Gram stain
A staining method that classifies bacteria into two groups.
Wikipedia / Wikimedia Commons
The Gram stain is a laboratory technique that sorts bacteria into two major categories—gram-positive and gram-negative—based on differences in their cell walls. It can also help diagnose fungal infections. The method is named after Danish bacteriologist Hans Christian Gram, who created it in 1884. The staining works by exploiting the chemical and physical traits of bacterial cell walls: gram-positive bacteria have a thick peptidoglycan layer that holds onto the primary stain, crystal violet, while gram-negative bacteria have a thinner peptidoglycan layer that allows the stain to be washed away with ethanol. After decolorization, a counterstain—usually safranin or fuchsine—turns gram-negative cells pink or red. Lugol's iodine is added after the crystal violet to form a stable complex that strengthens the stain's bond to the cell wall.
Gram staining is typically the first step in identifying a bacterial group. It is a valuable diagnostic tool in both clinical and research settings, but not all bacteria can be definitively classified this way, leading to gram-variable and gram-indeterminate groups.
**History** The technique is named after Hans Christian Gram (1853–1938), a Danish scientist who developed it while working with Carl Friedländer in the morgue of Berlin's city hospital in 1884. Gram did not originally aim to distinguish bacterial types; he wanted to make bacteria more visible in stained lung tissue. He noticed that some bacterial cells resisted decolorization. Based on this, he created the first Gram staining procedure, using Ehrlich's aniline-gentian violet, Lugol's iodine, absolute alcohol for decolorization, and Bismarck brown as a counterstain. He published his method in 1884, noting that the typhus bacillus did not retain the stain. Gram did not initially classify bacteria as gram-negative or gram-positive.
**Uses** Gram staining is a bacteriological lab technique that separates bacterial species into two groups (gram-positive and gram-negative) based on cell wall properties. It can also diagnose fungal infections. It is not used to classify archaea, as these microorganisms show widely varying responses that do not match their phylogenetic groups. Gram stains are performed on body fluid or biopsy samples when infection is suspected. The results come much faster than culturing, which is especially important when infection affects a patien
- born
- 1853
- died
- 1938
- field
- Bacteriology
- nationality
- Danish
- known_for
- Developing the Gram stain method
Lore & Background
Hans Christian Gram developed the technique while working with Carl Friedländer in the morgue of the city hospital in Berlin in 1884. Gram devised his technique not for the purpose of distinguishing one type of bacterium from another but to make bacteria more visible in stained sections of lung tissue. Gram noticed that some bacterial cells possessed noticeable resistance to decolorization. Based on these observations, Gram developed the initial gram staining procedure, initially making use of Ehrlich's aniline-gentian violet, Lugol's iodine, absolute alcohol for decolorization, and Bismarck brown for counterstain. He published his method in 1884, and included in his short report the observation that the typhus bacillus did not retain the stain. Gram did not initially make the distinction between Gram-negative and Gram-positive bacteria using his procedure.
Reader's Guide
Gram staining is almost always the first step in the identification of a bacterial group. While Gram staining is a valuable diagnostic tool in both clinical and research settings, not all bacteria can be definitively classified by this technique, giving rise to gram-variable and gram-indeterminate groups. Gram stains are performed on body fluid or biopsy when infection is suspected, yielding results much more quickly than culturing. They are especially important when infection would make an important difference in the patient's treatment and prognosis, such as in cerebrospinal fluid for meningitis and synovial fluid for septic arthritis. The method remains a cornerstone of bacteriology, though it is not used to classify archaea, since these microorganisms yield widely varying responses that do not follow their phylogenetic groups.
Did You Know?
- Gram staining can also be used to diagnose a fungal infection.
- The method uses crystal violet, iodine, ethanol or acetone, and a counterstain such as safranin or fuchsine.
- Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet stain, while gram-negative bacteria have a thinner layer and are counterstained pink or red.
- Hans Christian Gram developed the technique in 1884 while working in a morgue in Berlin.
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