Selective & Differential media

 Selective media

Media which favor the growth of particular microorganisms are selective media. They enhance the growth of a particular kind of microorganism and do not promote or even inhibit the growth of other microorganims that may be present. For example, bile salts or dyes like basic fuchsin and crystal violet can favor the growth of gram-negative bacteria by inhibiting the growth of gram-positive bacteria; the dyes have no effect on gram negative organisms.

Endo agar, eosin methylene blue agar, and MacConkey agar are three media widely used for the detection of E. coli and related bacteria in water supplies etc. These media contain dyes that suppress gram positive bacterial growth. MacConkey agar also contains bile salts which inhibits gram-positive bacteria

Bacteria can also may be selected by providing nutrients that they specifically can use. A medium containing only cellulose as a carbon and energy source is useful in the isolation of cellulose-digesting bacteria. Starch agar can be used to isolate amylase producing microorganisms. Thus, the possibilities for selection are many, and there are numerous special selective media in use.

 Differential media

Media that distinguish among different groups of microbes. They can be used for a preliminary identification of microorganisms based on their biological characteristics. Certain reagents or components may be added to medium to allow differentiation of bacteria based on their growth in such media. For example, if a mixture of bacteria is inoculated into a Blood agar plate, some bacteria may destroy red blood cells as they grow (hemolytic); some bacteria will not show hemolysis. Thus, they can be distinguished as hemolytic or non-hemolytic based on their growth in the same medium.

Blood agar is both a differential medium and an enriched one. It distinguishes between hemolytic and non-hemolytic bacteria. Hemolytic bacteria (e.g., many streptococci and staphylococci isolated from throats) produce clear zones around their colonies because of red blood cell destruction.

MacConkey agar is both differential and selective. Since it contains lactose and neutral red dye, lactose-fermenting colonies appear pink to red in color and are easily distinguished from colonies of nonfermenters.