There are innumerable microorganisms and therefore, it is most essential to prepare a medium to study their morphological, biochemical, diagnostic and other types of characteristics of basic and applied nature. Generally any liquid or solid preparation made specifically for the growth, storage or transportation of micro-organisms or another types of cells is referred to as a medium by students of microbiology.
The growth media may be used for initiation of a culture or a subculture for enrichment, identification or diagnostic tests. There are special media for the growth and maintenance of mammalian cells, plant cells, protoplasts and other types of cells in tissue culture.
There are many chemoautotrophic bacteria which can be grown in simple aqueous media containing mainly or only a mixture of inorganic salts. Nutritionally simple (undemanding) heterotrophs can be cultured on a large range of culture media as nutrient broth and peptone water. But nutritionally ‘fastidious’ heterotrophs can be grown only on an enriched medium.
Strict anaerobes are often cultured on pre-reduced media that means media poised at or below a particular redox potential (oxidation-reduction potential represented by Eh). The poising agents used in the media are ascorbic acid, cysteine hydrochloride and thioglycolate.
Poising is analogous to buffering in the context of pH, i.e., the Eh produced depends on the ratio oxidized: reduced poising agent. The stability of Eh is influenced by the absolute amount of poising agent in the medium. The solid media are prepared from liquid media that have been solidified or gelled with an agent such as agar or gelatin. The other gelling agents are alginate, gelrite and pluronic polyol F 127.
The medium which without supplement can support the growth of nutritionally undemanding species of bacteria is often referred to as basal medium. The medium in which all the ingredients including trace substances are known quantitatively is called defined medium.
The solid medium on which different types of organisms can be distinguished by their different forms of growth is termed differential medium. A medium which contains substances that encourage growth of the required organisms and/or inhibit the growth of other types of organisms is called enrichment medium, e.g., selenite broth and tetrathionate broth.
The medium used for the initial growth and the subsequent storage under conditions of minimal growth is known as maintenance medium. The maintenance medium for microorganisms is used to prepare a culture of the given organisms that can be stored either at ambient temperature or under refrigeration.
Its subcultures are required at intervals ranging from 1 to 12 months. The constituents of the maintenance medium are often of minimum consistence with the need to maintain viability of the organisms that is being cultured, e.g., cooked meat medium and Dorset’s egg.
The common media for cultivation of bacteria, often required in a laboratory are:
1. Chemically defined media:
Used to grow chemoautotrophs and photoautotrophs and for microbiological assays.
2. Complex media:
Used to grow most chemoheterotrophic organisms.
3. Anaerobic growth media:
Used to grow obligate anaerobes.
4. Selective media:
Used to suppress unwanted microbes and encourage desired microorganisms.
5. Differential media:
These are used to distinguish colonies of desired microorganisms from others.
6. Enrichment media:
These are similar to selective media but are designed to increase the number of desired microorganisms to a detectable level.