).