1. Synthetic Media: Whenever media are prepared to exact specifications, they are known as synthetic or chemically defined media. That is, they are composed of highly purified organic and inorganic substances of known amounts and molecular composition. The constituents of these media may include a variety of specific sugars, salts, amino acids, and vitamins. They can be formulated to grow microbes that call for specific nutritional requirements or growth factors. Since these media are standardized and highly reproducible, they are generally used in research labs. They are particularly useful when studying microbial metabolism. On the downside, however, these media can be very tedious to prepare and very expensive since they require the purchase of highly purified compounds.
2. Nonsynthetic
Media: When media contain one or more ingredients of
imprecise composition, they are
known as nonsynthetic
or complex media. The ingredients usually include complex
mixtures of organic and inorganic substances prepared from extracts. The extracts are made from yeasts or animal and plant
tissues. Beef extract, for example, is
prepared by boiling beef muscle and then concentrating this material into a
paste or dry powder. Present in extracts
are various amino acids, vitamins, minerals, and sugars of imprecise
quantities. Another possible constituent
of a complex medium is peptone. This substance is prepared from the partial
digestion of animal or plant proteins.
This results in a complex mixture of amino acids and polypeptides.
Overall,
a complex medium serves as a rich mixture of compounds that is likely to supply
the general nutritional requirements for a wide variety of microbes. Used routinely in microbiology labs, these
media are much cheaper and less time-consuming to prepare than synthetic media.