Introductory Microbiology

GROWTH MEDIA AND ITS CLASSIFICATION

Growth media
Growth media are the nutrient solutions used to grow microorganisms in the laboratory. Because laboratory culture is requires for the detailed study of a microorganism, careful attention must be paid to both selection and preparation of media for successful culture to takes place.

Classes of culture media
Two broad classes of culture media are used in microbiology; chemically defined and undefined (complex). Chemically defined media are prepared by adding precise amounts of highly purified inorganic or organic chemicals to distilled water. Therefore, the exact chemical composition of a defined medium is known. Of paramount importance in any culture medium is the carbon source, since all cells need large amounts of carbon to make new cell material. In a simple defined medium, a single carbon source is present.
The nature of carbon source and its concentration depends on the organism to be cultured. Chemically defined media are needed for the cultivation of autotrophs and also useful for defining the nutritional requirements of heterotrophs. However, for the routine cultivation of heterotrophs, chemically defined media are not generally used. Instead, certain complex raw materials such as peptones, meat extract, and yeast extract are used. And the resulting media support the growth of a wide variety of heterotrophic bacteria.
Agar is included as a non-nutritive solidifying agent when a solid medium is desired. A description of these raw materials is given in table. Examples of relatively simple liquid and solid media that support the growth of many common heterotrophs are nutrient broth and nutrients agar (Table). The addition of yeast extract to each of these formulas improves the nutrient quality.
Since, yeast extract contains several of the B vitamins and other growth promoting substances. Other complex supplements such as bovine rumen fluid, animal blood, blood serum or extract of plant and animal tissues may be required for the cultivation of fastidious heterotrophs.

Sl.No	Raw material	Characteristic	Nutritive value
1.	Beef extract	An aqueous extract of lean beef tissue concentrated a paste	Carbohydrates, organic nitrogen commands, water soluble, vitamins and salts.
2	Peptone	The product resting from the digestion of proteinaceous materials eg: meat, casein and gelatin	Contains organic nitrogen vitamins, carbon hydrates.
3.	Agar	Accomplish carbohydrates contain from marine algae	Used as a solidifying agent, agar dissolved in aqueous solutions, gels when temperature is reduced below 450 C
4	Yeast extract	An aqueous extract of yeast cells, available as a powder	A very rich source of the B vitamins; also contains organic nitrogen and carbon components.

Table: Characteristics of several complex materials used as ingredients of media

Nutrient broth	Nutrient agar
Beef extract -3.0g	Beef extent -3.0g
Peptone -5.0g	Peptone -5.0g
Water -1000ml	Agar -15.0g
	Water -1000ml

Table: Composition of nutrient broth and nutrient agar.
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