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Growth media (Culture media)
To identify and characterize bacteria you often (but not always) need to multiply them by culture and for this purpose liquid or solid media (substrates) are used. Media can be defined or undefined. In a defined medium we know of all chemical substances used, whereas an undefined (or semi-defined) medium may contain different types of commersial extracts from plant or animal tissue where it is not known exactly which nutrients are included. Furthermore the composition of these extracts may vary from batch to batch. A minimal medium contains only those ingredients that are absolutely essential for a particular bacterium to grow. An enriched medium contains many substances (e.g., amino acids and nucleotides, as well as vitamins), which allows the actual bacterium to grow faster.
Extracts, which are used in growth media
Liquid media are sterile nutrient solutions that are tailored to the bacteria you want to grow. An enrichment medium is a fluid medium, which is specially adapted to a particular bacterial species or group of closely related bacterial species. Enrichment media is used in order to be sure that a certain bacteria will grow, even if it is present in a small amounts only, in the original sample. Selective media can be liquid or solid and they contain substances (e.g. antibiotics or other bacterial inhibitors), which will prevent some bacteria to grow. Selective media are used to increase the chances to be able to cultivate a slow-growing bacteria, which otherwise would easily be outcompeted.
To get a solid substrates you mix a gel-forming substance with a liquid medium. Most commonly agar, but you can also use agarose, which is a more pure substance. In some special cultivations (e.g. of mycobacteria) egg yolk is used as gelling agent.