Growth media (Culture media)
To isolate, 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 all components used are known, 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. A riched medium contains many substances (e.g., serum, amino acids and nucleotides, as well as vitamins), which allows the actual bacterium to grow faster and to higher density (form bigger colonies).
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. In some cultivation for example Salmonella analysis pre-enrichment is used to stimulate the growth of all bacteria in the sample. Pre-enrichment is a non-selective enrichment and in Salmonella analysis it is used before the selective enrichment. Selective media can be liquid, semisolid 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 competed out.
To get a solid substrates you mix a gel-forming substance with a liquid medium and heat the mixture until the substance dissolves. Then the solution is cooled until it solidifies. Agar is the most common gel-forming substance, but agarose can also be used, which is purified from agar. In some special cultivations (e.g. of mycobacteria) egg yolk is used as gelling agent. Semisolid media are produced by using a smaller amount of gelifying agent.
In order to facilitate identification of bacteria, a pH indicator is often added to the growth medium to see if the bacterium is producing metabilites the change the pH of the medium. When adding a pH indicator to the solid medium and the bacterium in question produces a metabolite that changes the pH of the medium resulting in a colour change, it may seem as if colonies also change colour. However, it is usually only the medium that changes color, but colonies are often translucent or semi-transparent and therefore it may seem as if they change colour too.
There are other more specific additives, which are also used for identification and that lead to a chemical reaction in the bacterium or on the bacterial surface, where one can really say that the colony has changed colour. For instance tellurite in Baird-Parker agar and the X-glucoside in Brilliance Listeria agar medium.