Biology Forums - Study Force

Compare and contrast gram-positive and gram-negative cell walls with regard to (a) sensitivity to antimicrobial agents, (b) resistance to phagocytosis, (c) chemical composition, and (d) decolorization by alcohol.

Both gram-positive and gram-negative bacteria have a cell wall made up of peptidoglycan and a phospholipid bilayer with membrane-spanning proteins. However, gram-negative bacteria have a unique outer membrane, a thinner layer of peptidoglycan, and a periplasmic space between the cell wall and the membrane. In the outer membrane, gram-negative bacteria have lipopolysaccharides (LPS), porin channels, and murein lipoprotein all of which gram-positive bacteria lack.

As opposed to gram-positive cells, gram-negative cells are resistant to lysozyme (phagocytosis) and penicillin attack. The gram-negative outer membrane which contains LPS, an endotoxin, blocks antibiotics, dyes, and detergents protecting the sensitive inner membrane and cell wall.

LPS is significant in membrane transport of gram-negative bacteria. LPS, which includes O-antigen, a core polysaccharide and a Lipid A, coats the cell surface and works to exclude large hydrophobic compounds such as bile salts and antibiotics from invading the cell. O-antigen are long hydrophilic carbohydrate chains (up to 50 sugars long) that extend out from the outer membrane while Lipid A (and fatty acids) anchors the LPS to the outer membrane.

A staining technique used to classify bacteria in which a bacterial specimen is first stained with crystal violet, then treated with an iodine solution, decolorized with alcohol, and counterstained with safranine. Gram-positive bacteria retain the violet stain; gram-negative bacteria do not. That means gram-negative bacteria is decolorized with alcohol.

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Post Merge: 9 years ago

The last picture is about gram-painting. Sorry, because I forgot to add the picture to my last post.