Figure 3.18
Peptidoglycancable
Ribitol
Wall-associatedprotein
Teichoic acid Peptidoglycan
Lipoteichoicacid
Cytoplasmic membrane
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3.6 The Cell Wall of Bacteria: Peptidoglycan
• Prokaryotes That Lack Cell Walls– Mycoplasmas
• Group of pathogenic bacteria
– Thermoplasma• Species of Archaea
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3.7 The Outer Membrane
• Total cell wall contains ~10% peptidoglycan (Figure 3.20a)
• Most of cell wall composed of outer membrane (aka lipopolysaccharide [LPS] layer)
– LPS consists of core polysaccharide and O-polysaccharide
– LPS replaces most of phospholipids in outer half of outer membrane
– Endotoxin: the toxic component of LPS
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Figure 3.20a
O-polysaccharide
Peptidoglycan
Core polysaccharide
Lipid A Protein
Porin
Out
8 nm
Phospholipid
Lipopolysaccharide(LPS)
Lipoprotein
In
Outermembrane
Periplasm
Cytoplasmic membrane
Cellwall
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3.7 The Outer Membrane
• Porins: channels for movement of hydrophilic low-molecular weight substances (Figure 3.20b)
• Periplasm: space located between cytoplasmic and outer membranes
– ~15 nm wide
– Contents have gel-like consistency
– Houses many proteins
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3.8 Cell Walls of Archaea
• No peptidoglycan• Typically no outer membrane• Pseudomurein
– Polysaccharide similar to peptidoglycan (Figure 3.21)
– Composed of N-acetylglucosamine and N-acetyltalosaminuronic acid
– Found in cell walls of certain methanogenic Archaea
• Cell walls of some Archaea lack pseudomurein
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Figure 3.21
Lysozyme-insensitive
N-Acetylglucosamine
N-Acetyltalosaminuronicacid
N-Acetyl group
Peptidecross-links
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3.8 Cell Walls of Archaea
• S-Layers– Most common cell wall type among Archaea
– Consist of protein or glycoprotein
– Paracrystalline structure (Figure 3.22)
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Figure 3.22
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3.9 Cell Surface Structures
• Capsules and Slime Layers– Polysaccharide layers (Figure 3.23)
• May be thick or thin, rigid or flexible
– Assist in attachment to surfaces
– Protect against phagocytosis
– Resist desiccation
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Figure 3.23
Cell Capsule
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3.9 Cell Surface Structures
• Fimbriae– Filamentous protein structures (Figure 3.24)
– Enable organisms to stick to surfaces or form pellicles
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Figure 3.24
Flagella
Fimbriae
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3.9 Cell Surface Structures
• Pili – Filamentous protein structures (Figure 3.25)
– Typically longer than fimbriae
– Assist in surface attachment
– Facilitate genetic exchange between cells (conjugation)
– Type IV pili involved in twitching motility
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Figure 3.25
Virus-coveredpilus
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Figure 3.26
-carbon
Polyhydroxyalkanoate
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Figure 3.27
Polyphosphate
Sulfur
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Figure 3.28
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3.11 Gas Vesicles
• Gas Vesicles– Confer buoyancy in planktonic cells
(Figure 3.29)
– Spindle-shaped, gas-filled structures made of protein (Figure 3.30)
– Gas vesicle impermeable to water
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Figure 3.31Ribs
GvpA
GvpC
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3.12 Endospores
• Endospores– Highly differentiated cells resistant to heat, harsh
chemicals, and radiation (Figure 3.32)
– “Dormant” stage of bacterial life cycle (Figure 3.33)
– Ideal for dispersal via wind, water, or animal gut
– Only present in some gram-positive bacteria
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Figure 3.32
Terminalspores
Subterminalspores
Centralspores
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Figure 3.33
Vegetative cell
Developing spore
Sporulating cell
Mature spore
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3.12 Endospores
• Endospore Structure (Figure 3.35)– Structurally complex
– Contains dipicolinic acid
– Enriched in Ca2+
– Core contains small acid-soluble proteins (SASPs)
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Figure 3.35
Exosporium
Spore coat
Core wall
Cortex
DNA
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3.12 Endospores
• The Sporulation Process– Complex series of events (Figure 3.37)
– Genetically directed
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Figure 3.37
Free endospore
Stage VI, VIIStage V
Coat
Stage IV
Stage IIIStage II
Mother cell
Prespore
Septum
Sporulation stages
Cortex
Cell wall
Cytoplasmicmembrane
Vegetative cycle
Maturation,cell lysis
Celldivision
GerminationGrowth
Engulfment
Asymmetriccell division;commitmentto sporulation,Stage I
Spore coat, Ca2
uptake, SASPs,dipicolinic acid
Cortexformation
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3.13 Flagella and Motility
• Flagellum (pl. flagella): structure that assists in swimming
– Different arrangements: peritrichous, polar, lophotrichous (Figure 3.38)
– Helical in shape
Animation: The Prokaryotic FlagellumAnimation: The Prokaryotic Flagellum
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Figure 3.38
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3.13 Flagella and Motility
• Flagellar Structure– Consists of several components (Figure 3.41)
– Filament composed of flagellin
– Move by rotation
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Figure 3.41
Rod
C Ring
MS Ring
Motprotein
Mot proteinMot protein Fli proteins(motor switch)
45 nm
Cytoplasmicmembrane
Basalbody
Rod
C Ring
MS Ring
P Ring
Periplasm Peptidoglycan
L Ring
HookOutermembrane(LPS)
Flagellin
Filament
MS
LP
15—20 nm
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3.13 Flagella and Motility
• Flagella increase or decrease rotational speed in relation to strength of the proton motive force
• Differences in swimming motions (Figure 3.44)– Peritrichously flagellated cells move slowly in a
straight line
– Polarly flagellated cells move more rapidly and typically spin around
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Figure 3.44
Polar
Peritrichous
CW rotation
CCW rotation
Unidirectional flagella
Cellstops,reorients
Reversible flagella
Bundledflagella(CCW rotation)
Tumble—flagellapushed apart(CW rotation)
Flagella bundled(CCW rotation)
CW rotation
CW rotation
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3.15 Microbial Taxes
• Taxis: directed movement in response to chemical or physical gradients
– Chemotaxis: response to chemicals
– Phototaxis: response to light
– Aerotaxis: response to oxygen
– Osmotaxis: response to ionic strength
– Hydrotaxis: response to water
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3.15 Microbial Taxes
• Chemotaxis– Best studied in E. coli
– Bacteria respond to temporal, not spatial, difference in chemical concentration
– “Run and tumble” behavior (Figure 3.47)
– Attractants and receptors sensed by chemoreceptors
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Figure 3.47
No attractant present:Random movement
Attractant present:Directed movement
Tumble
Run
Tumble
Run
Attractant
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3.15 Microbial Taxes
Measuring Chemotaxis (Figure 3.48) Measured by inserting a capillary tube
containing an attractant or a repellent in a medium of motile bacteria
Can also be seen under a microscope
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Figure 3.48
Time
Repellent
Control
Attractant
Repellent
Control Attractant
Cel
ls p
er t
ub
e
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