Saprophytic bacteria are extremely important to biology as decomposers. Dead animals and plants decay through the action of bacteria, which consume the organic parts and return them, eventually, to the soil. Without the aid of bacteria, energy would remain locked up in undecomposed dead matter accumulated since life began. Many bacterial decompositions are put to good use by man. He purifies sewage, produces cheese, cures tobacco, and prepares flax with their help. But, at the same time, other bacteria turn meat rotten, sour milk, and make butter rancid, while they obtain food energy for their own purposes. One of the most important contributions of bacteria is the part they play in making nitrogen available to plants.Nitrogen gas is abundant in the atmosphere, but animals and plants have no means of converting it into useable forms. However, certain kinds of bacteria that live in the soil or in the roots of legume plants such as clover and beans, are able to convert nitrogen gas to nitrites and nitrates, which are eventually built up by plants into proteins.
Parasitic bacteria cause numerous serious diseases of man, including tetanus, tuberculosis, diphtheria, and various kinds of blood poisoning. Before the use of antiseptics, these so-called pathogenic bacteria caused misery and death in colossal proportions, and rendered even the simplest surgery hazardous. Nowadays, surgery is carried out in germ-free rooms with apparatus previously steamheated under pressure to kill all forms of bacteria.