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The Gram Stain

In 1884, Hans Christian Gram, a Danish doctor working in Berlin, accidentally stumbled on a method which still forms the basis for the identification of bacteria. While examining lung tissue from patients who had died of pneumonia, he discovered that certain stains were preferentially taken up and retained by bacterial cells. Over the course of the next few years, Gram developed a staining procedure which divided almost all bacteria into two large groups - the Gram stain.

Individual bacterial cells are hard to see, partly because they are small, but also because they are almost transparent. In addition to magnification under a microscope, optical tricks must also be used to be able to see them:

  • Staining

Either of these methods can make bacterial cells visible under the microscope. Other staining methods are described elsewhere in these documents, e.g. the Ziehl-Neelsen acid-fast staining procedure, but the Gram stain procedure is as follows:


  1. Place a slide with a bacterial smear on a staining rack.
  2. STAIN the slide with crystal violet for 1-2 min.
  3. Pour off the stain.
    Note: fingers stain Gram-positive - use forceps!
  4. Flood slide with Gram's iodine for 1-2 min.
  5. Pour off the iodine.
  6. Decolourize by washing the slide briefly with acetone (2-3 seconds).
  7. Wash slide thoroughly with water to remove the acetone - do not delay with this step.
  8. Flood slide with safranin counterstain for 2 min.
  9. Wash with water.
  10. Blot excess water and dry in hand over bunsen flame.
Gram stain procedure

Bacterial cell wallSo how does it work? Gram didn't know - he simply worked empirically. We now know that the Gram reaction is based on the structure of the bacterial cell wall.
In Gram-positive bacteria, the dark purple crystal violet stain is retained by the thick layer of peptidoglycan which forms the outer layer of the cell.
In Gram-negative bacteria, the thin peptidoglycan layer in the periplasm does not retain the dark stain, and the pink safranin counterstain stains the peptidoglycan layer.

In the video below you can see different species of bacteria distinguished both by their morphology and Gram-reaction:


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