D. radiodurans is a rather large, spherical bacterium, with a diameter of 1.5 to 3.5 µm. Four cells normally stick together, forming a tetrad. The bacteria are easily cultured and do not appear to cause disease.[3] Colonies are smooth, convex, and pink to red in color. The cells stain Gram positive, although its cell envelope is unusual and is reminiscent of the cell walls of Gram negative bacteria