Media Preparation for Culture

MEDIA PREPARATION

Microrganisms need nutrients, a source of energy and certain environmental conditions in order to grow and reproduce. In the environment, microbes have adapted to the habitats most suitable for their needs, in the laboratory, however, these requirements must be met by a culture medium. This is basically an aqueous solution to which all the necessary nutrients have been added. Depending on the type and combination of nutrients, different categories of media can be made.

Categories

Complex media are rich in nutrients, they contain water soluble extracts of plant or animal tissue (e.g., enzymatically digested animal proteins such as peptone and tryptone). Usually a sugar, often glucose is added to serve as the main carbon and energy source. The combination of extracts and sugar creates a medium which is rich in minerals and organic nutrients, but since the exact composition is unknown, the medium is called complex.

Defined media are media composed of pure ingredients in carefully measured concentrations dissolved in double distilled water i.e., the exact chemical composition of the medium is known. Typically, they contain a simple sugar as the carbon and energy source, an inorganic nitrogen source, various mineral salts and if necessary growth factors (purified amino acids, vitamins, purines and pyrimidines).

Selective/differential media are media based on either of the two categories above supplemented with growth-promoting or growth-inhibiting additives. The additives may be species- or organism-selective (e.g., a specific substrate, or an inhibitor such as cyclohexamide (artidione) which inhibits all eucaryotic growth and is typically used to prevent fungal growth in mixed cultures).

The mixture of necessary nutrients can be used as a liquid medium, or a solidifying agent can be added. "Agar agar" is a natural polysaccharide produced by marine algae and is the most commonly used solidifying agent added to media (end concentration usually 1.5 % w/v). If hydrolysis of the agar is suspected, a silica gel is used as a replacement solidifying agent. 

Media             Purpose

Complex         Grow most heterotrophic organisms

Defined          Grow Specific Heterotrophs and are often mandatory for chemoautotrophs, photoautotrophs and for microbilogical assays

Selective        Suppress unwanted microbes, or encourage desired microbes

Differential    Distinguish colonies of specific microbes from others

Enrichment    similar to selective media but designed to increase the numbers of desired microorganisms to a detectable         level without stimulating the rest of the bacterial population

Reducing       Growth of obligate anaerobes

Source: Media Preparation.Retrieved August 13,2011 from http://www.sigmaaldrich.com/analytical-chromatography/microbiology/microbiology-products.html?TablePage=8657623