Download Bacteria and the bacterial cell wall Aims of the Lecture

Bacteria and the
bacterial cell wall
Lecture 2
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Aims of the Lecture
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Review the molecules and structures of the cell envelope of
bacterial cells
Describe the roles played by the major components.
Discuss briefly the features and structures of the cell well that are
targets for current antibacterial drugs.
Bacterial morphology
The size,
size shape and arrangement of bacteria,
bacteria and other microbes,
microbes is
the result of their genes and thus is a defining characteristic called
morphology.
The most common bacterial shapes are the rod (bacilli) and spherical
(cocci) shaped bacteria.
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Enterococcus sp.
A coccoid
bacterium
SEM x27,235
© Dennis Kunkel
(www.denniskunkel.com)
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Bacillus anthracis
A rod-shaped
bacterium which
causes anthrax
SEM x24,200
© Dennis Kunkel
(www.denniskunkel.com)
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Bordetella holmesii
Coccobacillus
bacterium:
causes respiratory
infections
SEM x35,200
,
© Dennis Kunkel
(www.denniskunkel.com)
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Spirulina sp.
A filamentous
cyanobacterium
b t i
SEM x2,080
© Dennis Kunkel
(www.denniskunkel.com)
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Vibrio cholerae
A “curved rod”
(vibrio) shaped
prokaryote
SEM x15,575
© Dennis Kunkel
(www.denniskunkel.com)
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Staphylococcus aureus
A coccoid
b t i
bacterium
SEM x23,000
© Dennis Kunkel
(www.denniskunkel.com)
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Group Association
• Group association describes how microbial cells, in either liquid or
on solid medium, are arranged in relation to one another.
• They are helpful in classification (identification).
• Bacteria may exist mainly as single cells or as common grouping
such as chains (e.g. streptococci) or clusters of cells (e.g.
staphylococci), or uneven clusters, pairs, tetrads, octads etc.
• They may exist as masses embedded within a large capsule.
• There are square bacteria
bacteria, star-shaped bacteria
bacteria, stalked bacteria
bacteria,
budding bacteria that grow in net-like arrangements and many other
morphologies.
• These range of morphologies are also found in fungi ranging from
the single cell yeasts to the filamentous moulds and dermatophytes.
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Bacteria and the Gram Stain
• Bacteria are prokaryotic (i.e. no membrane enclosing a nucleus)
• They have a cell wall in addition to cell membrane (together making
the cell envelope).
envelope)
• Many bacteria can be assigned to one of two major groups, based
on the Gram-staining reaction. The differences in cell-wall
structure and composition account for the differential Gram reaction.
•
The cell wall is both a target for antibacterial drugs and a
defence against antibacterial drugs, i.e. a resistance factor.
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Cell Wall Ultrastructure
Differences in cell-wall structure and composition account for the
differential Gram staining process.
We need to know something about the composition and organisation
("ultrastructure") of the cell wall to understand why some antibacterial
drugs work and others do not..
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Cell Wall Structure
• A complex cell wall is present in prokaryotes (and not eukaryotic
mammalian cells).
• The outer membrane in GNB is more complex than the inner
cytoplasmic membrane and presents an extra barrier that drugs
must to pass through to get into bacterial cells. GNB are therefore
often resistant to drugs that GPB are susceptible to.
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Cell Wall Components
Gram-positive
• Major component (~50%) is peptidoglycan
• No
N lilipid
id and
d often
ft no protein
t i presentt
• Accessory polymers (teichoic acid and/or teichuronic acid) are
covalently linked to peptidoglycan
• Relatively thick and featureless (under electron microscope)
Gram-negative
• Cell envelope consists of a pair of membranes (cytoplasmic and
outer) with a thin, intermediate layer of peptidoglycan
• Outer membrane contains lipopolysaccharide (LPS) as well as
lipids and proteins.
• The protruding polysaccharide in LPS makes the bacteria appear
fuzzy under an electron microscope.
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Peptidoglycan:
a polysaccharide core
Lysozymes are glycosidases specific to the bond from MurNAc to GlcNAc. They
are commonly found in egg white and present in some secretions (tears) as part
of the body's defence. GPB are attacked and killed directly, but GNB are usually
resistant (outer membrane prevents access to the peptidoglycan).
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Peptidoglycan:
glycan chain cross-linked with peptides
Autolysins are glycosidases (for bond from MurNAc to GlcNAc), amidases (for
bond from MurNAc to the peptide) and peptidases (breaking bonds in the peptide or
the bridge) which are produced by bacteria to “remodel” the cell wall as the bacteria
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grows and divides.
The Peptide Side Chain
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Biosynthesis of peptidoglycan
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The Outer Membrane in GNB
Lipoproteins anchor the outer membrane to the peptidoglycan (not all of them are
attached). The lipopolysaccharide is important for the functions and properties of
the outer membrane. It is essential for structural integrity and viability of the
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bacteria and makes the outer membrane more rigid.
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Lipopolysaccharide
LPS contributes to the structural and functional properties of the OM and is
present in almost all GNB.
LPS is an endotoxin [cell-bound]. The Lipid A portion of LPS is responsible for
the endotoxic activities. Free, monomeric LPS, is released from the OM
naturally or through antibiotic action and interacts with receptors on macrophages
and other blood cells, causing the release of inflammatory agents.
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