Gram's stain


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Gram's stain,

laboratory staining technique that distinguishes between two groups of bacteria by the identification of differences in the structure of their cell walls. The Gram stain, named after its developer, Danish bacteriologist Christian Gram, has become an important tool in bacterial taxonomy, distinguishing between so-called gram-positive bacteria, which remain colored after the staining procedure, and gram-negative bacteria, which do not retain dye. In the staining technique, cells on a microscope slide are heat-fixed (killed) and stained with a basic dye, crystal violet, which stains all bacterial cells blue; then they are treated with an iodine-potassium iodide solution that allows the iodine to enter the cells and form a water-insoluble complex with the crystal violet dye. The cells are treated with alcohol or acetone solvent in which the iodine-crystal violet complex is soluble. Following solvent treatment, only gram-positive cells remain stained, possibly because of their thick cell wall, which is not permeable to solvent. After the staining procedure, cells are treated with a counterstain, i.e., a red acidic dye such as safranin or acid fuchsin, in order to make gram-negative (decolorized) cells visible. Counterstained gram-negative cells appear red, and gram-positive cells remain blue. Although the cell walls of gram-negative and gram-positive bacteria are similar in chemical composition, the cell wall of gram-negative bacteria is a thin layer sandwiched between an outer lipid-containing cell envelope and the inner cell membrane, whereas the gram-positive cell wall is much thicker, lacks the cell envelope, and contains additional substances, such as teichoic acids, polymers composed of glycerol or ribitol. The difference in reactivity between gram-positive and gram-negative bacteria is linked with differences in physiological properties of the two groups. Gram-positive bacteria are generally more sensitive to growth inhibition by dyes, halogens, many antibioticsantibiotic,
any of a variety of substances, usually obtained from microorganisms, that inhibit the growth of or destroy certain other microorganisms. Types of Antibiotics
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, and to attack by phagocytosis (see endocytosisendocytosis
, in biology, process by which substances are taken into the cell. When the cell membrane comes into contact with a suitable food, a portion of the cell cytoplasm surges forward to meet and surround the material and a depression forms within the cell wall.
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), and are more resistant to digestion by the enzymes pepsinpepsin,
enzyme produced in the mucosal lining of the stomach that acts to degrade protein. Pepsin is one of three principal protein-degrading, or proteolytic, enzymes in the digestive system, the other two being chymotrypsin and trypsin.
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 and trypsintrypsin,
enzyme that acts to degrade protein; it is often referred to as a proteolytic enzyme, or proteinase. Trypsin is one of the three principal digestive proteinases, the other two being pepsin and chymotrypsin.
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 and enzymes in animal sera.

Gram's stain

[′gramz ‚stān]
(microbiology)
A differential bacteriological stain; a fixed smear is stained with a slightly alkaline solution of basic dye, treated with a solution of iodine in potassium iodide, and then with a neutral decolorizing agent, and usually counterstained; bacteria stain either blue (gram-positive) or red (gram-negative).
References in periodicals archive ?
Some organisms fail to yield a typical Gram's stain reaction due to cell wall damage, which can cause Gram positives to appear Gram negative or Gram variable.
As with any test, Gram's stain has its flaws, so laboratorians must be mindful of potential problems.
Some organisms cannot be demonstrated by Gram's stain (eg, Mycoplasma spp.
Several other modifications have been made to improve Gram's stain results (see Table 2, left).
My annual examinations include an oral and fecal Gram's stain.
For adult birds in good health, I suggest annual crop and fecal Gram's stain and basic health blood work (CBC, biochemistry profile, and liver function testing) every 2-3 years.
I used to recommend more fecal Gram's stains, and I believe this was useful.
The overall agreement measured between culture and Gram's stain results for all bacteria was fair (weighted [kappa] = 0.
Most gram-positive bacteria identified on Gram's stain of cloacal swab samples in this population of 21 Hispaniolan Amazon parrots were gram-positive rods, with a rod to cocci ratio of 6.
In this study, a third of the clinically healthy birds sampled had gram-negative bacteria present on Gram's stain cytology, culture, or both.
Although it appears that Gram's stain results were more likely to identify specific groups of bacteria compared with microbiologic culture, the 95% CI suggests that there is no difference between the 2 techniques in this study.