Gram stain

Gram stain (Gram staining or Gram's method), is a method of staining used to classify bacterial species into two large groups: gram-positive bacteria and gram-negative bacteria. It may also be used to diagnose a fungal infection.[1] The name comes from the Danish bacteriologist Hans Christian Gram, who developed the technique in 1884.[2]

Gram staining differentiates bacteria by the chemical and physical properties of their cell walls. Gram-positive cells have a thick layer of peptidoglycan in the cell wall that retains the primary stain, crystal violet. Gram-negative cells have a thinner peptidoglycan layer that allows the crystal violet to wash out on addition of ethanol. They are stained pink or red by the counterstain,[3] commonly safranin or fuchsine. Lugol's iodine solution is always added after addition of crystal violet to form a stable complex with crystal violet that strengthens the bonds of the stain with the cell wall.[4]

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. This gives rise to gram-variable and gram-indeterminate groups.

  1. ^ "Gram Stain: MedlinePlus Medical Test". medlineplus.gov.
  2. ^ Colco, R. (2006). Gram Staining. Current Protocols in Microbiology. Vol. Appendix 3. pp. Appendix 3C. doi:10.1002/9780471729259.mca03cs00. ISBN 978-0471729259. PMID 18770544. S2CID 32452815.
  3. ^ Cite error: The named reference Beveridge_and_Davies_1983 was invoked but never defined (see the help page).
  4. ^ Libenson, L.; McIlroy, A. P. (1955-07-01). "On the Mechanism of the Gram Stain". Journal of Infectious Diseases. 97 (1): 22–26. doi:10.1093/infdis/97.1.22. ISSN 0022-1899. PMID 13242849.
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