Chromogenic Agar Plate
Using different chromogenic substrates may result in a variation in the variety or hue of color displayed. The mechanism for this reaction can vary and may or may not require an operon depending on the organism, the chromophore, and the substrate used. In the case of the lac operon, an inducer binds to the control region of the operon. The lac operon contains 3 genes: LacZ, lacY and lacA, which encode functional enzymes necessary for lactose metabolism. LacZ encodes β-galactosidase, which hydrolyzes the ether or ester bond found in β-galactosides (or di and trisaccarides) into two parts, galactose and a sugar or a chromophore. When these bonds are hydrolyzed, a colorless molecule called indolol is released. In the presence of oxygen, this molecule dimerizes, causing the pigment indigo to form. Color variations of blues, greens and even red and purple are possible by substituting halogen groups on the indolol molecule. LacY and lacA encodes the gene to initiate the formations of permease, and acetylase transferase, respectively.