Gram test

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Gram staining is a well know preliminary diagnosis test to classify and characterize bacteria, enabling these microorganisms to be examined using a light microscope, then putting their morphology and arrangement in evidence. This differential staining technique is based on the chemical and structural cell wall diferences that influences the ability of bacteria to retaining the crystal violet dye during solvent treatment, allowing us to conceive two major groups of bacteria: Gram positive and Gram negative. The first ones disposing of a thick layer of peptidoglycan and lipoteichoic acid, and the second ones presenting a thinner layer of peptidoglican, and an additional outer membrane containing lipoproteins, phospholipids, proteins, and lipopolysaccharides. Due to the treatment with alcohol-acetone, Gram negative cell walls permeability are increased during the staining process, thus washing away the crystal violet-iodine (CV-I) complex stain that distinguish Gram positive cells. Those ones are then able to retain a counter stain (generally safranin), wich explains their pinky distinctive aspect. On the other hand, alcohol rinse makes that the CV-I complex gets tightly bound into the multi-layered, highly cross-linked gram positive cell wall, thus staining the cells purple. Besides Gram’s stain there’s a wide range of other staining methods available. By using appropriate dyes, different parts of the bacteria such as capsules, flagella, granules, and spores can be stained.

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