* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download 351 CHAPTER 21 Gram-Positive Cell Wall
Survey
Document related concepts
Biochemical switches in the cell cycle wikipedia , lookup
Cell encapsulation wikipedia , lookup
Cell nucleus wikipedia , lookup
Cellular differentiation wikipedia , lookup
Cell culture wikipedia , lookup
Extracellular matrix wikipedia , lookup
Signal transduction wikipedia , lookup
Cell growth wikipedia , lookup
Organ-on-a-chip wikipedia , lookup
Cell membrane wikipedia , lookup
Cytokinesis wikipedia , lookup
Transcript
THE NATURE OF BACTERIA CHAPTER 21 351 P The Gram-negative cell wall PM OM The Gram-positive cell wall PM W Peptidoglycan Plasma membrane Cell wall Cell wall M Outer membrane Peptidoglycan Plasma membrane P Periplasmic space FIGURE 21–4. Gram positive and Gram negative cell walls. M—peptidoglycan or murein layer; OM, outer membrane; PM, plasma membrane; P, periplasmic space; W, Gram-positive peptidoglycan wall. (Reproduced with permission from Willey J, Sherwood L, Woolverton C (eds). Prescott’s Principles of Microbiology. New York: McGraw-Hill; 2008.) Virtually all bacteria with walls can now be assigned a Gram category even if they cannot be visualized with the stain itself for technical reasons. Examples include the causative agents of tuberculosis and syphilis. Mycobacterium tuberculosis (Gram-positive) has lipids in its cell wall that resist the uptake of most stains. Treponema pallidum (Gram-negative) takes stains poorly but is also too thin to be resolved in the light microscope without special illumination. In these cases, the Gram categorization is based on electron microscopy (Figure 21–4) and chemical analysis of the cell wall. Poorly staining bacteria still have a Gram category Gram-Positive Cell Wall The Gram-positive cell wall contains two major components, peptidoglycan and teichoic acids, plus additional carbohydrates and proteins, depending on the species. A generalized scheme illustrating the arrangement of these components is shown in Figure 21–5. The chief component is peptidoglycan, which is found nowhere except in prokaryotes. Peptidoglycan consists of a linear glycan chain of two alternating sugars, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) (Figure 21–6). Each muramic acid residue bears a tetrapeptide of alternating l- and d-amino acids. Adjacent glycan chains are cross-linked into sheets by peptide bonds between the third amino acid of one tetrapeptide and the terminal d-alanine of another. The same cross-links between other tetrapeptides connect the sheets to form a three-dimensional, rigid matrix. The crosslinks involve perhaps one third of the tetrapeptides and may be direct or may include a peptide bridge, as, for example, a pentaglycine bridge in Staphylococcus aureus. The crosslinking extends around the cell, producing a scaffold-like giant molecule. Peptidoglycan is much the same in all bacteria, except that there is diversity in the nature and frequency of the cross-linking bridge and in the nature of the amino acids at certain positions of the tetrapeptide. The peptidoglycan sac derives its great mechanical strength from the fact that it is a single, covalently bonded structure. Most enzymes found in mammalian hosts and other biologic systems do not degrade peptidoglycan; one important exception is lysozyme, the hydrolase in tears and other secretions, which cleaves the β-1,4 glycosidic bond between muramic acid and glucosamine residues. The role of the peptidoglycan component of the cell wall in conferring osmotic resistance and shape on the cell is easily demonstrated by Ryan_CH21_p345-386.indd 351 Major components of Grampositive walls are peptidoglycan and teichoic acid Peptidoglycan comprises glycan chains cross-linked by peptide chains Scaffold-like sac surrounds cell Components of peptidoglycan provide resistance to most mammalian enzymes 9/18/09 8:55:13 PM 352 PART I I I PATHOGENIC BACTERIA Teichoic acid Peptidoglycan Lipoteichoic acid Plasma membrane Perplasmic space FIGURE 21–5. Gram-positive envelope. (Reproduced with permission from Willey J, Sherwood L, Woolverton C (eds). Prescott’s Principles of Microbiology. New York: McGraw-Hill; 2008.) Loss of cell wall leads to lysis or production of protoplasts removing or destroying it. Treatment of a Gram-positive cell with penicillin (which blocks formation of the tetrapeptide cross-links) destroys peptidoglycan sac, and the wall is lost. Prompt lysis of the cell ensues. If the cell is protected from lysis by suspension in a medium approximately isotonic with the cell interior, such as 20% sucrose, the cell becomes round and forms a sphere called a protoplast. ::: Penicillin action, p. 406 A second component of the Gram-positive cell wall is a teichoic acid. These compounds are polymers of either glycerol phosphate or ribitol phosphate, with various sugars, amino sugars, and amino acids as substituents. The lengths of the chain and the nature and location of the substituents vary from species to species and sometimes among strains within a species. Up to 50% of the wall may be teichoic acid, some of N-Acetylmuramic acid N-Acetylglucosamine FIGURE 21–6. Peptidoglycan structure. A schematic diagram of one model of peptidoglycan. Shown are the polysaccharide chains, tetrapeptide side chains and peptide bridges. (Reproduced with permission from Willey J, Sherwood L, Woolverton C (eds). Prescott’s Principles of Microbiology. New York: McGraw-Hill; 2008.) Ryan_CH21_p345-386.indd 352 Peptide chain Pentaglycine interbridge 9/18/09 8:55:15 PM THE NATURE OF BACTERIA which is covalently linked to occasional NAM residues of the peptidoglycan. Of the teichoic acids made of polyglycerol phosphate, much is linked not to the wall but to a glycolipid in the underlying cell membrane. This type of teichoic acid is called lipoteichoic acid and seems to play a role in anchoring the wall to the cell membrane and as an epithelial cell adhesin. Besides the major wall components—peptidoglycan and teichoic acids—Gram-positive walls usually have lesser amounts of other molecules characteristic of their species. Some are polysaccharides, such as the group-specific antigens of streptococci; others are proteins, such as the M protein of group A streptococci. ::: M protein, p. 446 CHAPTER 21 353 Teichoic and lipoteichoic acids promote adhesion and anchor wall to membrane Other cell wall components related to species Gram-Negative Cell Wall The second kind of cell wall found in bacteria, the Gram-negative cell wall, is depicted in Figure 21–7. Except for the presence of peptidoglycan, there is little chemical resemblance to cell walls of Gram-positive bacteria, and the architecture is fundamentally different. In Gram-negative cells, the amount of peptidoglycan has been greatly reduced, with some of it forming a single-layered sheet around the cell and the rest forming a gel-like substance, the periplasmic gel, with little cross-linking. External to this periplasm is an elaborate outer membrane. The proteins in solution in the periplasm consist of enzymes with hydrolytic functions, sometimes antibiotic-inactivating enzymes, and various binding proteins with roles in chemotaxis and in the active transport of solutes into the cell. Oligosaccharides secreted into the periplasm in response to external conditions serve to create an osmotic pressure buffer for the cell. The periplasm is an intermembrane structure, lying between the cell membrane and a special membrane unique to Gram-negative cells, the outer membrane. This has an overall structure similar to most biologic membranes with two opposing phospholipid–protein O-specific side chains of LPS Porin Thin peptidoglycan sac is imbedded in periplasmic gel Periplasmic proteins have transport, chemotactic, and hydrolytic roles Lipopolysaccharide (LPS) Lipoprotein Outer membrane Periplasmic space and peptidoglycan Plasma membrane Phospholipid Integral protein Peptidoglycan FIGURE 21–7. Gram-negative envelope. (Reproduced with permission from Willey J, Sherwood L, Woolverton C (eds). Prescott’s Principles of Microbiology. New York: McGraw-Hill; 2008.) Ryan_CH21_p345-386.indd 353 9/18/09 8:55:17 PM