The Gram stain is a differential staining technique that differentiates nearly all bacterial species into two large groups, based on structural differences in the cell wall.
The cell wall is essential, as it protects and shape the bacteria, and is involved in inducing disease in the infected host. It can also be a target for certain antibiotics. The gram staining takes no longer than several minutes to perform and analyse the bacterial sample.

- Bacteria that have a thick peptidoglycan layer retain the primary stain, Crystal Violet, and are said to be Gram-positive.
- Those that have a thin peptidoglycan layer and an outer membrane do not retain the primary stain and are said to be Gram-negative.

The Gram stain does not identify individual species within a group, and to do this, we would need to run additional microbiological tests.
Still, the Gram stain is widely used as it allows an experienced microbiologist to detect the presence of bacteria, and decide if a bacteria is
Gram-positive or Gram-negative, in a matter of minutes.
Knowing the Gram type, and seeing the morphological shape of a bacteria aid in selecting the initial antibiotic needed to treat a patient.

Following a Gram stain, Gram-positive bacteria appear purple, while Gram-negative bacteria appear pink.

Microscopic images of gram positive and gram negative bacteria after gram staining, The image on the top presents gram positive bacteria shown as clusters of small, purple spheres on the white background. The image below presents gram negative bacteria shown as small pink spheres joined together like beads on a string, on white background.
Figure 1: Purple, Gram-positive bacteria as viewed with a light microscope, following the Gram stain. Pink, Gram-negative bacteria as viewed with a light microscope, following the Gram stain.