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It is because of the thickness of the peptidoglycan cell wall of the bacteria.

Gram positive bacteria have a larger/thicker cell wall.

It is this cell wall which retains the stain of the crystal violet(primary stain) and carbol fuschin(counterstain).

Gram negative have a very thin cell wall. So when the Acetone solution is applied in between stains the crystal violet is washed out of the gram negative bacteria. As it is only left on for a few seconds the gram positive bacteria can still retain the crystal violet solution due to the cell wall thickness.

The counterstain is then added. because the gram negative bacteria have been 'unstained' by the acetone the fuschin (pink stain) is absorbed and therefore shown when looked at under a microscope.

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14y ago
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13y ago

Gram positive bacteria appear blueish and Gram negative bacteria appear redish, after staining with Gram stain.

The reason why these bacteria react differently to gram staining procedure is because of differences in the structure of the cell wall between gram positive and negative organisms. Alcohol is used to counterstain the cell and gram positives retain the stain, whereas gram negative don't.

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13y ago

The cell wall of a bacterium is made up of short chain amino acids and sugars linked together called peptidoglycans. Depending on their thickness they will take up the grams stain. Gram+ve bacteria have a layer of peptidoglycans up to 5x thicker than Gram-ve bacteria. This layer retains the Crystal Violet/Iodine stain in Gram+ve bacteria whereas in Gram-ve it does no but they will take up the methyl red counterstain. Also Gram-ve bacteria have extra layers of lipo-polysaccharides and these also retain the counterstain.

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10y ago

Gram-positive bacteria have a thick mesh-like cell wall which is made up of peptidoglycan (50-90% of cell wall), which stains purple. Peptidoglycan is mainly a polysaccharide composed of two subunits called N-acetyl glucosamine and N-acetyl muramic acid. As adjacent layers of peptidoglycan are formed, they are cross linked by short chains of peptides by means of a transpeptidase enzyme, resulting in the shape and rigidity of the cell wall. The thick peptidoglycan layer of Gram-positive organisms allows these organisms to retain the crystal violet-iodine complex and stains the cells as purple

Gram-negative bacteria have a thinner layer of peptidoglycan (10% of the cell wall) and lose the crystal violet-iodine complex during decolorization with the alcohol rinse, but retain the counter stain Safranin, thus appearing reddish or pink. They also have an additional outer membrane which contains lipids, which is separated from the cell wall by means of periplasmic space.

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12y ago

B'coz gm positive bacteria get stained with the colour of safranine...

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Q: Why is the difference color between gram positive and gram negative bacteria?
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What cell structure is significantly different between gram-positive and gram-negative bacteria?

The difference between a gram positive and gram negative bacteria is the thickness/presence of the peptidoglycan layer secreted on the outside of the plasma membrane


What is the difference between positive gram and negative gram?

The major difference between gram positive and the gram negative bacteria is the structure of their cell wall. The gram positive bacteria have two layers a plasma membrane and outer to it a peptidoglycan layer. But the gram negative bacteria have another layer other than the cell wall. While the gram negative bacteria has three layers. It has a plasma membrane that is called as the inner membrane, then a peptidoglycan layer and finally an outer membrane of phospholipids and lipopolysaccharides. There is space between the inner membrane and the peptidoglycan layer called as the periplasmic space. Peptidoclycan layer in the gram positive bacteria are much more thicker than that of the gram negative bacteria. The molecule teichoic acid is present in gram positive bacteria and the porin proteins are characteristically present in the gram negative bacteria. Gram positive bacteria usually produce exotoxins and the gram negative bacteria usually produces endotoxins. Gram positive bacteria gives Violet colour to the gram staining and the gram negative bacteria gives pink colour to the gram staining. In gram staining the bacteria is fixed on slides and then they are treated with Crystal Violet. After that they are treated with iodine and then they are decolourised. If the bacteria is gram negative then it shows Violet colour and if it is gram negative then it shows pink colour. Initially both strains show Violet colour after Crystal Violet treatment but on the treatment with iodine and decolourisation the gram negative bacteria loose colour and become pink. This is because in the gram positive bacteria the Violet crystals bind firmly to the peptidoglycan layer and the colour is not removed in later stages. In gram positive bacteria the colour does not penetrate much because of outer membrane and is lost much during decolourisation.


What is the difference between cell wall of gram positive and gram negative bacteria.?

Gram positive bacteria cell walls have a thick layer of Peptidoglycan and no periplasmic space. Gram negative bacteria cell walls have inner and outter cytoplasmic membranes with a periplasmic space in between. these also have a thin layer of Peptidoglycan. The outter cytoplasmic membrane of gram negative bacteria contains lipopolysaccharides.


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Can treatment with Transpeptidase eliminate gram positive bacteria from a mixture of gram positive bacteria and gram negative bacteria?

No. A treatment of ethylenediaminetetraacetic acid (EDTA) and the enzyme, lysozyme, will cause gram negative bacteria to lyse. Some gram positive bacteria can also be lysed by doing this. The EDTA stabilizes the solution and allows lysozyme to cut the bonds between the carbohydrates that make up the cell wall wall in bacteria. This will lyse the cell and cause the contents of the cell to be spilled out. This technique is primarily used for DNA assays. A good way to isolate either gram positive or gram negative is by using different types of agar. The MacConkey agarmight be the most useful for what you want to do. It will only allow gram negative to grow, isolating just the gram negatives only.

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