The differentiation of species in bacteria is based chiefly upon their properties, there being too little difference in form and size to distinguish species.

The introduction of methods of culture was followed by an immediate advance of our knowledge concerning them.
This method consists in the use of fluid and solid substances which contain the necessary salts and other ingredients for their food, and in or on which they are planted.

The use of a solid or gelatinous medium for growth has greatly facilitated the separation of single species from a mixture of bacteria; a culture fluid containing sufficient gelatine to render it solid when cooled is sown with the bacteria to be tested by placing in it while warm and fluid, a small portion of material containing the bacteria, and after being thoroughly mixed the fluid is poured on a glass plate and allowed to cool.

The bacteria are in this way separated, and each by its growth forms a single colony which can be further tested.

It is self-evident that all culture material must be sterilized by heat before using, and in the manipulations care must be exercised to avoid contamination from the air.