A culture medium is a laboratory technique consisting of a gel or a solution containing the nutrients necessary to allow, under favorable conditions of pH and temperature, the growth of viruses, microorganisms, cells, plant tissues or even small plants. Depending on what you want to grow, the medium will require some or other conditions. Desiccants in the form of fine or granular powders are generally present before being prepared; Already prepared may be in the solid, semi-solid or liquid state. The ultimate objective of the crop is varied: antibiogram, identification, multiplication.
One of the most important systems for the identification of microorganisms is to observe their growth in artificial food substances prepared in the laboratory. The food material in which microorganisms grow is the Medium of Cultivation and the growth of microorganisms is the Cultivation. More than 10,000 different culture media have been prepared.
For bacteria to grow properly in an artificial culture medium it must meet a number of conditions such as: temperature, suitable humidity and oxygen pressure, as well as a correct degree of acidity or alkalinity. A culture medium must contain the necessary nutrients and growth factors and must be free of any contaminating microorganisms.
Viruses, for example, are forced intracellular parasites, so they need a medium that contains living cells.
Classification of culture media
According to their physical qualities
According to its origin
• natural: are preparations made from natural substances of animal or vegetable origin such as tissue extracts or infusions and whose chemical composition is not exactly known.
• Synthetics: means that contain a defined qualitative and quantitative chemical composition. They are used to obtain reproducible results.
• semi-synthetic: they are synthetics to which groweth factors are added in the form of a complex organic extract, such as yeast extract.
According to its formulation
• chemically defined: the exact amount of each of the compounds in the medium is known.
• complexes: made from natural extracts (yeast extract, blood, etc.); It is not known exactly what the composition of the medium is; However, it has the advantage that all or almost all the elements that a cell may require are already present.
According to its use
• general medium: medium in which all types of microorganisms grow, except those that need special conditions (eg CLED agar).
• selective medium: allows selection of the growth of a particular species or group (fungi, enteric bacteria, protozoa …).
• differential means: it allows the identification of one species with another, both in the same medium. It can be by its growth, its metabolism, its breathing, etc. (For example, McConkey medium).
• enrichment medium: it contains the nutrients necessary to support the growth of a wide variety of microorganisms, it is used for harvesting different types of microorganisms in the same medium.
• minimum medium: contains the minimum amount of nutrients possible that allows the growth of a species;
• means of transport: prepared to serve as temporary storage for specimens transported or transferred; Maintain their viability and concentration;
Generally, after analyzing a culture, a bacterial count is performed, in order to know the density of microbial population found in the medium.
An agar plate – an example of a bacterial growth medium*: Specifically, it is a streak plate; the orange lines and dots are formed by bacterial colonies.