Growth medium

A growth medium is an object in which microorganisms or cells can experience growth. There are different sorts of media for growing different sorts of cells. The biggest difference in growth media are between those used for cell culture uses specific cell types derived from plants or animals, and microbiological culture used for growing microorganisms, usually bacteria or yeast. These differences arise due to the fact that cells derived from whole organisms and grown in culture are often incapable of growth without the provision of certain requirements, such as hormones or growth factors which usually occur in vivo. In the case of animal cells these requirements are often provided by the addition of blood serum to the medium. These media are often red or pink due to the inclusion of pH indicators. In the case of microorganisms, there are no such limitations as they are often unicellular organisms. One other major difference is that animal cells in culture are often grown on a flat surface to which they attach, and the medium is provided in a liquid form, which covers the cells. Bacteria such as Escherichia coli may be grown on solid media or in liquid media, liquid nutrient medium is commonly called nutrient broth. The most common growth media for microorganisms are nutrient broth or Lysogeny broth (L-B medium). Bacteria grown in liquid cultures often form colloidal suspensions. When agar, a substance which sets into a gel, is added to a liquid medium it can be poured into petri dishes where it will solidify, called agar plates, and provide a solid medium on which microbes may be cultured.

Another important distinction between different growth media is that of a defined and an undefined medium. A defined medium will have known quantities of all ingredients, for microorganisms they consist of providing trace elements, any vitamins required by the microbe and especially a defined carbon source and nitrogen source (for example glucose or glycerol are often used as carbon sources, ammonium salts or nitrates as inorganic nitrogen sources), minimal media are defined media. Nutrient media are not defined media as they contain ingredients such as yeast extract which vary in composition depending on the source. This definition applies to cell culture media as well, where any medium containing, for example, animal blood serum is undefined, as the composition of the serum will vary from supplier to supplier and batch to batch.

A good example of a growth medium is the wort used to make beer. The wort contains all the nutrients required for yeast growth, and under anaerobic conditions alcohol is produced. When the fermentation process is complete the microbe is removed from the medium which is now beer and ready for consumption.

Some organisms require specialized environments, for example, since viruses are obligatory intracellular parasites, they require a growth medium composed of living cells. Often special media are required for microorganism and cell culture growth. Selective media are used to grow cells which possess a desired selectable trait. For example if a microorganism is resistant to a certain antibiotic, then that antibiotic can be added to the medium in order to prevent other cells, which do not possess the resistance, from growing.

Differential media are used to distinguish one microorganism type from another growing on the same media. While selective media are used to allow the growth of only select microoganisms, differential media allow the growth of multiple types, but result in distinguishing characteristics (such as the production of a brightly coloured or phosphorescent dye). Growth media can be both selective and differential.

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