(i) Nutrient media : It is a source of amino acids and nitrogen (e.g., beef, yeast extract). This is an undefined medium because the amino acid source contains a variety of compounds with the exact composition being unknown. Nutrient media contain all the elements that most bacteria need for growth and are non-selective, so they are used for the general cultivation and maintenance of bacteria kept in laboratory culture collections. An undefined medium (also known as a basal or complex medium) is a medium that contains :
• a carbon source such as glucose for bacterial growth
• various salts needed for bacterial growth
Defined media (also known as chemically defined media or synthetic media)
• all the chemicals used are known
• does not contain any yeast, animal or plant tissue Differential medium
• some sort of indicator, typically a dye, is added, that allows for the differentiation of particular chemical reactions occurring during growth.
(ii) Sterlization of instruments : The metallic instruments (e.g., forceps, scalpels,needles,spatula etc.) are flame sterilized i.e., dipping them in 75% ethanol followed by flaming and cooling. It is called incineration.
Sterlization of transfer area : Floor and walls of culture room are first washed with detergent and then 2% sodium hypochlorite or 95% ethanol. Larger surface area is sterilized by exposure to UV light.