Peptidoglycan layer

IV-Acetylglucosamine and IV-Acetylmuraniic acid Sugar derivatives in the peptidoglycan layer of bacterial cell walls. 

Figure 6.7 Formation of cross-linkage between individual peptide chains in the peptidoglycan layer of S. aureus.

Gram-negative bacteria have a more complex cell surface. The peptidoglycan layer is also the outer layer with respect to the cytoplasmic membrane. However, besides this, they have another outer polysaccharide membrane. This outer membrane is built out of lipopolysac-charides and lipoproteins, and can be a serious barrier for permeating hydrophilic molecules. 

The glycopeptides include vancomycin and teico-planin. They are bactericidal antibiotics. Their mechanism of action is based on inhibition of bacterial cell-wall synthesis by blocking the polymerization of glycopeptides. They do not act from within the peptidoglycan layer, as the beta-lactam antibiotics do, but intracellularly. The indications are mainly restricted to the management of severe or resistant staphylococcal infections, especially those caused by coagulase negative staphylococcal species such as S. epidermidis. 

Cephalosporins Have a nucleus that is similar to that of (3-lactams and similarly dismpt the synthesis of the peptidoglycan layer. This layer is important in maintaining cell wall integrity in gram-positive bacteria. Omnicef (Cefdinir) Cefzil (Cefprozil) Cephalexin (Keflex) Bioavailabihty is dose dependent is only about 16-21%. About 90% is excreted as the parent compound. It is well absorbed by the body and 90% of the dmg is excreted unchanged. 

Cephalosporins were first isolated from a fungus Cephalosporium sp. in the late 1940s. They have a nucleus that is analogous to that of penicillins and other (3-lactams. As a matter of fact, they have the same mode of action but disrupt the synthesis of the peptidoglycan layer of gram-positive bacterial cells. They are some of the most widely prescribed antibiotics, and, because of their broad-spectrum nature, cephalosporins are frequently dispensed to people who are allergic to penicillins. 

Vancomycin acts by inhibiting the correct synthesis of cell walls in gram-positive bacteria by specifically inhibiting the incorporation of V-acetylmuramic acid (NAM) and A-acetylglucosamine (NAG), two important peptide subunits that are present in the peptidoglycan layer of these types of bacteria. 

Gram-negative tougher peptidoglycan layer extensive internal membrane system with photosynthetic pigments... 

No outer membrane peptidoglycan layer outside plasma membrane... 

Teichoic and teichuronic acids. The cell walls of gram-positive bacteria are composed of a thick peptidoglycan layer which also contains proteins and additional polymers known as teichoic acids and teichuronic acids. In some species these account for 50% of the dry weight of the cells.598 599 Teichoic acids are high polymers of the following general types ... 

Nearly 800 of the foregoing tetrasaccharide units are joined to give a large, densely packed, 500-kDa polymer. The reducing end of the polymer is covalently attached to the peptidoglycan layer. 

Polysaccharide chains in the peptidoglycan layer (Fig. 8-29) of the cell walls of bacteria are attacked and cleaved by lysozymes,55 enzymes that occur in tears and other body secretions and in large amounts in egg white. Some bacteria and fungi, and even viruses, contain lysozymes.56 Their function is usually to protect against bacteria, but lysozyme of phage T4 is a component of the baseplate of the virus tail (Box 7-C). 

Group A streptococci, which are serious human pathogens, form a-helical coiled-coil threads whose C termini are anchored in the cell membrane. They protrude through the peptidoglycan layers and provide a hairlike layer around the bacteria. A variable region... 

The mycobacterial cell wall, discussed in Chapter 8, contains mycolic acids bound covalently at the nonreducing ends of arabinogalac-tans that are attached to the inner peptidoglycan layer,e as well as phosphatidylinositol-anchored lipoarabinomannans. Other unusual lipids that are also present and account for some of the difficulty of treatment with antibiotics include esters of mycocerosic acid with long-chain diols known as phenolphthiocerols and phthiocerols.f/fg... 

Bacteria utilize both D-alanine and D-glutamate in the synthesis of their peptidoglycan layers (Fig. 8-29). Both D-amino acids are formed by racemases. That of... 

As described in the first section of this chapter, D-alanine is an essential constituent of the peptidoglycan layer of bacterial cell wall, but it is not a common metabolite in mammalian cells. Therefore, alanine racemase has been recognized as a suitable target for antibiotics, and a variety of natural and synthetic inhibitors (inactivators) of the enzyme have been reported. 

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