Gram Staining

Topics: Bacteria, Staining, Microbiology Pages: 5 (1268 words) Published: November 4, 2013
Gram staining
From Wikipedia, the free encyclopedia

A Gram stain of mixed Staphylococcus aureus (Staphylococcus aureus ATCC 25923, Gram-positive cocci, in purple) andEscherichia coli (Escherichia coli ATCC 11775, Gram-negative bacilli, in red), the most common Gram stain reference bacteria Gram staining (or Gram's method) is a method of differentiating bacterial species into two large groups (Gram-positive and Gram-negative). The name comes from its inventor, Hans Christian Gram. 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.[1] A Gram-positive results in a purple-blue color while a Gram-negative results in a pink-red color. 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. Contents

1 History
2 Uses
2.1 Medical
2.2 Staining mechanism
3 Examples
3.1 Gram-positive bacteria
3.2 Gram-negative bacteria
3.3 Gram-indeterminate bacteria
4 See also
5 References
6 External links
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 make bacteria more visible in stained sections of lung tissue.[2] He published his method in 1884, and included in his short report the observation that the Typhus bacillus did not retain the stain.[3] Uses[edit]

Gram staining is a bacteriological laboratory technique[4] used to differentiate bacterial species into two large groups (Gram-positive and Gram-negative) based on the physical properties of their cell walls.[5] Gram staining is not used to classify archaea, formerly archaeabacteria, since these microorganisms yield widely varying responses that do not follow theirphylogenetic groups.[6] 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.[7] Medical[edit]

See also: Gram-negative bacterial infection and Gram-positive bacterial infection 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.[4][8] Staining mechanism[edit]

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...
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