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LAB 3, M-265

DIFFERENTIAL STAINS

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GRAM STAINING PROCEDURE

Gram staining is a common technique used todifferentiate two large groups of bacteria based on theirdifferent cell wall constituents . The Gram stainprocedure distinguishes between Gram positive andGram negative groups by coloring these cells red orviolet . Gram positive bacteria stain violet due to thepresence of a thick layer of peptidoglycan in their cellwalls, which retains the crystal violet these cells arestained with. Alternatively, Gram negative bacteria stainred, which is attributed to a thinner peptidoglycan wall,which does not retain the crystal violet during thedecoloring process.

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How Does Gram Staining Work? Gram staining involves three processes: staining with a water-soluble dye called crystal

violet, decolorization, and counterstaining, usually with safanin. Due to differences in thethickness of a peptidoglycan layer in the cell membrane between Gram positive and Gramnegative bacteria, Gram positive bacteria (with a thicker peptidoglycan layer) retain crystal

violet stain during the decolorization process, while Gram negative bacteria lose thecrystal violet stain and are instead stained by the safranin in the final staining process. Theprocess involves three steps:

Cells are stained with crystal violet dye. Next, a Gram's iodine solution (iodine andpotassium iodide) is added to form a complex between the crystal violet and iodine. Thiscomplex is a larger molecule than the original crystal violet stain and iodine and isinsoluble in water.

A decolorizer such as ethyl alcohol or acetone is added to the sample, which dehydratesthe peptidoglycan layer, shrinking and tightening it. The large crystal violet-iodine complexis not able to penetrate this tightened peptidoglycan layer, and is thus trapped in the cell inGram positive bacteria. Conversely, the the outer membrane of Gram negative bacteria isdegraded and the thinner peptidoglycan layer of Gram negative cells is unable to retain thecrystal violet-iodine complex and the color is lost.

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1. A counterstain, such as the weakly water soluble safranin, is added to thesample, staining it red. Since the safranin is lighter than crystal violet, itdoes not disrupt the purple coloration in Gram positive cells. However,the decolorized Gram negative cells are stained red.

2. Reagentsa. Primary stain: Crystal violet (primary stain) 1 minb . Mordant: Iodine solution/Gram's Iodine (mordant that fixes crystal

violet to cell wall). -1 minc . Decolorizer : acetone alcohol (50%-50%) or ethanol 95%d . Counter stain :Safranin (secondary stain) 1 min

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Make a slide of cell sample to be stained (a smear) . Heat fix thesample to the slide by carefully passing the slide

with a drop orsmall piece of sample on it through a Bunsen burner three times.

Add the primary stain (crystal violet) to the sample/slide and

incubate for 1 minute. Rinse slide with a gentle stream of water fora maximum of 5 seconds to remove unbound crystal violet. Add Gram's iodine for 1 minute- this is a mordant, or an agent that

fixes the crystal violet to the bacterial cell wall. Rinse sample/slide with acetone or alcohol for 10-20 seconds and

rinse with a gentle stream of water . The acetone-alcohol willdecolorize the sample if it is Gram negative, removing the crystalviolet. However, if the alcohol remains on the sample for toolong, it may also decolorize Gram positive cells .

Add the secondary stain, safranin, to the slide and incubate for 1minute. Wash with a gentle stream of water for a maximum of 5seconds. If the bacteria is Gram positive, it will retain the primarystain (crystal violet) and not take the secondary stain (safranin),causing it to look violet/purple under a microscope. If the bacteriais Gram negative, it will lose the primary stain and take thesecondary stain, causing it to appear red when viewed under amicroscope. Observe your smears in the microscope under oilimmersion.

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HELPFUL SUGGESTIONS : a) DO NOT make your smears too thick ! b) Be very careful when you decolorize . c) Be sure your cultures are young, preferably less than 24 hours old. Older cultures tend to lose the ability to retain stains d)Fresh cultures must be used

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Gram staining

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Results of Gram staining Gram (-) Gram (+)