Transcript Gram positive cell wall

Gram positive
cell wall
Dr Olga Perovic, CMID/NHLS/WITS
2007
Bacterial structure
•Capsule
•Extracellular
polysaccharides
outside of cell
walls.
•Can’t be stained.
•Formed biofilm
•Protects from
phagocytosis and
death.
Classification of bacteria with
Gram positive cell wall
Gram Positive
cocci
Staphylococci
Streptococci
Gram Positive
bacilli
Anaerobic
gram positive
cocci
Aerobic
Gram Positive
bacilli
Anaerobic
Gram Positive
cocci
Corynebacterium
and other
non-spore
-forming
gram-positive
bacilli
The bacterial cell wall definition
and functions
• The bacterial cell wall provides structural integrity
to the cell.
• Protects the cell from internal pressure caused by
the much higher concentrations of proteins and
other molecules inside the cell compared to
outside the cell.
• The bacterial cell wall differs from all other
organisms by the presence of peptidoglycan
(poly-N-acetylglucosamine and N-acetylmuramic
acid).
• Peptidoglycan is responsible for the rigidity of the
bacterial cell wall and for the determination of cell
shape.
• It is relatively porous and does not prevent for
small substrates to pass trough.
The Gram positive cell wall
structural components
• Teichoic acids-deepseated in the Gram
positive cell wall, and are
polyalcohols.
– They are strongly antigenic,
but are generally absent in
Gram-negative bacteria.
• Lipoteichoic acid –
teichoic acid linked with
lipid, goes to the
cytoplasmic membrane
and links the
peptidoglycan to the
cytoplasmic membrane.
– They are antigenic,
cytotoxic and adhesins
Why they are named Gram
positive bacteria?
• Named by Danish doctor in 1884,
Hans Christian Gram.
• Gram developed a staining procedure
which divided almost all bacteria into
two large groups.
• Depending on their retention of
specific basic dyes there are:
– Gram positive and
– Gram negative.
The Characteristics of Gram
positive cell wall organisms
– The Gram positive cell wall is
characterized by the presence of a very
thick peptidoglycan layer, which is
responsible for the retention of the
crystal violet dyes during the Gram
staining procedure, opposite to Gram
negative cell wall which doesn’t.
Application of Gram stain
– Routine use for primary microscopic
examination of clinically significant
specimens.
– To characterize bacteria growth from
culture.
Comparison of the thick
cell wall of Grampositive bacteria with
the comparatively thin
cell wall of Gramnegative bacteria
The Gram stain and bacterial cell
walls
Property
Gram-positive
Gram-negative
Thickness of wall
thick (20-80 nm)
thin (10 nm)
Number of layers
1
2
Peptidoglycan (murein) content
>50%
10-20%
Teichoic acids in wall
present
absent
Lipid and lipoprotein content
0-3%
58%
Protein content
0
9%
Lipopolysaccharide content
0
13%
Sensitivity to Penicillin G
yes
no (1)
Sensitivity to lysozyme
yes
no (2)
Importance of cell wall structure
for selective activity of antibiotics
• The formation of the peptide bond
between chains of peptidoglycan is
blocked by a group of antibiotics of
the beta lactam class,
– Penicillin, cephalosporin, and carbapenems.
• The beta lactam antibiotics prevent the
assembly of the bacterial cell wall.
• The wall becomes progressively weaker
and weaker until the cell lyses or
ruptures.
Assembly of Gram-positive
peptidoglycan and activity of
antibiotics
A bridge of amino acids links
the peptide side chains to
one another-Gram-positive interpeptide bridge, and is
blocked by the beta lactam
antibiotics.
Gram positive cocci
Enterococci
•
Catalase
•
•
Coagulase
Staphylococci
•
•
+
S.aureus
•
•
•
•
•
•
•
•
•
•
haemolysis
Streptococci
Coagulase negative


non staphylococci
S.epidermidis
haemolytic
optochin
Lancefield
S.saprophyticus
groups
S
R
A,B,C,D
S.pneumoniae
F,G
viridans
group
e.g.
S.pyogenes (A)
S.agalactiae (B)
Gram Positive Bacilli
Corynebacterium
diphteriae
Reference:
• Greenwood David, Medical
Microbiology, 2002, 6th Edition