Gram Positive vs Gram Negative: Key Differences

 Bacteria have cell walls made up of polysaccharides. That give them strength and rigidity. This is important since bacteria often experience variations in osmotic pressure due to the different solutions. They encounter and it is their cell walls which prevent them from shrinking or swelling. As a reminder as Osmosis is the process by which solvent molecules pass through a semipermeable membrane from a less concentrated solution to a more concentrated one equalising the concentration on either side of the membrane.

Nearly all bacterial cell walls have a peptide polysaccharide layer called peptidoglycan also known as moraine. Peptidoglycan is a polymer made up of sugars and amino acids, which forms a kind of mesh.

Bacteria can be classified based on their reaction to the Gram stain. Which identifies them as gram positive or gram-negative based on the chemical and physical properties of their cell walls.

Gram positive bacteria have a thick cell wall which consists of up to around 30 layers of peptidoglycan. this cell wall surrounds a mono derm which is a single plasma membrane.

Gram-negative bacteria have a much thinner cell wall consisting of a single layer of peptidoglycan. this layer of peptidoglycan is sandwiched between two lipid bilayer membranes called dye terms.

we can differentiate between gram-positive and gram-negative bacteria by dyeing them with crystal violet and then washing them with a decolorizing solution. Then a counter stain is added for example safranine or fruit scene.

gram-positive bacteria will retain the crystal violet dye and remain purple. while the gram negative bacteria will be stained pink.

"note" that gram positive bacteria also pick up the pink color of the counter stain. However this is not visible when they are dyed with the darker purple color of the crystal violet stain. the reason for these staining differences is due to differences in cell wall structure which is the chief difference between gram-positive and gram-negative bacteria.

the Gram stain detects peptidoglycan and since gram positive bacteria have a thick multi-layered peptidoglycan layer they retain the crystal violet dye. gram-negative bacteria do not retain the dye for two reasons.
1) They have an outer membrane getting in the way of the crystal violet and
2) They lack peptidoglycan to retain the stain.

Although both gram positive and gram-negative bacteria can be pathogenic. gram-negative bacteria are more resistant to antibodies because of their impenetrable cell wall. unfortunately these bacteria also develop resistance more quickly. Not all bacteria can be reliably classified through gram staining.

Acid-fast bacteria and gram variable bacteria for example do not respond to gram staining. acid-fast bacteria are bacteria whose cell walls retain stains particularly well although they aren't closely related to gram positive bacteria. They can't appear purple after the Gram stain test. Gram variable bacteria show a mix of pink and purple cells when stained. 

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