Microbiology lab courses 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. They often describe the morphology, or form, of the bacterial colonies as an important first step in the identification process.
Some bacteria produce vivid pigments, colouring their colonies red, purple, yellow or even black. Unlike these bacteria, 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 colour of the centre may darken slightly.
Bacterial colonies exhibit a variety of shapes and textures ranging from round to filamentous. However, E. coli colonies possess a rather nondescript appearance. 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 centre. Colonies of E. coli show a basic, convex elevation form that is as nondescript as the colony shape.
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.