Gram stain

Gram stain is a method of staining used in microbiology to differentiate bacterial species into two large groups (Gram-positive and Gram-negative). The name comes from the Danish bacteriologist Hans Christian Gram, who developed the technique.

History

Hans Christian Gram developed the Gram stain in 1884 to differentiate bacterial cells in lung tissue samples. He noticed that certain stains were retained by some cells but not others, leading to the classification of bacteria into Gram-positive and Gram-negative.

Procedure

The Gram stain procedure involves four steps:

1. Application of a primary stain (Crystal Violet)
2. Application of a mordant (Gram's Iodine)
3. Decolorization with alcohol or acetone
4. Counterstaining with a secondary stain (Safranin)

Interpretation

Gram-positive bacteria retain the crystal violet dye, and thus they are stained violet, while the Gram-negative bacteria do not retain this stain and are thus stained red from the safranin counterstain.

Applications

The Gram stain is almost always the first step in the preliminary identification of a bacterial organism. It helps in the classification and differentiation of microorganisms. It is also used in the initial steps of diagnosing an infection.

Limitations

While Gram staining is a valuable diagnostic tool in both clinical and research settings, not all bacteria can be definitively classified by this technique, these are known as Gram-variable and Gram-indeterminate.

See also

• Microbiology
• Bacteria
• Hans Christian Gram
• Gram-positive bacteria
• Gram-negative bacteria

References

Gram stain Resources
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