Download 351 CHAPTER 21 Gram-Positive Cell Wall

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
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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
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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.)
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Peptide
chain
Pentaglycine
interbridge
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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
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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.)
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