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Bacterial Cell Wall: Gram-Negative & Gram-Positive

Bacterial Cell Wall: Gram-Negative & Gram-Positive

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    * Most bacteria associated with infectious disease can be categorized as gram-positive or gram-negative, which refers to whether they retain crystal violet stain, and depends on the composition of their cell walls.
Gram-positive
In a microscopic sample, we can see the bright purple stain – think Purple Positive*.
  • Cell wall has thick layer of peptidoglyclan (aka, murein and mucopeptide)
– It comprises a network of sugars and amino acids, and is the target of some antibiotic drugs. – We indicate two acids that are present in the cell wall: teichoic acids, which attach to the peptidoglycan, and, lipoteichoic acid, which are attached to the cytoplasmic membrane. – These surface acids contribute to the cell wall structure and charge.
Gram-negative
  • In a microscopic sample, gram negative bacteria appear reddish-pink.
  • They have a thin peptidoglycan layer, which is covered by the outer membrane.
  • The outer membrane is unique to gram negative bacteria and comprises lipopolysaccharide, aka, endotoxin.
– Endotoxin contributes to disease symptoms such as fever and shock.
Other stain types
  • Gram-staining is not appropriate for all bacterial strains.
  • We show an image of mycobacteria, which does not have a cell wall and is visualized with acid-fast staining methods.
  • We show Treponema pallidum, which is a bacterial strain that has very thin cell wall but can be seen with dark-field microscopy, as in our image, or fluorescent antibodies.
  • Some strains, such as the intracellular Chlamydiae, can be seen with hematoxylin and eosin (H&E) staining.

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