How does a gram positive and gram negative bacteria differ in cell structure's and How does this contribute to their differential staining properties?

Answer 1

Gram positive cells withold the crystal violet stain whilst gram negative cells are unable to and thus when washed with ethanol the stain washes out. This enables them to be free to take up the counterstain safranin.

http://en.wikipedia.org/wiki/Gram-negative_bacteria

OK, that is reason #1 why you don't take your information from Wiki...

Some history:
The Gram stain was developed in 1884 by the Danish bacteriologist Hans Christian Gram. It is one of the most useful staining procedures used today because it classifies bacteria into two large groups: your Gram-positive and your Gram-negative.

The steps in Gram staining:
1) A heat-fixed smear is covered with a basic purple dye, usually crystal violet. Because the purple stain imparts its color to all cells, it is referred to as a primary stain.

2) After a short time, the purple dye is washed off, and the smear is covered with iodine, a mordant, When the iodine is washed off, both gram-positive and gram-negative bacteria appear dark violet or purple.

3) Next, the slide is washed with alxohol of and alcohol-acetone solution. This solution is a decolorizing agent, which removes the purple from the cells of some species but not from others. (will go into detail in a second)

4) The alcohol is rinsed off, and the slide is then stained with safranin, a basic red dye. The smear is washed again, blotted dry, and examined microscopically.(this is where you decide if its gram-negative or positive.)

The Why:
The purple dye and the iodine combine in the cytoplasm of each bacterium and color it dark violet or purple. Bacteria that retain this color after the alcohol has attempted to decolorize them are classified as gram-positive; bacteria that lose the dark violet or purple color after colorization are classified as gram-negative. Because gram-negative bacteria are colorless after the alcohol wash, they are no longer visible. This is why the basic dy safranin is applied; it turns the gram-negative bacteria pink. Stains suxh as safranin that have contrasting color to the primary stain are called counterstains. Because gram-positive bacteria retain the original purple stain, they are not affected by the safranin counterstain.

The structure of G(-) and G(+):
Now that you know about the gram staining procedure i can tell you about the cell wall...

The cell wall of the bacterial cell is a complex, semi-rigid structure responsible for the shape of the cell.
The bacterial cell wall is composed of a macromolecular network called Peptidoglycan (AkA Murein), which is bresent either alone or in a combination with other substances. Peptidoglycan consists of a repeating disaccharide attached by polypeptides to form a lattice that surrounds and protects the entire cell. The disaccharide portion is made up of monosaccharides called N-acetylgluconamine (NAG) and N-acetylmuramic acid (NAM), with are related to glucose. Alternating NAM and NAG molecules are linked in rows of 10 to 65 sugars to form a carbohydrate "backbone" (the glycan portion of peptidoglycan). Adjacent rows are linked by polypeptides (the peptide portion of peptidoglycan). Althoguh the structure of the polypeptide link varies, is always included tetrapeptide side chains, which consist of four amino acids attached to NAMs in the backbone. The amino acids occur in an alternating pattern of D and L forms. This is unique because the amino acids found in other proteins are L forms. Parallel tatrapeptide side chains may be directly bonded to each other or linked by a peptide cross-bridge, consisting of a short chain of amino acids.

G(+) Cell Walls:
In most gram-positive bacteria, the cell wall consists of many layers of peptidoglycan, forming a thick, rigid structure. By contrast, gram-negative cell walls contain only a thin layer of peptidoglycan. In addition, the cell walls of gram-positive bacteria contain teichoic acids, which consists primarily of an alcohol and a phosphate. There are two classes of teichoic acids: lipoteichoic acid, which spans the peptidoglycan layer and is linked to the plasma membrane, and wall teichoic acid, which is linked to the peptidoglycan layer.

G(-) Cell Walls:
The cell walls of gram-negative bacteria consist of one or a very few layers of peptidoglycan and an outer membrane. The peptidoglycan is bonded to lipoproteins in the outer membrane and is in the periplasm, a gel-like fluid between the outer membrane and the plasma membrane. the periplasm contains a high concentration of degradative enzymes and transport proteins, Gram-negative cell walls do not contain teichoic acids. Because the cell walls of gram-negative bacteria contain only a small amount of peptidoglycan, they are more susceptible to mechanical breakage.

HOW IT ALL WORKS TOGETHER:
Now that you know about the Gram stain and the chemistry of the bacterial cell wall, it is easier to understand the mechanism of the Gram stain. The mechanism is based on differences in the structure of the cell walls of gram-positive and gram-negative bacteria and how each reacts to the various reagents. Crystal Violet, the primary stain, stains both gram-positive and gram-negative cells purple because the dye enters the cytoplasm of both types of cells. When iodine is applied, it forms large crystals with the dye that are too large to escape through the cell wall. The application of alcohol dehydrates the peptidoglycan of gram-positive cells to make it more impermeable to the crystal violet-iodine. The effect on gram-negative cells is quite different; alcohol dissolves the outer membrane of gram-negative cells and even leaves small holes in the thin peptidoglycan layer through which crystal violet-iodine diffuse. Because gram-negative bacteria are colorless after the alcohol wash, the addition of safranin turns the cells pink. Safranin provides a contrasting color to the primary stain (crystal violet). Although gram-positive and gram-negative cells both absorb safranin, the pink color of safranin is masked by the darker purple dye previously absorbed by gram-positive cells.

(hope it helped!)