Structure and Function of Bacterial Cells (page 5)
(This chapter has 10 pages)
© Kenneth Todar, PhD
bacteria deserve special
for several reasons:
1. They are an essential structure for viability,
as described above.
2. They are composed of
unique components found
nowhere else in nature.
3. They are one of the most
important sites for
attack by antibiotics.
4. They provide ligands for
sites for drugs or viruses.
5. They cause symptoms of
disease in animals.
6. They provide for
and immunological variation among strains of bacteria.
Most procaryotes have a
rigid cell wall.
The cell wall is an essential structure that protects the cell
from mechanical damage and from osmotic rupture or lysis.
usually live in relatively dilute environments such that the
of solutes inside the procaryotic cell cytoplasm greatly exceeds the
solute concentration in the outside environment. Thus, the osmotic
against the inside of the plasma membrane may be the equivalent of
atm. Since the membrane is a delicate, plastic structure, it must be
by an outside wall made of porous, rigid material that has high tensile
strength. Such a material is murein, the ubiquitous component
bacterial cell walls.
Murein is a unique type of
peptidoglycan, a polymer of disaccharides (glycan) cross-linked
by short chains
amino acids (peptide). Many types of peptidoglycan exist. All Bacterial
peptidoglycans contain N-acetylmuramic acid, which is the
component of murein. The cell walls of Archaea may be
of protein, polysaccharides, or peptidoglycan-like molecules, but never
do they contain murein. This feature distinguishes the Bacteria
from the Archaea.
Bacteria (those that retain the purple crystal violet dye when
to the Gram-staining procedure), the cell wall consists
of several layers of peptidoglycan. Running perpendicular to
the peptidoglycan sheets is a group of molecules called teichoic acids
which are unique to the Gram-positive cell wall (Figure 14).
Structure of the Gram-positive bacterial cell wall. The wall is
relatively thick and consists of many layers of peptidoglycan
interspersed with teichoic acids that run perpendicular to the
In the Gram-negative
Bacteria (which do not retain the crystal violet), the cell wall
composed of a single layer of
surrounded by a membranous structure called the outer membrane.
The outer membrane of Gram-negative bacteria invariably contains a
component, lipopolysaccharide (LPS or endotoxin),
which is toxic to animals. In Gram-negative bacteria the outer membrane
is usually thought of as part of the cell wall (Figure
15. Structure of the Gram-negative cell wall. The wall is relatively
thin and contains much less peptidoglycan than the Gram-positive wall.
Also, teichoic acids are absent. However, the Gram negative cell wall
consists of an outer membrane that is outside of the peptidoglycan
layer. The outer membrane is attached to the peptidoglycan sheet by a
unique group of lipoprotein molecules.
Bacteria, the cell wall is thick
consisting of several layers of peptidoglycan. In the Gram-negative
Bacteria the cell wall is
relatively thin (10 nanometers) and is composed of a single layer of
surrounded by an outer membrane.
and arrangement in E.
coli is representative of all Enterobacteriaceae, as well
Gram-negative bacteria. The glycan backbone is made up of
molecules of N-acetylglucosamine (G) and N-acetylmuramic acid (M)
by a beta 1,4-glycoside bond. The 3-carbon of N-acetylmuramic acid (M)
substituted with a lactyl ether group derived from pyruvate. The lactyl
ether connects the glycan backbone to a peptide side chain that
L-alanine, (L-ala), D-glutamate (D-glu), Diaminopimelic acid (DAP), and
D-alanine (D-ala). MurNAc is unique to bacterial cell walls, as is
DAP and D-ala. The muramic acid subunit of E. coli is shown in
Figure 16. The structure of
the muramic acid subunit of the peptidoglycan of Escherichia coli.
This is the type of murein found in most Gram-negative bacteria. The
backbone is a repeat polymer of two amino sugars, N-acetylglucosamine
and N-acetylmuramic acid (M). Attached to the N-acetylmuramic acid is a
tetrapeptide consisting of L-ala-D-glu-DAP-D-ala. b. Abbreviated
of the muramic acid subunit. c. Nearby tetrapeptide side chains may be
linked to one another by an interpeptide bond between DAP on one chain
and D-ala on the other. d. The polymeric form of the molecule.
Strands of murein
are assembled in the
periplasm from about 10 muramic acid subunits. Then the strands are
to form a continuous glycan molecule that encompasses the cell.
their proximity allows it, the tetrapeptide chains that project from
glycan backbone can be cross-linked by an interpeptide bond
a free amino group on DAP and a free carboxy group on a nearby D-ala.
assembly of peptidoglycan on the outside of the plasma membrane is
by a group of periplasmic enzymes, which are transglycosylases,
and carboxypeptidases. The mechanism of action of penicillin and
beta-lactam antibiotics is to block transpeptidase and
enzymes during their assembly of the murein cell wall. Hence, the
lactam antibiotics are said to "block cell wall synthesis" in the
The glycan backbone of
can be cleaved by an enzyme called lysozyme that is present in
serum, tissues and secretions, and in the phagocytic lysosome. The
of lysozyme is to lyse bacterial cells as a constitutive defense
bacterial pathogens. Some Gram-positive bacteria are very sensitive to
lysozyme and the enzyme is quite active at low concentrations.
secretions (tears) can be diluted 1:40,000 and retain the ability to
certain bacterial cells. Gram-negative bacteria are less vulnerable to
attack by lysozyme because their peptidoglycan is shielded by the outer
membrane. The exact site of lysozymal cleavage is the beta 1,4 bond
N-acetylmuramic acid (M) and N-acetylglucosamine (G) , such that the
acid subunit shown in Figure 16(a) is the result of the action of
on bacterial peptidoglycan.
bacteria there are numerous
peptide arrangements among peptidoglycans. The best studied is the
of Staphylococcus aureus shown in Figure 17 below. In place of
E. coli) is the diamino acid, L-lysine (L-lys), and in place
of the interpeptide bond (in Gram-negatives) is an interpeptide
of amino acids that connects a free amino group on lysine to a free
group on D-ala of a nearby tetrapeptide side chain. This arrangement
allows for more frequent cross-bonding between nearby tetrapeptide side
chains. In S. aureus, the interpeptide bridge is a peptide
of 5 glycine molecules (called a pentaglycine bridge). Assembly
of the interpeptide bridge in Gram-positive murein is inhibited by the
beta lactam antibiotics in the same manner as the interpeptide bond in
Gram-negative murein. Gram-positive bacteria are more sensitive to
than Gram-negative bacteria because the peptidoglycan is not protected
by an outer membrane and it is a more abundant molecule. In
bacteria, peptidoglycans may vary in the amino acid in place of DAP or
L-lys in position 3 of the tetrapeptide, and in the exact composition
the interpeptide bridge. At least eight different types of
exist in Gram-positive bacteria.
Figure 17. Schematic diagram
of the peptidoglycan sheet of Staphylococcus aureus. G =
M = N-acetyl-muramic acid; L-ala = L-alanine; D-ala = D-alanine; D-glu
= D-glutamic acid; L-lys = L-lysine. This is one type of murein found
Gram-positive bacteria. Compared to the E. coli peptidoglycan
7) there is L-lys in place of DAP (diaminopimelic acid) in the
The free amino group of L-lys is substituted with a glycine
(gly-gly-gly-gly-gly-) which then becomes an interpeptide bridge
a link with a carboxy group from D-ala in an adjacent tetrapeptide side
chain. Gram-positive peptidoglycans differ from species to species,
in regards to the amino acids in the third position of the tetrapeptide
side chain and in the amino acid composition of the interpeptide bridge.
may contain a single
layer of murein in their cell walls while Gram-positive bacteria are
to have several layers or "wraps" of peptidoglycan. Closely associated
with the layers of peptidoglycan in Gram-positive bacteria are a group
of molecules called teichoic acids. Teichoic acids are linear
of polyglycerol or polyribitol substituted with phosphates and a few
acids and sugars. The teichoic acid polymers are occasionally anchored
to the plasma membrane (called lipoteichoic acid, LTA) apparently
outward at right angles to the layers of peptidoglycan. The functions
teichoic acid are not known. They are essential to viability of
bacteria in the wild. One idea is that they provide a channel of
negative charges for threading positively charged substances through
complicated peptidoglycan network. Another theory is that teichoic
are in some way involved in the regulation and assembly of muramic acid
subunits on the outside of the plasma membrane. There are instances,
in the streptococci, wherein teichoic acids have been implicated in the
adherence of the bacteria to tissue surfaces.