Outer membrane of Gram-negative bacteria

As shown in Figure 9.24, the outer membrane of Gram-negative bacteria is coated with a highly complex lipopolysaccharide, which consists of a lipid group (anchored in the outer membrane) joined to a polysaccharide made up of long chains with many different and characteristic repeating structures... 

Endotoxins are the lipopolysaccharides (LPS) of the outer membrane of Gram-negative bacteria. They trigger inflammatory reactions in the infected organism, activate complement and cause fever or even a septic shock. They act on toll-like receptors. 

The outer membrane of gram-negative bacteria is a permeability barrier that allows the passive diffusion of small hydrophilic antibiotics only through aqueous channels, the porins. Drugs larger than 800 Da are... 

A great deal of our current understanding of the structure and function of the outer membrane of Gram-negative bacteria has come from studies with Escherichia coli and Salmonella typhimurium. The permeability barrier function of the outer membrane can... 

Figure 7.7 Structure of a generalized LPS molecule. LPS constitutes the major structural component of the outer membrane of Gram-negative bacteria. Although LPSs of different Gram-negative organisms differ in their chemical structure, each consists of a complex polysaccharide component, linked to a lipid component. Refer to text for specific details...

TRANSPORT ACROSS THE OUTER MEMBRANE OF GRAM-NEGATIVE BACTERIA... 

Figure 2.7. The outer membrane of Gram-negative bacteria. Abbreviations LB, LamB protein LP, lipoprotein O, OmpA protein MP, membrane protein. The peptidoglycan backbone consists of alternating residues of A-acetylglucosamine and /V-acetylmuramic acid, which are cross-linked via short peptides.

The insertion of (i-barrel precursors is one of the two translocation processes, besides the sorting of inner membrane and IMS proteins, that are clearly derived from a eubacterial translocation system. (1-Barrel proteins are exclusively found in the outer membranes of Gram-negative bacteria and of endosymbiotic organelles such as mitochondria and plastids (Wim-... 

The lipid membranes are a few nanometers thick. They contain proteins whose role is to actively transport particular target chemicals across these nonpolar barriers. The outer membranes of gram negative bacteria also have protein channels called porins that allow passage of small polar and charged substrates. 

Lipopolysaccharides are the dominant surface feature of the outer membrane of gram-negative bacteria such as Escherichia coli and Salmonella ty-phimurium. These molecules are prime targets of the antibodies produced by the vertebrate immune system in response to bacterial infection and are therefore important determinants of the serotype of bacterial strains (serotypes are strains that are distinguished on the basis of antigenic properties). The lipopolysaccharides of S. typhimurium contain six fatty acids bound to two... 

Porins. The outer membranes of gram-negative bacteria contain several 34- to 38-kDa proteins known... 

Transport and Cell Penetration. One of the causes of bacterial resistance to the cephalosporins is poor transport of the antibiotic through tlie outer membrane of gram-negative bacteria. This lipid-bilayer membrane carries receptor proteins for the recognition and transport of essential nutrients, but provides an effective barrier to large molecules. In the case of the cephalosporins there can be a considerable difference between the concentration required to inhibit intact cells and the concentrations required to saturate the target enzymes in broken cell piepaiations. 

Some membranes contain relatively large pores which allow for the free passage of molecules with molecular weights up to about 600. For example the outer membranes of gram negative bacteria contain pores with diameters of about 10 A which are formed from proteins called porins. 

While gramicidin and other channel formers can show high transport rates, they do not show the high selectivity that characterizes natural channels. There is much interest at present in a class of proteins called porins, which form natural pores in the outer membranes of Gram-negative bacteria. Several different porin proteins have been isolated from Escherichia coli. These form water-filled channels of various sizes in membranes. Thus the proteins OmpC and OmpF seem to be cation-specific channels while other proteins give larger diameter channels that seem to be specific for anions.34,35... 

Helenius and Simmons (1975) discussed the solubilization of bacteria cell membranes with detergents or surfactants, and Nixdorff (1978) discussed the interaction of lipopolysaccharides with detergents and the possible role in the detergent resistance of the outer membranes of gram-negative bacteria. 

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