Microorganisms 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. A microbiological culture, or microbial culture, is a method of multiplying microbial organisms by letting them reproduce in predetermined culture media under controlled laboratory conditions. Microbial cultures are used to determine the type of organism, its abundance in the sample being tested, or both.
Basic Constituents of Culture Media
Culture medium has the following constituents:
These are enzymatic digest of meat, casein, soyameal, gelatin and other protein sources.
- Meat extracts
These are mixture of proteins, carbohydrates, lipids and other biochemical constituents obtained mostly from beef tissue.
All bacteria require some amount of water. Water is the source of hydrogen and oxygen.
Agar is a complex carbohydrate extracted from marine algae that solidifies below temperatures of 45⁰C. It is not a nutritional component. It is added to nutrient solution to form fairly stiff gels.
Types of Media
Culture media can be classified using different criteria like chemical composition, physical state and utility purpose.According to their use. Culture media may be categorized in to the following types:
Classification based on Physical Characteristics
- Solid Media
On solid media, bacteria produce distinct colony morphology and characteristics properties such as pigmentation, haemolysis, etc. which are useful in the identification of the bacteria. Each colony would constitute a homogenous culture, termed pure or axenic culture. A pure culture is normally considered to consist of clone cells i.e. a polulation derived from a single ancestral cell. Agar is used as a solid media for culturing microbes.Earlie gelatin solution was used but it could not be found useful as it easily melted at the incubation temperature.
- Semi-solid media
These are the media with gelatinous consistency and contain a small amount of agar or some other solidifying agent like corn meal.The semi solid are employed for specific purposes like cultivation of micoaerophilic bacteria and for studying the motile structures of microorganisms.
- Liquid media
Liquid media are used for culturing organisms from specimens like blood or water where large volumes have to be tested. It is most commonly used medium used for the isolation of bacteria.
Classification based on Constituents
- Defined/Synthetic media
These media are of known composition and concentration and are exclusively composed of pure chemical substances. These media are quite useful for nutritional and metabolic studies of various groups of microorganisms. For example photolithotrophic autotrophs such as cynobacteria and eukaryotic algae can be grown in synthetic media containing CO 2 as a carbon source, nitrate and ammonia as a nitrogen source, sulphur, phosphorus and various minerals.
- Selective media
It contains added components which will inhibit the growth of certain types or species of bacteria and promote the growth of desired species.
- Differential medium
A culture medium may also be a differential medium if it allows distinguishing among different types of bacteria on the basis of differences in their pattern of growth on the medium
Classification based on Use or Application
Many special purpose media are needed to facilitate recognition, enumeration, and isolation of certain types of bacteria. To meet these needs, numerous media are available.These include basal media, enriched media, selective/enrichment media, indicator/differential media, transport media and holding media.
- Simple or basal media
This is the simplest routine laboratory medium. Basal media are basically simple media that supports most non fastidious bacteria. Peptone water, nutrient broth and nutrient agar are considered basal media.
- Enriched media
This is basal medium containing natural proteins like blood, serum, egg and sugars like glucose.
- Selective and enrichment media
These media favour the growth of a particular bacterium by inhibiting the growth of undesired bacteria and allowing growth of desirable bacteria. While selective media are agar based, enrichment media are liquid in consistency. Both these media serve the same purpose. Any agar media can be made selective by addition of certain inhibitory agents that don’t affect the pathogen of interest. Various approaches to make a medium selective include addition of antibiotics, dyes, chemicals, alteration of pH or a combination of these.
- Differential media or Indicator media
Certain media are designed in such a way that different bacteria can be recognized on the basis of their colony colour. Various approaches include incorporation of dyes, metabolic substrates etc, so that those bacteria that utilize them appear as differently coloured colonies. Such media are called differential media or indicator media. Exmples:
- MacConkey’s agar
- CLED agar
- TCBS agar
XLD agar etc