It is only in modern times that bacteria have themselves become known, though the effects of their living have been experienced probably always and recognized more or less since ancient times. Bacteria abound everywhere in air, water, soil. Wherever they are they are doing something'. They break down the substances they live on. Human beings are the host upon which disease-bacteria subsist.

The work of non-disease bacteria in nature is real and of unique value, as they have a necessary part in life, not shared by other organisms or done by other agencies. Bacteria are living organisms, so require the conditions necessary to their life in order to live; nitrogenous food, moisture and usually air.

Bacteria utilize the nitrogenous substances that are the waste-products of the living of higher forms of physical life and that are not directly usable by vegetation. These they break up into simpler nitrogen compounds that plants can use. They also free some nitrogen to the atmosphere. But for such bacterial activity, nitrogen in the forms needed everywhere in life would not be available.

Until the microscope was invented bacteria were not seen. They are such minute organisms that they have to be enlarged (some, 1200 times) to be visible to the human eye. Though not wholly like non-microscopic plants, they have more plant characteristics than animal. The use of bacteria to both plants and animals is their life-activity. This makes the products of living of animals of use to plants that in turn themselves make food for animals and humankind.

Bacteria in drop of milk; multi-plication in 12 hrs. (After Russell).