Transcript History of Microbiology

Differentiation of Bacteria by
Cell Wall Composition
The cell wall is the outer most layer
of the cell. In many cases the cell
wall comes in direct contact with the
environment.
Function
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Protection of the cell
Maintains the shapes of the cell
Maintains the osmotic integrity of
the cell
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Provide structure and shape and
protect cell from osmotic forces
Assist some cells in attaching to
other cells or in eluding
antimicrobial drugs
Not present in animal cells, so can
target cell wall of bacteria with
antibiotics
Bacteria and archaea have different
cell wall chemistry
Consider the three morphological
distinctions of bacteria.
Which of these shapes would need
to be flexible in order to exhibit
the shape of the bacterium?
Bacteria are classified into
two groups based on the
composition of their cell
walls.
The two groups are known as
gram positive and gram negative.
◦ Most have cell wall composed of
peptidoglycan
◦ Peptidoglycan is composed of
sugars, NAG, and NAM
◦ Chains of NAG and NAM
attached to other chains by
tetrapeptide crossbridges
 Bridges may be covalently
bonded to one another
 Bridges may be held together
by short connecting chains of
amino acids
Gram-positive cell walls
 Relatively thick layer of
peptidoglycan
 Contain unique polyalcohols
called teichoic acids
 Some covalently linked to
lipids, forming lipoteichoic
acids that anchor
peptidoglycan to cell
membrane
 Retain crystal violet dye in
Gram staining procedure; so
appear purple
The gram positive cell wall is referred to as a peptidoglycan
layer and is composed of alternating subunits of two
carbohydrates: n-acetyl muramic acid and n-acetyl glucosamine.
The gram positive cell wall is referred to as a peptidoglycan
layer and is composed of alternating subunits of two carbohydrates:
n-acetyl muramic acid (NAM) and n-acetyl glucosamine (NAG).
◦ Gram-negative cell walls
 Have only a thin layer of
peptidoglycan
 Bilayer membrane outside the
peptidoglycan contains
phospholipids, proteins, and
lipopolysaccharide (LPS)
 May be impediment to the
treatment of disease
 Appear pink following Gram
staining procedure
The gram negative cell wall consists of two distinct layers. The
outer most layer is referred as the outer membrane and is
composed of lipopolysaccarhides (LPS). The second layer a
peptidoglycan layer that is homologous to a gram positive cell
wall.
Why does the gram stain work?
How is the differences of cell wall
types used to distinguish the
different bacterial types?
Scenario
You have been give a slide with a mixture of gram positive
(coccus) and gram negative bacteria (spirochete).
Think about the gram stain procedure as it relates to the
cell wall differences. What would the end result of each
step of the gram stain be for each of the bacteria types?
Gram Staining Procedure
Reagent
Function
Crystal Violet
Primary Stain
Iodine
Mordant
Acetone-Alcohol
Decolorizer
Safranin
Counter Stain
Results
Why does the gram stain work?
Because of the differences of cell wall composition.
The gram positive cell wall is referred to as a peptidoglycan
layer and is composed of alternating subunits of two
carbohydrates: n-acetyl muramic acid and n-acetyl
glucosamine.
The gram negative cell wall consists of two distinct layers.
The outer most layer is referred as the outer membrane and
is composed of lipopolysaccarhides (LPS). The second layer
a peptidoglycan layer that is homologous to a gram positive
cell wall.
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Archaeal Cell Walls
◦ Do not have peptidoglycan
◦ Contains variety of
specialized polysaccharides
and proteins
◦ Gram-positive archaea stain
purple
◦ Gram-negative archaea stain
pink