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(iii) Semi-solid agar media The semi-solid media are prepared by adding 0.5% of agar in the media. This type of media may be selective as it promotes the growth of one organism and may retard the growth of the other organism. (C)On the basis of their application(s) and functions, the culture media are classified into the following types (i) Selective media The selective media favours the growth of a particular type of microorganism. They provide nutrients that increase the growth of a particular type of microorganism and suppresses all other microorganisms that are present in the culture. This medium is useful for isolating a specific microorganism from a mixed population. Thus, for the isolation of cellulose-digesting bacteria, a medium containing only cellulose as a carbon and energy source is used. Endo agar, eosin methylene blue agar, and MacConkey agar media are used for the growth of E. coli and similar bacteria. Certain media that contain bile salts or dyes, such as basic fuchsin and crystal violet are used to suppress the growth of Gram-positive bacteria but encourage the growth of Gram-negative bacteria. (ii) Differential media Differential media are used to distinguish different groups of microorganisms based on the appearance on the medium as well as their biological characteristics, which permit the identification of microorganisms. For example, if a mixture of bacteria is inoculated on to a blood-containing agar medium (blood agar), some of the bacteria may hemolyze (destroy) the red blood cells, and others do not. Thus, this medium distinguishes between hemolytic and non- hemolytic bacteria. Therefore, blood agar medium is both a differential medium and an enriched one. MacConkey agar is also kept under both differential and selective media. Certain indicators are present in this medium, which may allow such differentiation. For example, the MacConkey’s agar containing lactose and a dye (which changes colour when pH drops). The microorganisms that ferment lactose, 152 BiotechnologyXII Reprint 2025-26