Gram-negative cell walls

•        Much more complex than gram-positive walls

•        The thin peptidoglycan layer - not more than 5 to 10% of the wall weight

•        (E. coli - 2 nm thick; only one or two layers or sheets of peptidoglycan)

•        The outer membrane lies outside the thin peptidoglycan layer

•         The most abundant membrane protein is Braun’s lipoprotein, a small lipoprotein joined to the peptidoglycan and extends to the outer membrane by its hydrophobic end 

                        

•        Outer membrane contains large, complex lipopolysaccharides (LPSs) and consist of three parts: (1) lipid A, (2) the core polysaccharide, and (3) the O side chain.

•        The lipid A region buried in the outer membrane and part of the LPS molecule projects from the surface

•        The core polysaccharide is joined to lipid A; In Salmonella, it is constructed of 10 sugars, many of them unusual in structure.

•        The O side chain or O antigen is a polysaccharide chain extending outward from the core; varies in composition between bacterial strains

•        O side chains readily recognized by host antibodies- gram- negative bacteria overcome host defenses by rapidly changing the nature of their O side chains to avoid detection.

•        Antibody interaction with the LPS before reaching the outer membrane protect the cell wall from direct attack

•        LPS contributes to the negative charge on the bacterial surface (core polysaccharide usually contains charged sugars and phosphate )

•        stabilizes membrane structure

•        Lipid A is toxic; LPS can act as an endotoxin; gram-negative bacterial infections

•        Protective barrier- prevents or slows the entry of bile salts, antibiotics, and other toxic substances

•        Outer membrane more permeable than the plasma membrane; permits the passage of small molecules like glucose and other monosaccharides

•        Presence of special porin proteins (Larger molecules such as vitamin B12 must be transported across the outer membrane by specific carriers )

•        The outer membrane also prevents the loss of constituents like periplasmic enzymes

•        A space is seen between the plasma membrane and the outer membrane in gram- negative bacteria, and between the plasma membrane and cell wall in gram- positive bacteria-periplasmic space containing periplasm

•        The periplasmic space contains many proteins, for example, hydrolytic enzymes, transport proteins etc

•         The periplasmic space also contains enzymes involved in peptidoglycan synthesis and the modification of toxic compounds that could harm the cell.