Gram stain
Say it: GRAM stayn
The Gram stain is a lab staining method that sorts bacteria into Gram-positive (purple) and Gram-negative (pink) groups based on their cell walls.
Also: Gram staining, Gram's method
Definition
What is the Gram stain?
The Gram stain is a differential stain, a staining method that colors different kinds of cells in different ways, and it is often the first test run on a bacterial sample. The Danish scientist Hans Christian Gram devised it in 1884. It divides most bacteria into two large groups according to the structure of their cell walls, and it also reveals their shape, such as round cocci or rod-shaped bacilli.
A smear of cells is dried and heat-fixed on a slide, then treated in four steps. Crystal violet stains every cell purple. Iodine, acting as a mordant, binds the dye into a larger complex. An alcohol or acetone decolorizer then rinses the dye out of cells that have a thin peptidoglycan layer, while cells with a thick peptidoglycan layer hold on to it. Finally, the red dye safranin counterstains the cells that lost their color. Gram-positive cells end up purple and Gram-negative cells pink or red.
In the clinic, a Gram stain of sputum, urine, wound fluid or other samples gives a quick first clue to the cause of an infection and helps guide early treatment while culture results are awaited. Fresh cultures give the most reliable results, because aging Gram-positive cells can lose the dye and look Gram-negative.
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- Gram-positive bacteriaGram-positive bacteria have a thick peptidoglycan cell wall and no outer membrane, so they keep the purple crystal violet dye in a Gram stain.
- Gram-negative bacteriaGram-negative bacteria have a thin peptidoglycan wall wrapped in an outer membrane, so they lose the purple dye in a Gram stain and look pink.
- PeptidoglycanPeptidoglycan is the mesh-like polymer of sugars and short peptides that forms the bacterial cell wall and gives bacteria their shape and strength.
- BacteriumA bacterium is a single-celled prokaryote with no nucleus, found in almost every habitat on Earth, including in and on the human body.
- AntibioticAn antibiotic is a drug that kills bacteria or stops them from growing, used to treat or prevent bacterial infections; it has no effect on viruses.
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