Microbial Cell Structure Function
Content
Microbial Cell Structure and
Function
01-20-15
I. Microscopy
• 2.1 Discovering Cell Structure: Light
Microscopy
• 2.2 Improving Contrast in Light Microscopy
• 2.3 Imaging Cells in Three Dimensions
• 2.4 Probing Cell Structure: Electron
Microscopy
Microbial Size
http://learn.genetics.utah.edu/content/begin/cells/scale/
Microscopy for Different Size Scales
• Different microscopes are required to resolve
various cells and subcellular structures
2.1 Discovering Cell Structure: Light Microscopy
• Compound light microscope uses visible light
to illuminate cells
• Many different types of light microscopy:
– Bright-field
– Phase-contrast
– Dark-field
– Fluorescence
Animation: Light Microscopy
2.1 Discovering Cell Structure: Light Microscopy
• Bright-field scope
– Specimens are visualized
because of differences in
contrast (density) between
specimen and surroundings
Ocular
lenses
• Two sets of lenses form the
image
– Objective lens and ocular lens
– Total magnification = objective
magnification ocular
magnification
– Maximum magnification is
~2,000
Objective lens
Stage
Condenser
Focusing knobs
Light source
Specimen on
glass slide
Magnification Light path
100, 400,
1000
10
Visualized
image
Eye
Ocular lens
Intermediate image
(inverted from that
of the specimen)
10, 40, or
100 (oil)
Objective lens
Specimen
None
Condenser lens
Light source
2.1 Discovering Cell Structure: Light Microscopy
• Resolution: the ability to distinguish two adjacent
objects as separate and distinct
– Resolution is determined by the wavelength of light
used and numerical aperture of lens
– Limit of resolution for light microscope is about
0.2 m
2.2 Improving Contrast in Light Microscopy
• Improving contrast results in a better final
image
• Staining improves contrast
– Dyes are organic compounds that bind to
specific cellular materials
– Examples of common stains are methylene
blue, safranin, and crystal violet
Animation: Microscopy & Staining Overview
Animation: Staining
I. Preparing a smear
Spread culture in thin
film over slide
Dry in air
II. Heat fixing and staining
Pass slide through
flame to heat fix
Flood slide with stain;
rinse and dry
III. Microscopy
Slide
Oil
Place drop of oil on slide;
examine with 100
objective lens
2.2 Improving Contrast in Light Microscopy
• Differential stains: the Gram stain
• Differential stains separate bacteria into groups
• The Gram stain is widely used in microbiology
– Bacteria can be divided into two major groups:
Gram-positive and Gram-negative
– Gram-positive bacteria appear purple and Gramnegative bacteria appear red after staining
Step 1
Flood the heat-fixed
smear with crystal
violet for 1 min
Result:
All cells purple
Step 2
Add iodine solution
for 1 min
Result:
All cells
remain purple
Step 3
Decolorize with
alcohol briefly
— about 20 sec
Result:
Gram-positive
cells are purple;
gram-negative
cells are colorless
Step 4
G-
Result:
Gram-positive
(G+) cells are purple;
gram-negative (G-) cells
are pink to red
Counterstain with
safranin for 1–2 min
G+
2.2 Improving Contrast in Light Microscopy
• Phase-Contrast Microscopy
– Phase ring amplifies differences in the refractive index of cell and
surroundings
– Improves the contrast of a sample without the use of a stain
– Allows for the visualization of live samples
– Resulting image is dark cells on a light background
• Dark-Field Microscopy
–
–
–
–
Light reaches the specimen from the sides
Light reaching the lens has been scattered by specimen
Image appears light on a dark background
Excellent for observing motility
2.2 Improving Contrast in Light Microscopy
• Fluorescence Microscopy
– Used to visualize specimens that fluoresce
• Emit light of one color when illuminated with
another color of light
– Cells fluoresce naturally (autofluorescence) or
after they have been stained with a
fluorescent dye like DAPI
– Widely used in microbial ecology for
enumerating bacteria in natural samples
bright-field
fluorescence
DAPI-stained
2.3 Imaging Cells in Three Dimensions
• Differential Interference Contrast (DIC)
Microscopy
– Uses a polarizer to create two distinct beams
of polarized light
– Gives structures such as endospores, vacuoles,
and granules a three-dimensional appearance
– Structures not visible
using bright-field
microscopy are
sometimes visible
using DIC
2.3 Imaging Cells in Three Dimensions
• Confocal Scanning Laser Microscopy (CSLM)
– Uses a computerized microscope coupled with a
laser source to generate a
three-dimensional image
– Computer can focus the laser
on single layers of the
specimen
– Different layers can then be
compiled for a threedimensional image
– Resolution is 0.1 m for CSLM
2.4 Electron Microscopy
• Electron microscopes use electrons instead
of photons to image
cells and structures
• Two types of electron
microscopes:
– Transmission electron
microscopes (TEM)
– Scanning electron
microscopes (SEM)
Electron
source
Evacuated
chamber
Sample
port
Viewing
screen
2.4 Electron Microscopy
• Transmission Electron Microscopy (TEM)
– Electromagnets function as lenses
– System operates in a vacuum
– High magnification and resolution
(0.2 nm)
– Enables visualization of structures at
the molecular level
– Specimen must be very thin (20–60 nm) and
stained
Cytoplasmic
membrane
Septum Cell wall
DNA
(nucleoid)
Animation: Electron Microscopy
2.4 Electron Microscopy
• Scanning Electron Microscopy (SEM)
– Specimen is coated with a thin film of heavy metal
(e.g., gold)
– An electron beam scans the object
– Scattered electrons are collected by a detector and
an image is produced
– Even very large specimens
can be observed
– Magnification range of
15–100,000
II. Cells of Bacteria and Archaea
• 2.5 Cell Morphology
• 2.6 Cell Size and the Significance of Being
Small
2.5 Cell Morphology
• Morphology = cell shape
• Major cell morphologies
– Coccus (pl. cocci): spherical or ovoid
– Rod: cylindrical shape
– Spirillum: spiral shape
• Cells with unusual shapes
– Spirochetes, appendaged bacteria, and
filamentous bacteria
• Many variations on basic morphological types
Coccus
Spirochete
Stalk
Rod
Hypha
Budding and
appendaged bacteria
Spirillum
Filamentous bacteria
2.5 Cell Morphology
• Morphology typically does not predict
physiology, ecology, phylogeny, etc. of a
prokaryotic cell
• Selective forces may be involved in setting the
morphology
– Optimization for nutrient uptake (small cells and
those with high surface-to-volume ratio)
– Swimming motility in viscous environments or
near surfaces (helical or spiral-shaped cells)
– Gliding motility (filamentous bacteria)
2.6 Cell Size and the Significance of Being Small
• Size range for prokaryotes: 0.2 µm to >700 µm
in diameter
– Most cultured rod-shaped
bacteria are between 0.5 and
4.0 µm wide and <15 µm long
– Examples of very large
prokaryotes
• Epulopiscium fishelsoni
• Thiomargarita namibiensis
• Size range for eukaryotic cells:
10 to >200 µm in diameter
2.6 Cell Size and the Significance of Being Small
• Surface-to-Volume
Ratios, Growth Rates,
and Evolution
– Advantages to being
small
• Small cells have more
surface area relative to cell
volume than large cells (i.e.,
higher S/V)
– Support greater nutrient
exchange per unit cell
volume
– Tend to grow faster than
larger cells
r = 1 m
r = 1 m
Surface area (4r2) = 12.6 m2
Volume (43 r3) = 4.2 m3
Surface
=3
Volume
r = 2 m
r = 2 m
Surface area = 50.3 m2
Volume = 33.5 m3
Surface
= 1.5
Volume
2.6 Cell Size and the Significance of Being Small
• Lower Limits of Cell Size
– Cellular organisms < 0.15 µm in diameter are
unlikely
– Open oceans tend to contain small cells (0.2–0.4
µm in diameter)
II. The Cytoplasmic Membrane and Transport
• 2.7 Membrane Structure
• 2.8 Membrane Functions
• 2.9 Nutrient Transport
2.7 Membrane Structure
• Cytoplasmic membrane:
– Thin structure that surrounds the cell
– 6–8 nm thick
– Vital barrier that separates cytoplasm from
environment
– Highly selective permeable barrier
• Allows concentration of specific metabolites
• Excretion of waste products
2.7 Membrane Structure
• Composition of Membranes
– General structure is phospholipid bilayer
• Contain both hydrophobic and hydrophilic components
– Can exist in many different chemical forms as a
result of variation in the groups attached to the
glycerol backbone
– Hydrophobic fatty acids point inward
– Hydrophilic portions remain exposed to external
environment or the cytoplasm
Animation: Membrane Structure
Glycerol
Fatty acids
Phosphate
Ethanolamine
Hydrophilic
region
Fatty acids
Hydrophobic
region
Hydrophilic
region
Glycerophosphates
Fatty acids
2.7 Membrane Structure
• Cytoplasmic Membrane
– Embedded proteins
– Stabilized by hydrogen bonds and hydrophobic interactions
– Mg2+ and Ca2+ help stabilize membrane by forming ionic
bonds with negative charges on the phospholipids
– Somewhat fluid
Out
Phospholipids
Hydrophilic
groups
6–8 nm
Hydrophobic
groups
In
Integral
membrane
proteins
Phospholipid
molecule
2.7 Membrane Structure
• Membrane Proteins
– Outer surface of cytoplasmic membrane can
interact with a variety of proteins that bind
substrates or process large molecules for transport
– Inner surface of cytoplasmic membrane interacts
with proteins involved in energy-yielding reactions
and other important cellular functions
– Integral membrane proteins
• Firmly embedded in the membrane
– Peripheral membrane proteins
• One portion anchored in the membrane
2.7 Membrane Structure
• Archaeal Membranes
– Ether linkages in phospholipids of Archaea
– Bacteria and Eukarya that have ester linkages in
phospholipids
– Archaeal lipids lack fatty acids, have isoprenes instead
– Major lipids are glycerol diethers and tetraethers
– Can exist as lipid monolayers, bilayers, or mixture
Ester
Ether
isoprene
Bacteria
Eukarya
Archaea
Glycerol diether
CH3 groups
Isoprene unit
Biphytanyl
Diglycerol tetraethers
Crenarchaeol
Out
Out
Glycerophosphates
Phytanyl
Biphytanyl or
crenarchaeol
Membrane protein
In
In
2.8 Membrane Function
• Permeability Barrier
– Polar and charged molecules must be transported
– Transport proteins accumulate solutes against the
concentration gradient
• Protein Anchor
– Holds transport proteins in place
• Energy Conservation
Permeability barrier:
Protein anchor:
Energy conservation:
Prevents leakage and functions
as a gateway for transport of
nutrients into, and wastes out
of, the cell
Site of many proteins that
participate in transport,
bioenergetics, and chemotaxis
Site of generation and use of the
proton motive force
2.9 Nutrient Transport
• Carrier-Mediated Transport Systems
Rate of solute entry
– Show saturation effect
– Highly specific
– Highly regulated
Transporter saturated
with substrate
Transport
Simple diffusion
External concentration of solute
2.9 Nutrient Transport
• Transport systems in
prokaryotes
In
Out
– Simple transport
• Driven by the energy in the proton
motive force
– Group translocation
Transported
substance
• Chemical modification of the
transported substance driven by
phosphoenolpyruvate
– ABC system
• Periplasmic binding proteins are
involved and energy comes from
ATP
• All require energy in some
form, usually proton motive
force or ATP
1
2
3
2.9 Nutrient Transport
• Three transport events are possible: uniport,
symport, and antiport
– Uniporters transport in one direction across the
membrane
– Symporters function as co-transporters
– Antiporters transport a
molecule across
the membrane while
simultaneously
transporting another
molecule in the
opposite direction
2.9 Nutrient Transport
• Simple Transport:
– Lac Permease of Escherichia coli
• Lactose is transported into E. coli by the simple
transporter lac permease, a symporter
• Activity of lac permease is energy driven
• Other symporters, uniporters, and antiporters
Out
In
Sulfate
symporter
Potassium
uniporter
Phosphate
symporter
Sodium-proton
antiporter
Lac permease
(a symporter)
3.5 Transport and Transport Systems
• The Phosphotransferase System in E. coli
– Type of group translocation: substance transported is
chemically modified during transport across the
membrane
– Best-studied system
– Moves glucose, fructose, and mannose
– Five proteins required
Glucose
– Energy derived from
Out
phosphoenolpyruvate
Cytoplasmic
membrane
Nonspecific components
Specific components
Enz
IIC
PE
Enz
HPr
Enz
IIa
Direction of P transfer
Enz
IIb
In
Glucose 6–P
Direction
of glucose
transport
3.5 Transport and Transport Proteins
• ABC (ATP-Binding Cassette) Systems
– >200 different systems identified in prokaryotes
– Often involved in uptake
of organic compounds
(e.g., sugars, amino
acids), inorganic nutrients
(e.g., sulfate, phosphate),
and trace metals
– Typically display high
substrate specificity
– Contain periplasmic
binding proteins
IV. Cell Walls of Bacteria and Archaea
• 2.10 Peptidoglycan
• 2.11 LPS: The Outer Membrane
• 2.12 Archaeal Cell Walls
2.10 Peptidoglycan
Peptidoglycan
– Rigid layer that provides
strength to cell wall
– Polysaccharide composed of
• N-acetylglucosamine and Nacetylmuramic acid
• Amino acids
• Lysine or diaminopimelic acid
(DAP)
• Cross-linked differently in gramnegative bacteria and grampositive bacteria
N-Acetylmuramic acid
N-Acetyl
group
Peptide
cross-links
Lysozymesensitive
bond
L-Alanine
D-Glutamic acid
Diaminopimelic
acid
D-Alanine
Glycan tetrapeptide
N-Acetylglucosamine
Polysaccharide
backbone
Interbridge
Peptides
Escherichia coli
(gram-negative)
Staphylococcus aureus
(gram-positive)
Peptide bonds
Y
X
Glycosidic bonds
2.10 Peptidoglycan
• Gram-Positive Cell Walls
– Can contain up to 90% peptidoglycan
– Common to have teichoic acids (acidic
substances) embedded in the cell wall
• Lipoteichoic acids: teichoic acids covalently bound
Wall-associated Teichoic acid
Peptidoglycan Lipoteichoic
to membrane lipids protein
acid
Peptidoglycan
cable
Cytoplasmic membrane
2.11 LPS: The Outer Membrane
• Gram-Negative Cell Walls
– Total cell wall contains ~10% peptidoglycan
– 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
O-specific
polysaccharide
Core polysaccharide
Lipid A
Protein
Out
Lipopolysaccharide
(LPS)
Porin
Outer
membrane
8 nm
Cell
wall
Phospholipid
Periplasm
Peptidoglycan
Lipoprotein
Cytoplasmic
membrane
In
2.11 LPS: The Outer Membrane
• Porins: channels for movement
of hydrophilic low-molecular
weight substances
• Periplasm: space located
between cytoplasmic and outer
membranes
– ~15 nm wide
– Contents have gel-like
consistency
– Houses many proteins
Periplasm
Cytoplasmic
membrane
Outer membrane
2.12 Archeal Cell Walls
• No peptidoglycan
• Typically no outer membrane
• Pseudomurein
– Polysaccharide similar to peptidoglycan
– Composed of N-acetylglucosamine and Nacetyltalosaminuronic acid
– Found in cell walls of certain methanogenic
Archaea
• Cell walls of some Archaea lack pseudomurein
N-Acetyltalosaminuronic
acid
Lysozyme-insensitive
N-Acetylglucosamine
Peptide
cross-links
N-Acetyl
group
2.12 Archaeal Cell Walls
• S-Layers
– Most common cell
wall type among
Archaea
– Consist of protein or
glycoprotein
– Paracrystalline
structure
V. Other Cell Surface Structures and
Inclusions
•
•
•
•
2.13 Cell Surface Structures
2.14 Cell Inclusions
2.15 Gas Vesicles
2.16 Endospores
2.13 Cell Surface Structures
• Capsules and Slime Layers
– Polysaccharide layers
• May be thick or thin, rigid or
flexible
– Assist in attachment to
surfaces
– Protect against phagocytosis
– Resist desiccation
2.13 Cell Surface Structures
• Fimbriae
– Filamentous protein structures
– Enable organisms to stick to surfaces or form
pellicles
Flagella
Fimbriae
2.13 Cell Surface Structures
• Pili
–
–
–
–
Filamentous protein structures
Typically longer than fimbriae
Assist in surface attachment
Facilitate genetic exchange between cells
(conjugation)
– Type IV pili involved in twitching motility
Viruscovered
pilus
2.14 Cell Inclusions
• Carbon storage polymers
-carbon
– Poly--hydroxybutyric acid
(PHB): lipid
– Glycogen: glucose
polymer
• Polyphosphates:
accumulations of
inorganic phosphate
• Sulfur globules:
composed of elemental
sulfur
• Magnetosomes: magnetic
storage inclusions
Polyhydroxyalkanoate
Sulfur
2.16 Endospores
• Endospores
– Highly differentiated cells resistant
heat, harsh chemicals, and radiation
Vegetative cell
– “Dormant” stage of
bacterial life cycle
– Ideal for dispersal
Developing spore
via wind, water,
or animal gut
– Only present
Sporulating cell
in some
gram-positive
bacteria
Mature spore
to
VI. Microbial Locomotion
• 2.17 Flagella and Motility
• 2.18 Gliding Motility
• 2.19 Microbial Taxes
2.17 Flagella and Swimming Motility
• Flagellum (pl. flagella): structure that
assists in swimming
– Different arrangements: peritrichous, polar,
lophotrichous
– Helical in shape
Animation: The Prokaryotic Flagellum
2.17 Flagella and Swimming Motility
15—20 nm
L
P
Filament
Flagellin
MS
• Flagellar structure
Hook
Outer
membrane
(LPS)
L Ring
Rod
– Consists of several
components
– Filament composed of
flagellin
– Move by rotation
P Ring
Periplasm
Peptidoglycan
Rod
MS Ring
MS Ring
Basal
body
C Ring
Cytoplasmic Mot protein Fli proteins Mot protein
(motor switch)
membrane
45 nm
C Ring
Mot
protein
2.17 Flagella and Swimming Motility
• Flagella increase or
decrease rotational
speed in relation to
strength of the proton
motive force
• Differences in
swimming motions
– Peritrichously
flagellated cells
move slowly in a
straight line
– Polarly flagellated
cells move more
rapidly and typically
spin around
Tumble—flagella
pushed apart
(CW rotation)
Bundled
flagella
(CCW rotation)
Flagella bundled
(CCW rotation)
Peritrichous
Reversible flagella
CCW rotation
CW rotation
Unidirectional flagella
CW rotation
Polar
Cell
stops,
reorients
CW rotation
2.19 Chemotaxis and Other Taxes
• Taxis: directed movement in response to
chemical or physical gradients
– Chemotaxis: response to chemicals
• Best studied in E. coli
• Bacteria respond to temporal, not spatial, difference
in chemical concentration
• “Run and tumble” behavior
• Attractants sensed
by
chemoreceptors
Tumble
Tumble
Run
Run
No attractant present:
Random movement
Attractant present:
Directed movement
Attractant
2.19 Chemotaxis and Other Taxes
• Measuring chemotaxis
– 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
Cells per tube
Control
Attractant
Repellent
Attractant
Control
Repellent
Time
Function
01-20-15
I. Microscopy
• 2.1 Discovering Cell Structure: Light
Microscopy
• 2.2 Improving Contrast in Light Microscopy
• 2.3 Imaging Cells in Three Dimensions
• 2.4 Probing Cell Structure: Electron
Microscopy
Microbial Size
http://learn.genetics.utah.edu/content/begin/cells/scale/
Microscopy for Different Size Scales
• Different microscopes are required to resolve
various cells and subcellular structures
2.1 Discovering Cell Structure: Light Microscopy
• Compound light microscope uses visible light
to illuminate cells
• Many different types of light microscopy:
– Bright-field
– Phase-contrast
– Dark-field
– Fluorescence
Animation: Light Microscopy
2.1 Discovering Cell Structure: Light Microscopy
• Bright-field scope
– Specimens are visualized
because of differences in
contrast (density) between
specimen and surroundings
Ocular
lenses
• Two sets of lenses form the
image
– Objective lens and ocular lens
– Total magnification = objective
magnification ocular
magnification
– Maximum magnification is
~2,000
Objective lens
Stage
Condenser
Focusing knobs
Light source
Specimen on
glass slide
Magnification Light path
100, 400,
1000
10
Visualized
image
Eye
Ocular lens
Intermediate image
(inverted from that
of the specimen)
10, 40, or
100 (oil)
Objective lens
Specimen
None
Condenser lens
Light source
2.1 Discovering Cell Structure: Light Microscopy
• Resolution: the ability to distinguish two adjacent
objects as separate and distinct
– Resolution is determined by the wavelength of light
used and numerical aperture of lens
– Limit of resolution for light microscope is about
0.2 m
2.2 Improving Contrast in Light Microscopy
• Improving contrast results in a better final
image
• Staining improves contrast
– Dyes are organic compounds that bind to
specific cellular materials
– Examples of common stains are methylene
blue, safranin, and crystal violet
Animation: Microscopy & Staining Overview
Animation: Staining
I. Preparing a smear
Spread culture in thin
film over slide
Dry in air
II. Heat fixing and staining
Pass slide through
flame to heat fix
Flood slide with stain;
rinse and dry
III. Microscopy
Slide
Oil
Place drop of oil on slide;
examine with 100
objective lens
2.2 Improving Contrast in Light Microscopy
• Differential stains: the Gram stain
• Differential stains separate bacteria into groups
• The Gram stain is widely used in microbiology
– Bacteria can be divided into two major groups:
Gram-positive and Gram-negative
– Gram-positive bacteria appear purple and Gramnegative bacteria appear red after staining
Step 1
Flood the heat-fixed
smear with crystal
violet for 1 min
Result:
All cells purple
Step 2
Add iodine solution
for 1 min
Result:
All cells
remain purple
Step 3
Decolorize with
alcohol briefly
— about 20 sec
Result:
Gram-positive
cells are purple;
gram-negative
cells are colorless
Step 4
G-
Result:
Gram-positive
(G+) cells are purple;
gram-negative (G-) cells
are pink to red
Counterstain with
safranin for 1–2 min
G+
2.2 Improving Contrast in Light Microscopy
• Phase-Contrast Microscopy
– Phase ring amplifies differences in the refractive index of cell and
surroundings
– Improves the contrast of a sample without the use of a stain
– Allows for the visualization of live samples
– Resulting image is dark cells on a light background
• Dark-Field Microscopy
–
–
–
–
Light reaches the specimen from the sides
Light reaching the lens has been scattered by specimen
Image appears light on a dark background
Excellent for observing motility
2.2 Improving Contrast in Light Microscopy
• Fluorescence Microscopy
– Used to visualize specimens that fluoresce
• Emit light of one color when illuminated with
another color of light
– Cells fluoresce naturally (autofluorescence) or
after they have been stained with a
fluorescent dye like DAPI
– Widely used in microbial ecology for
enumerating bacteria in natural samples
bright-field
fluorescence
DAPI-stained
2.3 Imaging Cells in Three Dimensions
• Differential Interference Contrast (DIC)
Microscopy
– Uses a polarizer to create two distinct beams
of polarized light
– Gives structures such as endospores, vacuoles,
and granules a three-dimensional appearance
– Structures not visible
using bright-field
microscopy are
sometimes visible
using DIC
2.3 Imaging Cells in Three Dimensions
• Confocal Scanning Laser Microscopy (CSLM)
– Uses a computerized microscope coupled with a
laser source to generate a
three-dimensional image
– Computer can focus the laser
on single layers of the
specimen
– Different layers can then be
compiled for a threedimensional image
– Resolution is 0.1 m for CSLM
2.4 Electron Microscopy
• Electron microscopes use electrons instead
of photons to image
cells and structures
• Two types of electron
microscopes:
– Transmission electron
microscopes (TEM)
– Scanning electron
microscopes (SEM)
Electron
source
Evacuated
chamber
Sample
port
Viewing
screen
2.4 Electron Microscopy
• Transmission Electron Microscopy (TEM)
– Electromagnets function as lenses
– System operates in a vacuum
– High magnification and resolution
(0.2 nm)
– Enables visualization of structures at
the molecular level
– Specimen must be very thin (20–60 nm) and
stained
Cytoplasmic
membrane
Septum Cell wall
DNA
(nucleoid)
Animation: Electron Microscopy
2.4 Electron Microscopy
• Scanning Electron Microscopy (SEM)
– Specimen is coated with a thin film of heavy metal
(e.g., gold)
– An electron beam scans the object
– Scattered electrons are collected by a detector and
an image is produced
– Even very large specimens
can be observed
– Magnification range of
15–100,000
II. Cells of Bacteria and Archaea
• 2.5 Cell Morphology
• 2.6 Cell Size and the Significance of Being
Small
2.5 Cell Morphology
• Morphology = cell shape
• Major cell morphologies
– Coccus (pl. cocci): spherical or ovoid
– Rod: cylindrical shape
– Spirillum: spiral shape
• Cells with unusual shapes
– Spirochetes, appendaged bacteria, and
filamentous bacteria
• Many variations on basic morphological types
Coccus
Spirochete
Stalk
Rod
Hypha
Budding and
appendaged bacteria
Spirillum
Filamentous bacteria
2.5 Cell Morphology
• Morphology typically does not predict
physiology, ecology, phylogeny, etc. of a
prokaryotic cell
• Selective forces may be involved in setting the
morphology
– Optimization for nutrient uptake (small cells and
those with high surface-to-volume ratio)
– Swimming motility in viscous environments or
near surfaces (helical or spiral-shaped cells)
– Gliding motility (filamentous bacteria)
2.6 Cell Size and the Significance of Being Small
• Size range for prokaryotes: 0.2 µm to >700 µm
in diameter
– Most cultured rod-shaped
bacteria are between 0.5 and
4.0 µm wide and <15 µm long
– Examples of very large
prokaryotes
• Epulopiscium fishelsoni
• Thiomargarita namibiensis
• Size range for eukaryotic cells:
10 to >200 µm in diameter
2.6 Cell Size and the Significance of Being Small
• Surface-to-Volume
Ratios, Growth Rates,
and Evolution
– Advantages to being
small
• Small cells have more
surface area relative to cell
volume than large cells (i.e.,
higher S/V)
– Support greater nutrient
exchange per unit cell
volume
– Tend to grow faster than
larger cells
r = 1 m
r = 1 m
Surface area (4r2) = 12.6 m2
Volume (43 r3) = 4.2 m3
Surface
=3
Volume
r = 2 m
r = 2 m
Surface area = 50.3 m2
Volume = 33.5 m3
Surface
= 1.5
Volume
2.6 Cell Size and the Significance of Being Small
• Lower Limits of Cell Size
– Cellular organisms < 0.15 µm in diameter are
unlikely
– Open oceans tend to contain small cells (0.2–0.4
µm in diameter)
II. The Cytoplasmic Membrane and Transport
• 2.7 Membrane Structure
• 2.8 Membrane Functions
• 2.9 Nutrient Transport
2.7 Membrane Structure
• Cytoplasmic membrane:
– Thin structure that surrounds the cell
– 6–8 nm thick
– Vital barrier that separates cytoplasm from
environment
– Highly selective permeable barrier
• Allows concentration of specific metabolites
• Excretion of waste products
2.7 Membrane Structure
• Composition of Membranes
– General structure is phospholipid bilayer
• Contain both hydrophobic and hydrophilic components
– Can exist in many different chemical forms as a
result of variation in the groups attached to the
glycerol backbone
– Hydrophobic fatty acids point inward
– Hydrophilic portions remain exposed to external
environment or the cytoplasm
Animation: Membrane Structure
Glycerol
Fatty acids
Phosphate
Ethanolamine
Hydrophilic
region
Fatty acids
Hydrophobic
region
Hydrophilic
region
Glycerophosphates
Fatty acids
2.7 Membrane Structure
• Cytoplasmic Membrane
– Embedded proteins
– Stabilized by hydrogen bonds and hydrophobic interactions
– Mg2+ and Ca2+ help stabilize membrane by forming ionic
bonds with negative charges on the phospholipids
– Somewhat fluid
Out
Phospholipids
Hydrophilic
groups
6–8 nm
Hydrophobic
groups
In
Integral
membrane
proteins
Phospholipid
molecule
2.7 Membrane Structure
• Membrane Proteins
– Outer surface of cytoplasmic membrane can
interact with a variety of proteins that bind
substrates or process large molecules for transport
– Inner surface of cytoplasmic membrane interacts
with proteins involved in energy-yielding reactions
and other important cellular functions
– Integral membrane proteins
• Firmly embedded in the membrane
– Peripheral membrane proteins
• One portion anchored in the membrane
2.7 Membrane Structure
• Archaeal Membranes
– Ether linkages in phospholipids of Archaea
– Bacteria and Eukarya that have ester linkages in
phospholipids
– Archaeal lipids lack fatty acids, have isoprenes instead
– Major lipids are glycerol diethers and tetraethers
– Can exist as lipid monolayers, bilayers, or mixture
Ester
Ether
isoprene
Bacteria
Eukarya
Archaea
Glycerol diether
CH3 groups
Isoprene unit
Biphytanyl
Diglycerol tetraethers
Crenarchaeol
Out
Out
Glycerophosphates
Phytanyl
Biphytanyl or
crenarchaeol
Membrane protein
In
In
2.8 Membrane Function
• Permeability Barrier
– Polar and charged molecules must be transported
– Transport proteins accumulate solutes against the
concentration gradient
• Protein Anchor
– Holds transport proteins in place
• Energy Conservation
Permeability barrier:
Protein anchor:
Energy conservation:
Prevents leakage and functions
as a gateway for transport of
nutrients into, and wastes out
of, the cell
Site of many proteins that
participate in transport,
bioenergetics, and chemotaxis
Site of generation and use of the
proton motive force
2.9 Nutrient Transport
• Carrier-Mediated Transport Systems
Rate of solute entry
– Show saturation effect
– Highly specific
– Highly regulated
Transporter saturated
with substrate
Transport
Simple diffusion
External concentration of solute
2.9 Nutrient Transport
• Transport systems in
prokaryotes
In
Out
– Simple transport
• Driven by the energy in the proton
motive force
– Group translocation
Transported
substance
• Chemical modification of the
transported substance driven by
phosphoenolpyruvate
– ABC system
• Periplasmic binding proteins are
involved and energy comes from
ATP
• All require energy in some
form, usually proton motive
force or ATP
1
2
3
2.9 Nutrient Transport
• Three transport events are possible: uniport,
symport, and antiport
– Uniporters transport in one direction across the
membrane
– Symporters function as co-transporters
– Antiporters transport a
molecule across
the membrane while
simultaneously
transporting another
molecule in the
opposite direction
2.9 Nutrient Transport
• Simple Transport:
– Lac Permease of Escherichia coli
• Lactose is transported into E. coli by the simple
transporter lac permease, a symporter
• Activity of lac permease is energy driven
• Other symporters, uniporters, and antiporters
Out
In
Sulfate
symporter
Potassium
uniporter
Phosphate
symporter
Sodium-proton
antiporter
Lac permease
(a symporter)
3.5 Transport and Transport Systems
• The Phosphotransferase System in E. coli
– Type of group translocation: substance transported is
chemically modified during transport across the
membrane
– Best-studied system
– Moves glucose, fructose, and mannose
– Five proteins required
Glucose
– Energy derived from
Out
phosphoenolpyruvate
Cytoplasmic
membrane
Nonspecific components
Specific components
Enz
IIC
PE
Enz
HPr
Enz
IIa
Direction of P transfer
Enz
IIb
In
Glucose 6–P
Direction
of glucose
transport
3.5 Transport and Transport Proteins
• ABC (ATP-Binding Cassette) Systems
– >200 different systems identified in prokaryotes
– Often involved in uptake
of organic compounds
(e.g., sugars, amino
acids), inorganic nutrients
(e.g., sulfate, phosphate),
and trace metals
– Typically display high
substrate specificity
– Contain periplasmic
binding proteins
IV. Cell Walls of Bacteria and Archaea
• 2.10 Peptidoglycan
• 2.11 LPS: The Outer Membrane
• 2.12 Archaeal Cell Walls
2.10 Peptidoglycan
Peptidoglycan
– Rigid layer that provides
strength to cell wall
– Polysaccharide composed of
• N-acetylglucosamine and Nacetylmuramic acid
• Amino acids
• Lysine or diaminopimelic acid
(DAP)
• Cross-linked differently in gramnegative bacteria and grampositive bacteria
N-Acetylmuramic acid
N-Acetyl
group
Peptide
cross-links
Lysozymesensitive
bond
L-Alanine
D-Glutamic acid
Diaminopimelic
acid
D-Alanine
Glycan tetrapeptide
N-Acetylglucosamine
Polysaccharide
backbone
Interbridge
Peptides
Escherichia coli
(gram-negative)
Staphylococcus aureus
(gram-positive)
Peptide bonds
Y
X
Glycosidic bonds
2.10 Peptidoglycan
• Gram-Positive Cell Walls
– Can contain up to 90% peptidoglycan
– Common to have teichoic acids (acidic
substances) embedded in the cell wall
• Lipoteichoic acids: teichoic acids covalently bound
Wall-associated Teichoic acid
Peptidoglycan Lipoteichoic
to membrane lipids protein
acid
Peptidoglycan
cable
Cytoplasmic membrane
2.11 LPS: The Outer Membrane
• Gram-Negative Cell Walls
– Total cell wall contains ~10% peptidoglycan
– 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
O-specific
polysaccharide
Core polysaccharide
Lipid A
Protein
Out
Lipopolysaccharide
(LPS)
Porin
Outer
membrane
8 nm
Cell
wall
Phospholipid
Periplasm
Peptidoglycan
Lipoprotein
Cytoplasmic
membrane
In
2.11 LPS: The Outer Membrane
• Porins: channels for movement
of hydrophilic low-molecular
weight substances
• Periplasm: space located
between cytoplasmic and outer
membranes
– ~15 nm wide
– Contents have gel-like
consistency
– Houses many proteins
Periplasm
Cytoplasmic
membrane
Outer membrane
2.12 Archeal Cell Walls
• No peptidoglycan
• Typically no outer membrane
• Pseudomurein
– Polysaccharide similar to peptidoglycan
– Composed of N-acetylglucosamine and Nacetyltalosaminuronic acid
– Found in cell walls of certain methanogenic
Archaea
• Cell walls of some Archaea lack pseudomurein
N-Acetyltalosaminuronic
acid
Lysozyme-insensitive
N-Acetylglucosamine
Peptide
cross-links
N-Acetyl
group
2.12 Archaeal Cell Walls
• S-Layers
– Most common cell
wall type among
Archaea
– Consist of protein or
glycoprotein
– Paracrystalline
structure
V. Other Cell Surface Structures and
Inclusions
•
•
•
•
2.13 Cell Surface Structures
2.14 Cell Inclusions
2.15 Gas Vesicles
2.16 Endospores
2.13 Cell Surface Structures
• Capsules and Slime Layers
– Polysaccharide layers
• May be thick or thin, rigid or
flexible
– Assist in attachment to
surfaces
– Protect against phagocytosis
– Resist desiccation
2.13 Cell Surface Structures
• Fimbriae
– Filamentous protein structures
– Enable organisms to stick to surfaces or form
pellicles
Flagella
Fimbriae
2.13 Cell Surface Structures
• Pili
–
–
–
–
Filamentous protein structures
Typically longer than fimbriae
Assist in surface attachment
Facilitate genetic exchange between cells
(conjugation)
– Type IV pili involved in twitching motility
Viruscovered
pilus
2.14 Cell Inclusions
• Carbon storage polymers
-carbon
– Poly--hydroxybutyric acid
(PHB): lipid
– Glycogen: glucose
polymer
• Polyphosphates:
accumulations of
inorganic phosphate
• Sulfur globules:
composed of elemental
sulfur
• Magnetosomes: magnetic
storage inclusions
Polyhydroxyalkanoate
Sulfur
2.16 Endospores
• Endospores
– Highly differentiated cells resistant
heat, harsh chemicals, and radiation
Vegetative cell
– “Dormant” stage of
bacterial life cycle
– Ideal for dispersal
Developing spore
via wind, water,
or animal gut
– Only present
Sporulating cell
in some
gram-positive
bacteria
Mature spore
to
VI. Microbial Locomotion
• 2.17 Flagella and Motility
• 2.18 Gliding Motility
• 2.19 Microbial Taxes
2.17 Flagella and Swimming Motility
• Flagellum (pl. flagella): structure that
assists in swimming
– Different arrangements: peritrichous, polar,
lophotrichous
– Helical in shape
Animation: The Prokaryotic Flagellum
2.17 Flagella and Swimming Motility
15—20 nm
L
P
Filament
Flagellin
MS
• Flagellar structure
Hook
Outer
membrane
(LPS)
L Ring
Rod
– Consists of several
components
– Filament composed of
flagellin
– Move by rotation
P Ring
Periplasm
Peptidoglycan
Rod
MS Ring
MS Ring
Basal
body
C Ring
Cytoplasmic Mot protein Fli proteins Mot protein
(motor switch)
membrane
45 nm
C Ring
Mot
protein
2.17 Flagella and Swimming Motility
• Flagella increase or
decrease rotational
speed in relation to
strength of the proton
motive force
• Differences in
swimming motions
– Peritrichously
flagellated cells
move slowly in a
straight line
– Polarly flagellated
cells move more
rapidly and typically
spin around
Tumble—flagella
pushed apart
(CW rotation)
Bundled
flagella
(CCW rotation)
Flagella bundled
(CCW rotation)
Peritrichous
Reversible flagella
CCW rotation
CW rotation
Unidirectional flagella
CW rotation
Polar
Cell
stops,
reorients
CW rotation
2.19 Chemotaxis and Other Taxes
• Taxis: directed movement in response to
chemical or physical gradients
– Chemotaxis: response to chemicals
• Best studied in E. coli
• Bacteria respond to temporal, not spatial, difference
in chemical concentration
• “Run and tumble” behavior
• Attractants sensed
by
chemoreceptors
Tumble
Tumble
Run
Run
No attractant present:
Random movement
Attractant present:
Directed movement
Attractant
2.19 Chemotaxis and Other Taxes
• Measuring chemotaxis
– 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
Cells per tube
Control
Attractant
Repellent
Attractant
Control
Repellent
Time
