Ans. D: All of the above Culture media may be classified into several categories depending on their composition or use. A chemically-defined (synthetic) medium is one in which the exact chemical composition is known. A complex (undefined) medium is one in which the exact chemical constitution of the medium is not known. Defined media are usually composed of pure biochemicals off the shelf; complex media usually contain complex materials of biological origin such as blood or milk or yeast extract or beef extract, the exact chemical composition of which is obviously undetermined. A defined medium is a minimal medium if it provides. only the exact nutrients (including any growth factors) needed by the organism for growth. A selective medium is one which has a component(s) added to it which will inhibit or prevent the growth of ceain types or species of bacteria and/or promote the growth of desired species. A culture medium may also be a differential medium if allows to distinguish between different types of bacteria based on some observable trait in their pattern of growth on the medium. Thus a selective, differential medium for the isolation of Staphylococcus aureus, contains a very high concentration of salt (which the staph will tolerate) that inhibits most other bacteria, mannitol as a source of fermentable sugar, and a pH indicator dye. From clinical specimens, only staph will grow. S. aureus is differentiated from S. epidermidis (a nonpathogenic component of the normal flora) on the basis of its ability to ferment mannitol. Mannitol-fermenting colonies (S. aureus) produce acid which reacts with the indicator dye forming a colored halo around the colonies; mannitol non-fermenters (S. epidermidis) use other non-fermentative substrates in the medium for growth and do not form a halo around their colonies. MacConkey's medium shows up lactose fermenters as pink colonies An enrichment medium contains some component that permits the growth of specific types or species of bacteria, usually because they alone can utilize the component from their environment. For example, Alkaline peptone water and Monsour's taurocholate Tellurite peptone water for vibrio cholera and Selenite F broth for dysentery bacilli.