Download The bacterial cell wall!

The bacterial cell wall!
Major functions:
Protection from osmotic changes
Determination and maintenance of cell
shape
Bacterial Cell Walls are made
of
•  PEPTIDOGLYCAN
•  Peptidoglycan has a polysaccharide
portion and a peptide portion.
•  The polysaccharide is a complex
polymer consisting of alternating
monosaccharides called
N-acetylglucosamine (NAG) and
N-acetyl muramic acid (NAM).
•  Short polypeptide chains cross-link, or
join together, many polysaccharide
chains; the polypeptides are always
attached to the NAM monosaccharide.
•  Also attached to the NAM is a
tetrapeptide side chain, which is
involved in the binding of the peptides
to hold together the whole structure.
Peptidoglycan Structure
The gram-positive cell wall
•  Consists of many
layers of
peptidoglycan
surrounding the
plasma membrane
•  Also contains
techoic acids and
lipotechoic acids
(probably function in
joining layers of
peptidoglycan and
anchoring wall to
plasma membrane)
The gram-negative cell wall
•  MORE COMPLEX!!
•  Consists of only a single (or very few
layers) of peptidoglycan surrounding the
plasma membrane, and
•  ANOTHER MEMBRANE (The outer
membrane)!
The outer membrane
•  The outer membrane is a phospholipid
bilayer like the plasma membrane, but
also contains
–  Lipoproteins which anchor it to the cell wall
and plasma membrane
–  Lipopolysaccharides (which contain Lipid A,
an endotoxin, and O polysaccharide, an
antigen)
–  Porin proteins, which allow transport of small
hydrophilic molecules across the outer
membrane
Outer membrane structures
and functions
•  Protect cell by excluding large toxic
molecules (like penicillin, to which they
are relatively resistant)
•  Creates a periplasmic space (between
the two membranes) in which there are
many enzymes and transport proteins better control of what enters and exits
the cell
•  No techoic acids!!!!!
Gram-negative cell walls
•  Because they are thin, gram negative
cell walls are more susceptible to
mechanical breakage than grampositive ones.
•  Because of the outer membrane, gramnegative cell walls are more successful
in avoiding poor osmotic conditions,
especially when an antibiotic like
penicillin is present.
Cell walls can be damaged by…
•  Lysozyme, an enzyme which breaks the glycosidic
bonds between NAG and NAM.
•  Penicillin, an antibiotic that interferes with cell wall
synthesis by inhibiting peptide bridge formation.
•  Gram-negative cells are much less susceptible to
both of these than gram-positive cells.
•  If cell wall is weakened or destroyed, cell lysis may
occur if the cell is in a hypotonic environment.
Structures internal to the cell
wall
The plasma membrane
The chromosome (nucleoid
region)
Ribosomes
Inclusions
Endospores (sometimes)
The plasma membrane
•  Consists of a phospholipid bilayer
associated with various proteins (the fluidmosaic model)
Major functions
•  Delineate cell boundary, and hold in the
contents
•  To control transportation of substances
into and out of the cell (they are
selectively permeable).
•  There are four ways for things to get
across a plasma membrane (next
lecture)
The Chromosome or
"nucleoid”
•  Bacteria always possess one circular
piece of DNA, their chromosome.
•  While they have no membrane bounded
nucleus, the chromosome is attached to
the plasma membrane and somewhat
localized; this region is sometimes
referred to as the "nucleoid” region.
Ribosomes
•  Ribosomes exist in all bacterial cells - they
are responsible for one and only one vital
job: MAKING PROTEINS!!! (Remember ribosomes have no membrane)
•  They consist of two subunits made of RNA
and Proteins
–  Many antibiotics kill bacteria by interfering with
ribosome function
–  A given cell may contain thousands of ribosomes
Endospores
•  Only certain gram-positive cells can form
endospores, dormant structures that can
survive essentially forever and are very
difficult to destroy. (Bacillus, Clostridium)
•  Endospores form when the environment is
unfriendly, in other words, when the cell
might otherwise die. Usually this is due to a
lack of an adequate carbon or nitrogen
source.
Endospores cont'd
•  Environmental signals trigger the beginning of sporulation
(spore formation).
•  A double plasma membrane surrounds the chromosome,
forming the core, and a layer of peptidoglycan forms
between the two membranes. In addition, a salt called
calcium dipicolinate is laid down. This has been shown to be
important in various aspects of endospore hardiness.
•  A very thick and impenetrable proteinaceous spore coat
forms.
•  So the spore forms INTERNAL to the plasma membrane, and
is then released to the environment, where it will wait for
conditions to improve.
Endospore Formation
Endospores are amazing
The presence of water and nutrients reverses the
dormancy, and the cell begins to grow and function
again.
25-million year old endospores trapped in amber
have germinated when placed in nutrient media!!!!!
It is almost impossible to "kill" an endospore, so the
bacteria that form them can be particularly
dangerous pathogens, for example, Bacillus
anthracis, which causes anthrax, and Clostridium
botulinum, which causes botulism.