Many types of biology labs commonly use Escherichia coli, better known as E. coli, as a study organism. In the microbiology lab, students identify bacteria based on a variety of physical and reactive characteristics. Molecular biologists need to know how natural E. coli colonies look to be able to distinguish them from modified E. coli.
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.
Bacterial colonies exhibit a variety of shapes and textures ranging from round to filamentous. E. coli colonies are round. The round colonies maintain both an entire margin (i.e., continuous, smooth) and a smooth surface.
Bacterial colonies also display a variety of forms of elevation, ranging from simple convex to umbonate forms raised only in the center. Colonies of E. coli show a basic, convex elevation form.
Some bacteria exhibit subtle growth patterns. Colonies of E. coli demonstrate a periodic growth pattern, growing in waves that result in concentric growth rings in the colony. Students may detect these rings under microscopic examination.