Gram-Positive and Gram-Negative Bacterial Cell Walls

Gram-Positive and Gram-Negative Bacterial Cell Walls

Gram-Positive Bacterial Cell Walls

Overview

  • Gram-positive bacteria are distinguished by the thick peptidoglycan layer in their cell wall.
  • This cell wall is responsible for the characteristic purple stain seen in Gram staining due to the retention of crystal violet dye.
  • The teichoic acids, embedded within or linked externally to the peptidoglycan, are responsible for the strength of the bacterial cell wall.

Structure and Components

  • The cell wall has a primary component called peptidoglycan, which makes up to 80-95% of the cell wall, providing its rigidity.
  • Teichoic and lipoteichoic acids are the other major components contributing to their characteristic properties.

Gram-Negative Bacterial Cell Walls

Overview

  • Gram-negative bacteria are encased by a thinner peptidoglycan shell, making them stain pink in a Gram stain due to inability to retain crystal violet dye.
  • The cell walls of gram-negative bacteria are structurally more complex, with multiple layers.

Structure and Components

  • Gram-negative cells have a thin peptidoglycan layer, only making up to 10% of the bacterial cell wall.
  • Additionally, they have an outer membrane which is not present in Gram-positive bacteria. This membrane encompasses the peptidoglycan layer and has lipopolysaccharides and proteins imbedded within it.

Lipopolysaccharides (LPS)

  • The outer leaflet of the outer membrane is made of lipopolysaccharides (LPS), which is a strong endotoxin causing inflammatory responses.
  • LPS has three parts: O-antigen, core polysaccharide, and lipid-A. The lipid-A component triggers much of the immune response to Gram-negative bacteria.

Comparison of Gram-Positive and Gram-Negative Cell Walls

  • The key difference between the two is the thickness of their peptidoglycan layer and the presence of an outer membrane in Gram-negative bacteria.
  • This difference in structure means that Gram-negative bacteria are typically more resistant to antibiotics than Gram-positive bacteria, as they have an extra layer that the drug needs to penetrate.
  • It also means that, generally speaking, Gram-negative bacteria are more likely to be pathogenic due to the immune response triggered by LPS.
  • The colour change during Gram staining is due to the differences in the cell wall structure of the two types.