![]() ![]() ![]() Gram’s method involves staining the sample cells dark blue, decolorizing those cells with a thin cell wall by rinsing the sample, then counterstaining with a red dye. As noted above, the decolorization step is critical to the success of the procedure. In addition to the clearly gram-positive and gram-negative, there are many species that are “gram-variable” with intermediate cell wall structure (Beveridge and Graham 1991). It is the thickness of the cell wall that characterizes the response of the cells to the staining procedure. In Gram-negative bacteria it also dissolves the outer membrane of the gram-negative cell wall aiding in the release of the dye. The decolorizing mixture dehydrates cell wall, and serves as a solvent to rinse out the dye-iodine complex. The crystal violet stain is small enough to penetrate through the matrix of the cell wall of both types of cells, but the iodine-dye complex exits only with difficulty (Davies et al. Gram-positive bacteria have a thick mesh-like cell wall, gram-negative bacteria have a thin cell wall and an outer phospholipid bilayer membrane. This cell wall provides rigidity to the cell, and protection from osmotic lysis in dilute solutions. Do not smear.īacteria have a cell wall made up of peptidoglycan.
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