Some bacteria are harmful to man. They cause illnesses ( such as tuberculosis ) which can possibly lead to death. But many bacteria are useful. we use bacteria to make cheese, wine and yogurt, and linen from flax. Some bacteria found in the human body help in the digestion of food. Bacteria also break down dead plants and animals into simple chemicals which can then be used again by living plants or animals> Finally, some bacteria change a chemical called nitrogen into a form which can be used by plants. Since plants need nitrogen to grow, without these bacteria plants could not grow.