Transcript Cell Envelope—Outer Covering 3 Basic layers: Glycocalyx, Cell wall

Cell Envelope—Outer Covering
3 Basic layers: Glycocalyx, Cell wall and Cell
membrane
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Glycocalyx
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Develops a coating of
macromolecules to protect cell
and sometimes help it stick to its
environment
Slime layer, loose, helps cell retain
nutrients and water
Capsules are tighter and made of
polysaccharides, proteins—gives
a mucoid character to the colony
Encapsulated bacteria have
greater pathogenicity because the
capsule protects the bacteria from
phagocytes (WBC) that would
engulf and destroy it
Some glycocalyces are so
adherent they are responsible for
persistent colonization of nonliving materials: plastic catheters,
IUD’s, metal pacemakers
Read pg 194, “Biofilms—The
Glue of Life”—outline and hand
in for reading comp/editing
assignment
Cell Wall
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Found below the glycocalyx,
determines shape of bacterium
Gives structural support due to the
macromolecule, peptidoglycan:
Made of long sugar chains (glycanblue & green) criss-crossed with short
peptide (protein-red lines) fragments
A, G, L & A—tetrapeptide chains
In aqueous bacteria the cell wall
prevents the absorption of too much
water—cause the cell to burst
Some antibiotics attack the peptide
cross-links weakening the
peptidoglycan, allowing cell to undergo
lysis and die
Some disinfectants
(alcohol/detergents) will do the same
Lysozyme, enzyme in saliva and tears,
also breaks down cell walls
Differences in cell
walls
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1844—Hans Christian Gram
discovered a staining technique
that distinguished between 2
major groups of bacteria
Difference was in their cell
envelopes
Gram positive cell: cell wall is a
thick layer of peptidoglycan and
then there is the cell membrane
This thick layer absorbs the
primary stain (crystal violet-purple)
Then Gram’s iodine is added and
it stabilizes the crystal violet to
form large crystals in the
peptidoglycan layer
When the ETOH is added, it
doesn’t wash out the purple color
When the counterstain is added
(safranin) its color (red) is NOT
absorbed
Gram negative cells
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Contains 3 layers:
Outer membrane (OM) acts as a sieve
to only allow small molecules in; made
of porin proteins—defense against
antibiotics
Thin peptidoglycan layer has a small
periplasmic space on both sides of it—
reaction site for substances
entering/leaving cell
Inner cell membrane
Gram stain: crystal violet stains the
cell purple
Gram’s iodine has no affect—due to
small peptidoglycan layer
ETOH partially dissolves the OM’s
lipids and the purple color is lost
Safranin (counterstain) now stains the
colorless cell
The extra layer in Gram- cells makes
them more impervious to some
antimicrobial chemicals—except
alcohol based ones
Cell Membrane
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Below the peptidoglycan layer
Phospholipid bilayer with proteins
embedded in it, EXCEPT:
Mycoplasmas—membranes
contain high amounts of sterols—
rigid lipids that reinforce the
membrane, AND…
Archaea—contain unique
branched hydrocarbons instead of
fatty acids
Can form internal folds w/in the
cytoplasm called mesosomes—
increases the internal surface area
available for membrane activities:
Regulates transport of nutrients in
and wastes out
Respiration and ATP synthesis
occurs here (no mitochondria)
Secretion of metabolic products in
cell’s environment