MacConkey's Agar is a culture medium designed to grow up Gram-negative bacteria and stain them for lactose fermentation. It contains bile salts, crystal violet dye (to inhibit Gram-positive bacteria), neutral red dye (which stains microbes fermenting lactose), lactose and peptone. This means that MacConkey agar is both selective and differential. Alfred Theodore MacConkey developed it while working as a bacteriologist for the Royal Commission on Sewage Disposal. A selective medium is a substance (usually agar-based) which grows a specific type of microbe. ... Bacteria that are Gram-negative are not stained dark blue or violet by Gram staining, in contrast to Gram-positive bacteria. ... Fermentation typically refers to the conversion of sugar to alcohol using yeast. ... Bile is also another name for Belenus, a god in Brythonic mythology. ... Structure of Methyl Violet 2B Methyl violet is the name given to a group of similar chemicals used as pH indicators and dyes. ... Neutral red Neutral Red (or toluylene red, Basic Red 5, or C.I. 50040) is an eurhodin dye used for staining in histology. ... Lactose Lactose is a disaccharide that makes up around 2-8% of the solids in milk. ... Peptones or peptides are chains of amino acids formed by condensation of the a. ...