Some bacteria produce vivid pigments, coloring their colonies red, purple, yellow or even black. Unlike these bacteria, natural E. coli lacks pigment altogether, although the shiny surface of the colony usually appears slightly whitish when it grows on plain agar. As the colony ages, the color of the center may darken slightly. Molecular biologists often transform E. coli, meaning that they cause the bacteria to take in a plasmid, a circle of DNA that contains a gene or gene fragment of interest to the biologist. The biologist will generally also include a color marker, so that E. coli that have successfully taken up the plasmid create colored colonies on certain media, such as blue colonies on medium containing a substance called Xgal
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