Gram staining differentiates bacteria by the chemical and physical properties of their cell walls and by detecting a thick layer of protein-sugar complexes called peptidoglycans, which are present in Gram-positive bacteria in a layer that makes up 60-90% of the cell wall. Peptidoglycans are present in the cells walls of Gram-negative organisms, but they only comprise 10-20% of the cell wall. In a Gram stain test, Gram-positive bacteria retain the crystal violet dye, while a counterstain (often safranin or fuchsine) added after the crystal violet gives all Gram-negative bacteria a red or pink coloring.
Gram staining 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 definitely classified by this technique.
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| Image Courtesy: Y. Tambe |
The image above shows both Gram-positive bacteria (in purple) and Gram-negative bacteria (in pink). The microscope image is of Staphylococcus aureus (Gram-positive, purple) and Escherichia coli (gram-negative, pink) and was captured under a biological microscope at 1000x magnification.
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| Gram stain of 3 types of bacteria captured at 400x under the RB30 biological microscope. |
The image above is of 3 different types of bacteria with a Gram stain applied to the prepared slide. The image was captured using a 40x Plan Fluor objective lens on the RB30 biological microscope.
