The components of the cell wall of Acid Fast Bacteria
Peptidoglycan:
Peptidoglycan is a complex, interwoven network that surrounds
the entire cell and is composed of a single covalently linked mac-
romoleculeUtis found only in bacterial cell walls.t provides rigid
support for the cell, is important in maintaining the characteristic
shape of the cell, and allows the cell to withstand low osmotic
pressure. A representative segment of the peptidoglycan layer is
shown in Figure 2-5,The term peptidoglycan is derived from
the peptides and the sugars (glycan) that make up the molecule
Synonyms for peptidoglycan are murein and mucopeptide.
Figure 2-5 illustrates the carbohydrate backbone, which
is composed of alternating N-acetylmuramic acid and N-
acetylglucosamine molecules. Attached to each of the muramic
acid molecules is a tetrapeptide consisting of both D- and
and
L-amino acids, the precise composition of which differs from
one bacterium to another, Two of these amino acids are wor-
thy of special mention: diaminopimelic acid, which is unique
to bacterial cell walls, and D-alanine, ywhich is involved in the
cross-links between the tetrapeptides and in the action of peni-
cillin. Note that this tetrapeptide contains, the rare D-1SOmers
of amino acids; most proteins'contain the L-isomer. The other
important component in this/network is the peptide cross-link
between the two tetrapeptides. T'he cross-links vary among spe-
cies; in Staphylococcus aureus, for example, five glycines link the
terminal D-alanine to the penultimate L-lysine.
Because
peptidoglycan is present in bacteria but not
in
human cells, it is a good target for antibacterial drugs. Several
of these drugs, such as penicillins, cephalosporins, and van-
comycin, inhibit the synthesis of peptidoglycan by inhibiting
the transpeptidase that makes the cross-links between the two
adjacent tetrapeptides (see Chapter 10).
Lysozynme, an enzyme present in human tears, mucus, and
saliva, can cleave the peptidoglycan backbone by breaking its
glycosyl bonds, thereby contributing to the natural resistance of
the host to microbial infection. Lysozyme-treated bacteria may
Swell and rupture as a result of the entry of water into thé cells,
which have a high internal osmotic pressure.
However, if the lysozyme-treated cells are in a solution with the same osmotic
pressure as that of the bacterial interior, they will survive as
spherical forms, called protoplasts, surrounded only by a cyto-
plasmic membrane.
Lipopolysaccharide:
The lipopolysaccharide (LPS) of the outer membrane of the cell
wall of gram-negative bacteria is endotoxin. It is responsible for
many of the features of disease, such as fever and shock (espe-
cially hypotension), caused by these organisms (see page 43). It
is called endotoxin because it is an integral part of the cell wall,
in contrast to exotoxins, which are actively secreted from the
bacteria. The constellation of symptoms caused by the endo-
toxin of one gram-negative bacterium is similar to another, but
the severity of the symptoms can differ greatly. In contrast,
the symptoms caused by exotoxins of different bacteria are usu-
ally quite different.
The LPS is composed of three distinct units (Figure 2-6):
(1) A phospholipid called lipid A, which is responsible for
the toxic effects.
(2) A core polysaccharide of five sugars linked through keto-
deoxyoctulonate (KD0) to lipid A.
(3) An outer polysaccharide consisting of up to 25 repeating
units of three to five sugars. This outer polymer is the impor-
tant somatic, or O, antigen of several gran-negative bacteria
that is used to identify certain organisms in the clinical labora-
tory. Some bacteria, notably members of the genus Neisseria,
have an outer lipooligosaccharide (LOS) containing very few
repeating units of sugars.
Teichoic Acid
Teichoic acids are fibers located in the outer layer of the
gram-positive cell wall and extend from it. They are composed
of polymers of either glycerol phosphate or ribitol phosphate.
Some polymers of glycerol teichoic acid penetrate the peptido-
glycan layer and are covalently linked to the lipid in the cyto-
plasmic membrane, in which case they are called lipoteichoic
acid; others anchor to the muramic acid of the peptidoglycan.
The medical importance of teichoic acids lies in their ability to
induce inflammation and septic shock when caused by certain
gram-positive bacteria; that is, they activate the same pathways
as does endotoxin (LPS) in gram-negative bacteria. Teichoic acids
also mediate the attachment of staphylococci to mucosal cells.
Gram-negative bacteria do not have teichoic acids.
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