Gram staining differentiates bacteria by the chemical and physical properties of their cell walls by detecting peptidoglycan, which is present in a thick layer in gram-positive bacteria. In a Gram stain test, gram-positive bacteria retain the crystal violet dye, while a counterstain (commonly safranin) added after the crystal violet gives all gram-negative bacteria a red or pink coloring.
The Gram stain is almost always the first step in the identification of a bacterial organism. 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 as well.
The method is named after its inventor, the Danish scientist Hans Christian Gram (1853–1938), who 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. He published his method in 1884, and included in his short report the observation that the Typhus bacillus did not retain the stain.
Gram staining is a bacteriological laboratory technique used to differentiate bacterial species into two large groups (gram-positive and gram-negative) based on the physical properties of their cell walls. Gram staining is not used to classify archaea, formerly archaeabacteria, since these microorganisms yield widely varying responses that do not follow their phylogenetic groups.
The Gram stain is not an infallible tool for diagnosis, identification, or phylogeny, and it is of extremely limited use in environmental microbiology. It still competes with molecular techniques even in the medical microbiology lab. Some organisms are gram-variable (that means, they may stain either negative or positive); some organisms are not susceptible to either stain used by the Gram technique. In a modern environmental or molecular microbiology lab, most identification is done using genetic sequences and other molecular techniques, which are far more specific and informative than differential staining.
Gram staining has proven as effective a diagnostic tool as PCR, particularly with regards to gonorrhoea diagnosis in Kuwait. The similarity of the results of both Gram stain and PCR for diagnosis of gonorrhea was 99.4% in Kuwait.
Gram stains are performed on body fluid or biopsy when infection is suspected. Gram stains yield results much more quickly than culture, and is especially important when infection would make an important difference in the patient's treatment and prognosis; examples are cerebrospinal fluid for meningitis and synovial fluid for septic arthritis.
Gram-positive bacteria have a thick mesh-like cell wall made of peptidoglycan (50–90% of cell envelope), and as a result are stained purple by crystal violet, whereas gram-negative bacteria have a thinner layer (10% of cell envelope), so do not retain the purple stain and are counter-stained pink by the Safranin. There are four basic steps of the Gram stain:
- Applying a primary stain (crystal violet) to a heat-fixed smear of a bacterial culture. Heat fixing kills some bacteria but is mostly used to affix the bacteria to the slide so that they don't rinse out during the staining procedure.
- The addition of iodine, which binds to crystal violet and traps it in the cell,
- Rapid decolorization with alcohol or acetone, and
- Counterstaining with safranin. Carbol fuchsin is sometimes substituted for safranin since it more intensely stains anaerobic bacteria, but it is less commonly used as a counterstain.
Crystal violet (CV) dissociates in aqueous solutions into CV+
and chloride (Cl−
) ions. These ions penetrate through the cell wall and cell membrane of both gram-positive and gram-negative cells. The CV+
ion interacts with negatively charged components of bacterial cells and stains the cells purple.
3) interacts with CV+
and forms large complexes of crystal violet and iodine (CV–I) within the inner and outer layers of the cell. Iodine is often referred to as a mordant, but is a trapping agent that prevents the removal of the CV–I complex and, therefore, color the cell.
When a decolorizer such as alcohol or acetone is added, it interacts with the lipids of the cell membrane. A gram-negative cell loses its outer lipopolysaccharide membrane, and the inner peptidoglycan layer is left exposed. The CV–I complexes are washed from the gram-negative cell along with the outer membrane. In contrast, a gram-positive cell becomes dehydrated from an ethanol treatment. The large CV–I complexes become trapped within the gram-positive cell due to the multilayered nature of its peptidoglycan. The decolorization step is critical and must be timed correctly; the crystal violet stain is removed from both gram-positive and negative cells if the decolorizing agent is left on too long (a matter of seconds).
After decolorization, the gram-positive cell remains purple and the gram-negative cell loses its purple color. Counterstain, which is usually positively charged safranin or basic fuchsin, is applied last to give decolorized gram-negative bacteria a pink or red color.
Some bacteria, after staining with the Gram stain, yield a gram-variable pattern: a mix of pink and purple cells are seen. The genera Actinomyces, Arthobacter, Corynebacterium, Mycobacterium, and Propionibacterium have cell walls particularly sensitive to breakage during cell division, resulting in gram-negative staining of these gram-positive cells. In cultures of Bacillus, Butyrivibrio, and Clostridium, a decrease in peptidoglycan thickness during growth coincides with an increase in the number of cells that stain gram-negative. In addition, in all bacteria stained using the Gram stain, the age of the culture may influence the results of the stain.
Gram-positive bacteria generally have a single membrane (monoderm) surrounded by a thick peptidoglycan. This rule is followed by two phyla—Firmicutes (except for the classes Mollicutes and Negativicutes) and the Actinobacteria. In contrast, members of the Chloroflexi (green non-sulfur bacteria) are monoderms but possess a thin or absent (class Dehalococcoidetes) peptidoglycan and can stain negative, positive or indeterminate. Members of the Deinococcus-Thermus group, stain positive but are diderms with a thick peptidoglycan.
Historically, the gram-positive forms made up the phylum Firmicutes, a name now used for the largest group. It includes many well-known genera such as Bacillus, Listeria, Staphylococcus, Streptococcus, Enterococcus, and Clostridium. It has also been expanded to include the Mollicutes, bacteria like Mycoplasma that lack cell walls and so cannot be stained by Gram, but are derived from such forms.
Normally, if Gram stain is done on acid-fast bacteria they show up as if they are gram-positive, mostly because of their thick cell wall.
Gram-negative bacteria generally possess a thin layer of peptidoglycan between two membranes (diderms). Most bacterial phyla are gram-negative, including the cyanobacteria, spirochaetes and green sulfur bacteria, and most Proteobacteria and Escherichia coli. (exceptions being some members of the Rickettsiales and the insect-endosymbionts of the Enterobacteriales).
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Gram-indeterminate bacteria, also known as gram-variable bacteria do not respond predictably to Gram staining and, therefore, cannot be determined as either gram-positive or gram-negative. They tend to stain unevenly, appearing partially gram positive and partially gram negative, or even unstained. Staining older cultures (over 48 hours) can lead to false gram-variable results, probably due to changes in the cell wall with aging. Gram-indeterminate bacteria are best stained using acid fast staining techniques. Examples include many species of Mycobacterium, including M. tuberculosis and M. leprae. 
- Bergey, David H.; John G. Holt; Noel R. Krieg; Peter H.A. Sneath (1994). Bergey's Manual of Determinative Bacteriology (9th ed.). Lippincott Williams & Wilkins. ISBN 0-683-00603-7.
- Austrian, R. (1960). "The Gram stain and the etiology of lobar pneumonia, an historical note". Bacteriol. Rev. 24 (3): 261–265. PMC 441053. PMID 13685217.
- Gram, HC (1884). "Über die isolierte Färbung der Schizomyceten in Schnitt- und Trockenpräparaten". Fortschritte der Medizin (in German) 2: 185–189..
English translation in: Brock, T.D. (1999). Milestones in Microbiology 1546–1940 (2 ed.). ASM Press. pp. 215–218. ISBN 1-55581-142-6..
Translation is also at: Brock, T.D. Pioneers in Medical Laboratory Science: Christian Gram 1884. Hoslink. Retrieved 2010-07-27.
- Ryan KJ, Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. pp. 232–3. ISBN 0-8385-8529-9.
- Madigan, MT; Martinko J, Parker J (2004). Brock Biology of Microorganisms (10th ed.). Lippincott Williams & Wilkins. ISBN 0-13-066271-2.
- Beveridge TJ (2001). "Use of the Gram stain in microbiology". Biotech Histochem 76 (3): 111–8. doi:10.1080/714028139. PMID 11475313.
- El-Garnal, A.H., Al-Otaibi, S.R., Alshamali, A., Abdulrazzaq, A., Najem, N., & Fouzan, A.A. Polymerase chain reaction is no better than Gram stain for diagnosis of gonococcal urethritis. Indian Journal of Dermatology, Venereology, and Leprology,(2009); 75, 101.
- Søgaard M, Nørgaard M, Schønheyder H (2007). "First notification of positive blood cultures: high accuracy of the Gram stain report (Epub ahead of publication)". J Clin Microbiol 45 (4): 1113–7. doi:10.1128/JCM.02523-06. PMC 1865800. PMID 17301283.
- Microbiology; J.G. Black Prentice Hall, 1993
- http://www.med-chem.com/procedures/GRAMSTAIN.pdf[dead link]
- Llewellyn, Brian D. (May 2005). "StainsFile – Stain theory – What a mordant is not". Retrieved 2009-09-10.
- Beveridge TJ, Davies JA (November 1983). "Cellular responses of Bacillus subtilis and Escherichia coli to the Gram stain". Journal of bacteriology 156 (2): 846–58. PMC 217903. PMID 6195148.
- Davies JA, Anderson GK, Beveridge TJ, Clark HC (November 1983). "Chemical mechanism of the Gram stain and synthesis of a new electron-opaque marker for electron microscopy, which replaces the iodine mordant of the stain". Journal of bacteriology 156 (2): 837–45. PMC 217902. PMID 6195147.
- Beveridge TJ (March 1990). "Mechanism of gram variability in select bacteria". Journal of bacteriology 172 (3): 1609–20. PMC 208639. PMID 1689718.
- Don J. Brenner, Noel R. Krieg, James T. Staley (July 26, 2005) . George M. Garrity, ed. Introductory Essays. Bergey's Manual of Systematic Bacteriology 2A (2nd ed.). New York: Springer. p. 304. ISBN 978-0-387-24143-2. British Library no. GBA561951.[page needed]
- Black, Jacquelyn (2012). Microbiology: Principles and exploration 8th edition. John Wiley & Sons. p. 68. ISBN 978-0-470-54109-8.
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