The medium is prepared by artificial methods, and is a mixed nutrient product specially used for the growth and reproduction of microorganisms. The pH of the medium is generally 7.2~7.6, and a small number of bacteria adjust the pH to be acidic or alkaline according to the growth requirements. Many bacteria decompose sugars to produce acid during metabolism, so buffers are often added to the medium to maintain a stable pH. The medium must be sterilized after preparation.
The culture medium is divided into the following categories according to its nutrient composition and purpose.
The basal medium contains the essential nutrients needed for the growth and reproduction of most bacteria. It is the basis for the preparation of special medium, and can also be used as a general medium. Such as nutrient broth, nutrient agar, protein, peptone water, etc.
If the special nutritional requirements of a certain bacterium are known, an enrichment medium that is suitable for the growth of this bacterium and not suitable for other bacteria can be formulated, and the bacteria group with the same nutritional requirements grows on this medium. It includes a general enrichment medium and a special enrichment medium. The former is a basal medium with appropriate growth factors or trace elements added to promote the growth and reproduction of some special bacteria, such as Streptococcus and Streptococcus pneumoniae. Or serum growth medium; the latter is also called selective enrichment medium, that is, in addition to the inherent nutrients, adding special inhibitors, is conducive to the growth and reproduction of target bacteria, such as alkaline peptone water for cholera Enrichment culture of Vibrio.
A chemical is added to the medium so that it inhibits the growth of some bacteria and favors the growth of others, thereby separating the latter from mixed specimens. This medium is called selective medium. For example, the agar for culturing enteropathogenic bacteria, in which bile salts can inhibit Gram-positive bacteria, and sodium citrate and yellow-green can inhibit Escherichia coli, thus making it easy to isolate pathogenic Salmonella and Shigella. If antibiotics are added to the medium, it can also play a selective role. In fact, the boundaries between some selective media and enrichment media are not very strict.
The media used to grow and differentiate between different bacterial species are called differential media. The ability of various bacteria to decompose carbohydrates and proteins and their metabolites are different, and specific substrates and indicators are added to the medium used for bacterial culture. Generally, no bacteriostatic agent is added, and the bacteria can be identified by observing the effect on the substrate after the bacteria grow in it. Such as the commonly used sugar fermentation tube, trisaccharide iron medium, eosin-methylene blue agar, etc. There are also some mediums that combine selection and identification functions, and play a certain identification role at the same time of selection, such as agar, in which the added substrate lactose and indicator neutral red play a role in identification.
The bacterial medium specially used for the isolation, cultivation and identification of anaerobic bacteria is called anaerobic medium. This medium is nutrient-rich, contains specific growth factors, has a low redox potential, and incorporates methylene blue as a redox indicator. Its central brain infusion, liver block, and meat residue contain unsaturated fatty acids, which can absorb oxygen in the medium; thioglycolate and cysteine are strong reducing agents; vitamin K hemin can promote certain Bacteroides growth. Commonly used meat culture medium thioglycolate broth, etc., and add Vaseline or liquid paraffin on the surface of the liquid medium to isolate the air.