The Gram-negative bacterial cell wall consists of three primary layers.

  1. The cytoplasmic membrane
  2. A thin peptidoglycan layer, located in the periplasmic space
  3. An outer membrane containing lipopolysaccharides and porin proteins

Cartoon at the top shows a diagram of lipid bilayer cytoplasmic membrane at the bottom embedded with membrane proteins. Above this is a thin purple rectangle representing the thin peptidoglycan layer. Above this is an outer membrane lipid bilayer embedded with channels called porins. Lipoproteins are seen embedded in both the outer membrane and the thin peptidoglycan layer and connecting them. On the surface of the outer membrane are long chains of hexagons floating up labelled lipopolysaccharides. Lower picture presents a cartoon of a lipopolysaccharide. At the bottom is a circle with two wiggly legs labelled Lipid A. Attached to the circle is a polysaccharide composed of linked hexagons and circles. The pink hexagons and green circles closest to the lipid circular head are the inner core. Beyond this is the outer core composed of 5 blue hexagons linked to the inner core. Beyond this it the O-antigen composed of repeating units of 5 light blue hexagons.
Figure 1. Upper image. Gram-negative bacterial cell wall. Lower image. Lipopolysaccharide

Cytoplasmic membrane: Surrounds the cytoplasm of Gram-positive and Gram-negative bacteria and regulates the passage of molecules in and out of the bacteria. It is similar to the one found in the mammalian cell membrane and consists mainly of proteins and lipids. Peptidoglycan: The peptidoglycan of the Gram-negative cell wall is a thin (5-10 nm) mesh of repeating subunits containing sugars and amino acids.

The Periplasmic space: lies between the outer membrane and the cytoplasmic membrane. It consists of a gel-like matrix containing a wide variety of proteins that are important for various cell functions.

The outer membrane protects the bacteria and acts as a barrier towards certain antibiotics. In the outer membrane, we find:
- Porin proteins penetrate the outer membrane of Gram-negative bacteria, forming pores which allow passive diffusion of hydrophilic molecules between the bacteria and the exterior
- Lipopolysaccharides are specific to the outer leaflet of the outer membrane. They carry a negative charge, which contributes to maintaining the shape of the bacteria.
Lipopolysaccharides are notorious because they have the potential of causing toxic shock in the infected host.