What are Gram negative cell walls?
The bacterial cells which give negative Gram’s stain test possess Gram negative cell walls. The peptidoglycan in them is joined with lipoproteins (lipids covalently linked to proteins) on the outer membrane. There is a space between the outer membrane and the plasma membrane which is referred to as periplasmic space. The high concentration of degradative enzymes and transport proteins are housed tn the periplasmic space. Gram negative cell walls are devoid of a substance called, teichoic acid, a polysaccharide found in Gram + ve cell walls.
The outer membrane of Gram -ve bacteria comprises lipoproteins, lipopolysaccharides and phospholipids. The outer membrane protects the cell from antibiotics like penicillin, digestive enzymes such as lysozyme, detergents heavy metals, bile salts and some dyes. Embedded in the outer membrane are certain proteins called porins which permit passage of molecules, e,g., nucleotides, disaccharides, peptides, amino acids, vitamin B12 and iron. The portion of lipopolysaccharide known as O-polysaccharide in the lipopolysaccharide of outer membrane provides the bacterial cell with its antigenic characteristics useful for identification and differentiation of Gram negative bacteria. The lipid portion of lipopolysaccharide known as Lipid A is an endotoxin and is toxic to the host’s blood stream or gastrointestinal tract. It is held responsible for fever and shock to the patient infected with pathogenic Gram negative bacteria.