Gram staining for differentiating bacterial species 

Gram staining (or Gram's method) is a method of differentiating bacterial species into two large groups (Gram-positive and Gram-negative). The name comes from its inventor, Hans Christian Gram.
Gram staining differentiates bacteria by the chemical and physical properties of their cell walls by detecting peptidoglycan, which is present in a thick layer in Gram positive bacteria.[1] A Gram positive results in a purple/blue color while a Gram negative results in a pink/red color.
The Gram stain is almost always the first step in the identification of a bacterial organism. While Gram staining is a valuable diagnostic tool in both clinical and research settings, not all bacteria can be definitively classified by this technique. This gives rise to Gram-variable and Gram-indeterminate groups as well.