Microorganisms, like all other living organisms require basic substances, for the sustenance of life. These substances are termed as nutrients.

In the laboratory conditions, the nutrients constitute a culture medium and the growth of microbes in it is termed as culture. Although all the microorganisms have the same basic nutritional requirements, there is diversity as to the use of organic and inorganic substances which comprise the culture medium.

The culture media vary in form and composition depending on the species to be cultured. Some media contain solutions of inorganic salts and may be supplemented with one or two organic compounds while some other media contain complex ingredients such as extracts of plant and animal tissues. Besides these ingredients, agar is used to prepare solid or semi solid media. Agar, normally liquefies on heating to 96° C and solidifies into a jelly on cooling to 40-45° C, and hence used to solidify broth or liquid media.

Media are classified variously using different criteria such as chemical composition, physical state or utility purpose. On the basis of their composition, there are three main types of culture media: i) natural or empirical culture media, ii) semi-synthetic type and iii) synthetic or chemically defined culture media.

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The exact chemical composition of natural medium is not known, (e.g.: milk, urine, malt extract, vegetable juices, potato extracts etc.).

The media whose chemical composition is partially known are called semi-synthetic media. Any medium that contains agar is a semi-synthetic one. Synthetic or chemically defined culture medium is composed of nutrients of known composition (e.g.: Czapek medium).

In microbiology, it is essential to have pure cultures of the microorganisms. It signifies a collection of cells that have arisen from a single cell.

A culture containing only one species of microbes is called pure culture. Microorganisms occur in mixed populations in nature it is possible to isolate a given type of organism by using certain defined technique. One of the first such techniques was enrichment culture technique.

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This method enables the growth of a given type of organism and this is achieved by providing suitable growth conditions. For example, we desire to isolate a sulphur oxidizing bacterium from the soil. This can be done by providing sulphur in the growth medium, which will enable only this group i.e. sulphur bacterium, to multiply.