Gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their cell wall.
Gram-positive bacteria take up the crystal violet stain used in the test, and then appear to be purple-coloured when seen through a microscope. This is because the thick peptidoglycan layer in the bacterial cell wall retains the stain after it is washed away from the rest of the sample, in the decolorization stage of the test.
Gram-negative bacteria cannot retain the violet stain after the decolorization step; alcohol used in this stage degrades the outer membrane of gram-negative cells making the cell wall more porous and incapable of retaining the crystal violet stain. Their peptidoglycan layer is much thinner and sandwiched between an inner cell membrane and a bacterial outer membrane, causing them to take up the counterstain (safranin or fuchsine) and appear red or pink.
Penicillin Bound to Penicillin Binding
Gram-positive bacteria take up the crystal violet stain used in the test, and then appear to be purple-coloured when seen through a microscope. This is because the thick peptidoglycan layer in the bacterial cell wall retains the stain after it is washed away from the rest of the sample, in the decolorization stage of the test.
Gram-negative bacteria cannot retain the violet stain after the decolorization step; alcohol used in this stage degrades the outer membrane of gram-negative cells making the cell wall more porous and incapable of retaining the crystal violet stain. Their peptidoglycan layer is much thinner and sandwiched between an inner cell membrane and a bacterial outer membrane, causing them to take up the counterstain (safranin or fuchsine) and appear red or pink.
Despite their thicker peptidoglycan layer, gram-positive bacteria are more receptive to antibiotics than gram-negative, due to the absence of the outer membrane (1)
Penicillin is effective only against Gram-positive bacteria because Gram
negative bacteria have a lipopolysaccharide and protein layer that
surrounds the peptidoglygan layer of the cell wall, preventing penicillin from attacking.
Penicillin kills bacteria by inhibiting the proteins which cross-link peptidoglycans in the cell wall
•When a bacterium divides in the presence of penicillin, it cannot fill in
the “holes” left in its cell wall.
•The bacterium is so filled with solutes compared to the surrounding solution
that the water rushes in, and without a full cell wall to support the bacterium,
it “pops” from the turgor pressurePenicillin Bound to Penicillin Binding
Protein 4 (PBP4) Penicillin (shown in the center of the protein as CPK and ball and stick format) acts on a group of proteins (in this case, PBP4) that help form new bacterial wall (3)
References:
1 - https://en.wikipedia.org/wiki/Gram-positive_bacteria
2- Medical microbiology blog: https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg_WYvslNVxhMBLGx7WB6d_-9x-sQ2UQDVwVsEz8l-1aI6VRev9TrYk0ksq8CECnkUWWEvcfVnXssI6D5YIEbtN_NthumjfQyBa1TgNqvDrV9rMmh4Fn7EE-Zdg6O3oge0L0JLGQLgOUMU/s1600/Gram-Staining-Procedure.jpg
3 - http://cbm.msoe.edu/includes/pdf/smart2007/mwp2007.pdf
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