The Gram stain provides a fundamental differentiation between types of bacteria. Gram positive bacteria retain the primary stain and are deep violet in color. Gram negative organisms are those that decolorize during the staining process and, after counterstaining, are pink in color. Bacterial organisms occur in two basic shapes: cocci, which are spherical, and bacilli, (also known as rods), which are elongated. Both cocci and rods may be either Gram positive or Gram negative. Examples of Gram positive cocci are shown in the upper image on the right and Gram negative cocci are shown in the lower image.