(nutrient medium) A mixture of nutrients used to support the growth of microorganisms or of the cells, tissues, or organs of animals or plants. Such media may be solid, if mixed with a gelling agent (e.g. *agar
), or liquid. Whenever possible media used in experimental work should be chemically defined, i.e. the identity and concentration of their constituents should be known. Sometimes however a culture can only be grown when complex undefined additional substances, e.g. coconut milk, are added. A culture medium must contain all the macro- and micronutnents essential to growth. A carbohydrate source is necessary and is usually supplied as sucrose. Carbohydrates are necessary even for plant tissue cultures since photosynthesis either does not occur or is insufficient in culture. Plant tissue cultures also require growth substances, normally an auxin or cytokinin or very often both in a particular ratio. Other factors, e.g. gibberellins or vitamins, are often necessary. Ideal media for plant tissue culture are usually also suitable for the growth of microorganisms. Culture must therefore be carried out under sterile conditions to avoid contamination.