GRAM POSITIVE VS GRAM NEGATIVE BACTERIA
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Bacteria have cell walls made of polysaccharides that give them strength and rigidity. This is important because bacteria often experience changes in osmotic force due to solutions in their environment and their cell walls prevent them from shrinking or swelling. As a reminder, osmosis is the process by which solvent molecules pass through a semi-permeable membrane from a less concentrated solution to a more concentrated solution, equalizing the concentration on either side of the membrane. Almost all bacterial cell walls have a peptide-polysaccharide layer called peptidoglycan, or sometimes murein. Peptidoglycan is a polymer made of sugars and amino acids that forms a sort of mesh.
Bacteria can be classified based on their response to Gram staining, which identifies them based on the chemical and physical properties of their cell walls, as Gram positive or Gram negative. Gram-positive bacteria have a thick cell wall, composed of approximately 30 layers of peptidoglycan. This cell wall surrounds a monoderm, which is a single plasma membrane. Gram-negative bacteria have a MUCH thinner cell wall, made up of a single layer of peptidoglycan. This peptidoglycan layer is sandwiched between two lipid bilayer membranes called didermis.
We can differentiate Gram-positive bacteria from Gram-negative bacteria by staining them with crystal violet and then washing them with a bleaching solution. Then a countercolor is added, for example safranin or fuchsin. Gram-positive bacteria will retain the crystal violet dye and remain purple, while Gram-negative bacteria will be colored pink. Note that Gram-positive bacteria also pick up the pink color of the counterstain, but this is not visible when dyed with the darker purple color of the crystal violet stain.
The reason for these differences in staining is due to differences in cell wall structure, which is the main difference between Gram-positive and Gram-negative bacteria. The Gram stain detects peptidoglycan, and because Gram-positive bacteria have a thick layer of multi-layered peptidoglycan, they retain the crystal violet dye. Gram-negative bacteria do not retain the dye for two reasons: they have an outer membrane that obstructs crystal violet, and 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 due to their impenetrable cell wall. Unfortunately, these bacteria also develop resistance more quickly.
Not all bacteria can be reliably classified by Gram stain. Acid-fast bacteria or Gram-variable bacteria, for example, do not respond to Gram staining. Acid-fast bacteria have cell walls that hold stains particularly well. Although they are not closely related to Gram-positive bacteria, they may appear purple after the Gram stain test. Gram-variable bacteria show a mixture of pink and purple cells when stained.
Bacteria can be classified based on their response to Gram staining, which identifies them based on the chemical and physical properties of their cell walls, as Gram positive or Gram negative. Gram-positive bacteria have a thick cell wall, composed of approximately 30 layers of peptidoglycan. This cell wall surrounds a monoderm, which is a single plasma membrane. Gram-negative bacteria have a MUCH thinner cell wall, made up of a single layer of peptidoglycan. This peptidoglycan layer is sandwiched between two lipid bilayer membranes called didermis.
We can differentiate Gram-positive bacteria from Gram-negative bacteria by staining them with crystal violet and then washing them with a bleaching solution. Then a countercolor is added, for example safranin or fuchsin. Gram-positive bacteria will retain the crystal violet dye and remain purple, while Gram-negative bacteria will be colored pink. Note that Gram-positive bacteria also pick up the pink color of the counterstain, but this is not visible when dyed with the darker purple color of the crystal violet stain.
The reason for these differences in staining is due to differences in cell wall structure, which is the main difference between Gram-positive and Gram-negative bacteria. The Gram stain detects peptidoglycan, and because Gram-positive bacteria have a thick layer of multi-layered peptidoglycan, they retain the crystal violet dye. Gram-negative bacteria do not retain the dye for two reasons: they have an outer membrane that obstructs crystal violet, and 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 due to their impenetrable cell wall. Unfortunately, these bacteria also develop resistance more quickly.
Not all bacteria can be reliably classified by Gram stain. Acid-fast bacteria or Gram-variable bacteria, for example, do not respond to Gram staining. Acid-fast bacteria have cell walls that hold stains particularly well. Although they are not closely related to Gram-positive bacteria, they may appear purple after the Gram stain test. Gram-variable bacteria show a mixture of pink and purple cells when stained.
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