Gram staining

A Gram stain of mixed Staphylococcus aureus gram-positive cocci (in purple) and Escherichia coli gram-negative bacilli (in red)

Gram staining (or Gram's method) is a way of classifying bacteria into two large groups: gram-positive and gram-negative. The name comes from its inventor, Hans Christian Gram.

Gram's method stains bacteria according to the chemical and physical properties of their cell walls. First, a violet dye is put on the bacteria. This dye stains peptidoglycan, a thick layer that is only found in gram-positive bacteria. After the first stain, another stain (usually safranin or fuchsin) gives all gram-negative bacteria a red or pink colour.

The Gram stain is almost always the first step in the identification of a bacterial organism. However, not all bacteria can be classified by this technique. Bacteria for which the method does not work are called 'gram-variable' or 'gram-indeterminate'.

Gram developed the technique working with another scientist, Carl Friedländer in a hospital in Berlin. However, Gram first used the test to make bacteria in the lungs easier to see. He published his finished method in 1884.

Gram Staining Media

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  Micrograph of a gram-positive coccus and a gram-negative rod.

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  Purple-stained gram-positive (left) and pink-stained gram-negative (right)

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  Gram-stain of gram-positive streptococci surrounded by pus cells

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  Gram negative Neisseria gonorrhoeae and pus cells