A culture medium is any nutrient liquid or solid that can be used in the laboratory & for the growth of micro organisms. Such a medium may resemble a natural substrate on which the micro organism will usually grow like blood serum for animal pathogen milk for milk micro organism. Distilled water is used to dissolve all to medium components. Peptones are used as a nitrogen source & it has another function in the medium that is buffering material, because it is an amphoteric compound it can maintain the medium PH Carbohydrates, peptides, amino acid and proteins are used as an energy source. Usually beef extract is used as a growth factor. Gelatin or agar agar is used as a solidifying agent. Media can be classified based on texture as liquid media, solid media, reversible to liquid, semi solid media. Selective media is used to isolate one species of micro organism and inhibit the growth of another micro organism. Differential media-in which we add a natural or chemical compounds differentiate microbial groups which may be formed in the same culture. Enriched media-in which we need to add blood serum or plant tissues to enrich the growth of specific groups of micro organism like heterophilic bacteria.