Structure and Function of Bacterial Cells (page 6)
(This chapter has 10 pages)
© Kenneth Todar, PhD
The Outer Membrane of
Of special interest as a
component of the
cell wall is the outer membrane, a discrete bilayered structure
on the outside of the peptidoglycan sheet (see Figure 18 below). For
bacterium, the outer membrane is first and foremost a permeability
but primarily due to its lipopolysaccharide content, it possesses many
interesting and important characteristics of Gram-negative bacteria.
outer membrane is a lipid bilayer intercalated with proteins,
resembling the plasma membrane. The inner face of the outer membrane is
composed of phospholipids similar to the phosphoglycerides that compose
the plasma membrane. The outer face of the outer membrane may contain
phospholipid, but mainly it is formed by a different type of
molecule which is composed of lipopolysaccharide
(LPS). Outer membrane
proteins usually traverse the membrane and in one case, anchor the
membrane to the underlying peptidoglycan sheet.
Figure 18. Schematic
of the outer membrane, cell wall and plasma membrane of a Gram-negative
bacterium. Note the structure and arrangement of molecules that
the outer membrane.
The LPS molecule that
constitutes the outer
of the outer membrane is composed of a hydrophobic region, called Lipid
A, that is attached to a hydrophilic linear polysaccharide region,
consisting of the core polysaccharide and the O-specific
Figure 19. Structure of LPS
The Lipid A head of the
molecule inserts into
the interior of the membrane, and the polysaccharide tail of the
faces the aqueous environment. Where the tail of the molecule inserts
the head there is an accumulation of negative charges such that a
cation is chelated between adjacent LPS molecules. This provides the
stability for the outer membrane, and explains why treatment of
bacteria with a powerful chelating agent, such as EDTA, causes
of LPS molecules.
lipopolysaccharides are toxic to
When injected in small amounts LPS or endotoxin activates
to produce pyrogens, activates the complement cascade causing
and activates blood factors resulting in intravascular coagulation and
hemorrhage. Endotoxins may play a role in infection by any
bacterium. The toxic component of endotoxin (LPS) is Lipid A. The
polysaccharide may provide ligands for bacterial attachment and confer
some resistance to phagocytosis. Variation in the exact sugar content
the O polysaccharide (also referred to as the O antigen) accounts for
antigenic types (serotypes) among Gram-negative bacterial pathogens.
even though Lipid A is the toxic component in LPS, the polysaccharides
nonetheless contribute to virulence of Gram-negative bacteria.
The proteins in the
outer membrane of Escherichia
coli are well characterized (see Table 5). About 400,00 copies of
lipoprotein are covalently attached to the peptidoglycan sheet at
end and inserted into the hydrophobic interior of the membrane at the
end. A group of trimeric proteins called porins form pores of a
fixed diameter through the lipid bilayer of the membrane. The
and omp F porins of E. coli are designed to allow
of hydrophilic molecules up to mw of about 750 daltons. Larger
or harmful hydrophobic compounds (such as bile salts in the intestinal
tract) are excluded from entry. Porins are designed in Gram-negative
to allow passage of useful molecules (nutrients) through the barrier of
the outer membrane, but to exclude passage harmful substances from the
environment. The ubiquitous omp A protein in the outer membrane
of E. coli has a porin like structure, and may function in
of specific ions, but it is also a receptor for the F pilus and an
site for bacterial viruses.
5. Functions of the
membrane components of Escherichia coli.
||Stabilizes LPS and is essential for its
||Anchors the outer membrane to peptidoglycan
|Omp C and Omp F porins
||proteins that form pores or channels through
for passage of hydrophilic molecules
|Omp A protein
||provides receptor for some viruses and
stabilizes mating cells during conjugation
S-layer proteins form the
outermost cell envelope component of a
spectrum of bacteria and archaea. S-layers are composed of a
protein or glycoprotein species (Mw 40-200 kDa) and exhibit either
oblique, square or hexagonal lattice symmetry with unit cell dimensions
in the range of 3 to 30 nm. S-layers are generally 5 to 10 nm thick and
show pores of identical size (diameter, 2 - 8 nm) and morphology.
Crystalline bacterial cell surface layer (S-layer) proteins have been
optimized during billions of years of biological evolution as
constituent elements of one of the simplest self-assembly systems in
nature. Isolated S-layer proteins have the intrinsic property to
recrystallize into two-dimensional arrays on a broad spectrum of
surfaces including silicon, metals and polymers, and to interfaces such
as planar lipid films and liposomes. The well defined arrangement of
functional groups on S-layer lattices allows the binding of molecules
and particles in defined regular arrays. S-layers also represent
templates for the formation of inorganic nanocrystal superlattices
composed of CdS, Au, Ni, Pt, or Pd.
The self-assembly of S-layers illustrates a basic building principle in
nature for generating large arrays of biomolecules with well-defined
geometrical and physicochemical surface properties.
Gram-negative and Gram-positive bacteria, as well a many archaea
possess a regularly structured layer called an S-layer attached to the
outermost portion of their cell wall. It is composed of protein or
glycoprotein and in electron micrographs, has a pattern resembling a
tiled surface. Transmission electron micrograph of a freeze-etched,
metal shadowed preparation of a bacterial cell with an S-layer with
symmetry. Bar = 100nm.
S-layers have been associated with a number of possible functions. The
S-layer may protect bacteria from harmful enzymes or changes in pH. It
may contribute to virulence by protecting the
bacterium against complement attack and phagocytosis. It is thought to
protect E. coli from attack
by the predatory bacterium, Bdellovibrio.
The S-layer can function as an adhesin, enabling the bacterium
to adhere to host cell membranes and environmental surfaces in order to
colonize. Many of the cell-associated protein adhesins used by
pathogens are components of the S-layer.
A correlation between
Gram stain reaction and
cell wall properties of bacteria is summarized in Table 6. The Gram
procedure contains a "destaining" step wherein the cells are washed
an acetone-alcohol mixture. The lipid content of the Gram-negative wall
probably affects the outcome of this step so that Gram-positive cells
a primary stain while Gram-negative cells are destained.
6. Correlation of
stain with other properties of Bacteria.
Cell Wall-less Forms
(1) A few Gram-negative bacteria
sensitive to natural penicillins. Many Gram-negative bacteria are
to some type of penicillin, especially semisynthetic penicillins.
bacteria, including E. coli, can be made sensitive to natural
by procedures that disrupt the permeability characteristics of the
|Thickness of wall
||thick (20-80 nm)
||thin (10 nm)
|Number of layers
|Peptidoglycan (murein) content
|Teichoic acids in wall
|Lipid and lipoprotein content
|Sensitivity to Penicillin G
|Sensitivity to lysozyme
(2) Gram-negative bacteria are
sensitive to lysozyme if pretreated by some procedure that removes the
outer membrane and exposes the peptidoglycan directly to the enzyme.
A few bacteria are able
to live or exist
a cell wall. The mycoplasmas are a group of bacteria that lack a cell
Mycoplasmas have sterol-like molecules incorporated into their
and they are usually inhabitants of osmotically-protected environments.
pneumoniae is the cause of primary atypical bacterial pneumonia,
in the vernacular as "walking pneumonia". For obvious reasons,
is ineffective in treatment of this type of pneumonia. Sometimes, under
the pressure of antibiotic therapy, pathogenic bacteria can revert
to cell wall-less forms (called spheroplasts or protoplasts) and persist or
in osmotically-protected tissues. When the antibiotic is withdrawn from
therapy the organisms may regrow their cell walls and reinfect