Bacteria are broadly divided into two groups, Rods and Cocci depending on the shape of their cells. Cocci refer to bacteria which are spherical, whereas Rods are rod shaped. An identification process known as Gram Staining shows the bacteria stained red/pink, which classes the bacteria as Gram negative or Gram positive.This procedure relies on the absorption of a crystal violet dye by the bacteria’s peptidoglycan layer. Gram positive bacteria have a thick layer of peptidoglycan which holds the Crystal Violet stain used in Gram Staining and so appear purple under the microscope. Gram negative bacteria have a thin layer of peptidoglycan held between two membranes. This is not able to hold the stain and therefore appear pink.
Gram-positive bacilli belong to a class of rod-shaped bacteria that acquire a violet color when subjected to the Gram staining method.