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SURVEY OF
PROKARYOTIC CELLS
Chapter 4
TYPICAL PROKARYOTIC CELL
Prokaryotic Cells: Shapes
• Average size: 0.2 –1.0 µm diameter & 2 – 8 µm length
• Shapes:
• Bacillus (rod-like), Coccus (spherical), Spiral (corkscrew, curved)
Figure 4.7a
Unusually Shaped Bacteria
Figure 4.5a
THE MOST COMMON BACTERIAL
SHAPES
Starting from the outside
Cell Extensions and
Surface Structures
Provide Motility
Flagella
Attachments/Channels
Axial filaments
Fimbriae
Coverings/ Glycocalyx
Pili
Slime layer
Capsule
MOBILITY
Flagella & Axial Filaments
Flagella
• Outside cell wall
• Made of chains of
•
•
•
•
flagellin (vs. tubulin)
Certain pathogenic
bacteria can be identified
by their flagellar proteins.
Attached to a protein
hook
Anchored to the wall and
membrane by the basal
body
animation
Figure 4.8
Polar, monotrichous flagellum
Pseudomonas (3,300X)
Polar, amphitrichous (am- fit-trichous) flagellum
Spirillum (694X)
Lophotrichous (le-fa-tri-kes)
flagella
Peritrichous (per-rit-tri-chous) flagella
Salmonella (1200X)
Flagella
• Usually too small to be seen with a typical microscope –
have to study live bacteria to detect mobility.
• Providing movement (in response to chemicals or light)
• Chemotaxis (move in response to chemical signals).
• Positive chemotaxis  more runs
• Negative chemotaxis  more tumbles
• Phototaxis
Axial Filaments
• Internal flagella
• In spirochetes
• Anchored at one end of a
cell
• Rotation causes cell to
move
• Video: Spirochetes the
cause Lyme disease
Figure 4.10a
ATTACHMENTS/CHANNEL
S
Pili & Fimbriae
Fimbriae
• Fimbriae are small bristlelike fibers that allow attachment
E. Coli colonizes the intestine by using the fimbriae to attach to each
other and the cells lining of the intestinal track
Biofilms! (plaque on teeth or scum in showers).
Figure 4.11
Pili
• Pili - usually longer than
fimbriae and only one or two
per cell.
• Facilitate transfer of DNA from
one cell to another – join bacterial
cells together.
Sympathy for Bacteria
EXTERNAL COATING
The glycocalyx
Glycocalyx (for protection & adherence):
Capsules
•Only certain bacteria are
capable of forming capsules
(many pathogenic ones – like
pneumonia)
•Chemical composition of each
capsule is unique to the strain
of bacteria that secreted it
•Capsules prevent phagocytosis
(process of engulfing a microbe) by
blocking the mechanism used by
the white blood cell to attach.
Slime Layer
• Less tightly bound to the cell
wall and is usually thinner
than a capsule
• Protects the cell against
drying, traps nutrients and
binds cells together (biofilm)
Pathogenicity: Adhesion
• Adhesion molecules on glycocalyx, pili, fimbriae, and
flagella attach directly to host cell receptors
• Can form biofilms, very complex, many layers thick, resist
disinfectants and antibiotics.
Slime Layer/Biofilm: Bacteria growing on tooth enamel
• Prevents osmotic lysis
(prevents cell rupture)
• Maintains cell shape
• Made of peptidoglycan
(in bacteria)
• CLINICAL
SIGNIFICANCE:
•
•
Cell wall composition can
contribute to the ability
of the cell to cause
disease
Also the site of action for
some antibiotics.
Figure 4.6a, b
Components of Bacterial Cell Walls

Peptidoglycan: The single most important component!
This polymer is made up of two alternating sugar units:
1.
2.

N-acetylglucosamine (NAG)
N-acetylmuramic acid (NAM)
The sugars are joined by short peptide chains that
consist of four amino acids (tetrapeptides)
Peptidoglycan
• Polymer of disaccharide
(“glyco” portion):
• N-acetylglucosamine (NAG)
• N-acetylmuramic acid (NAM)
Figure 4.12
3D VIEW
Peptidoglyccan Structure
Arrangements
• Pairs: Diplococci,
diplobacilli
• Clusters: Staphylococci
• Chains: Streptococci,
streptobacilli
Figures 4.1a, 4.1d, 4.2b, 4.2c
Can you figure
out the first part
of the name of
this bacteria by
describing it’s
shape?
Streptococcus mutans –
bacterial responsible for
causing cavities!
Q&A
• Advertisements tell you that
bacteria and viruses are all
over your home and that you
need to buy antibacterial
cleaning products. Should you?
MICROSCOPES &
PREPARING SLIDES
Chapter 3
Observing Microorganisms
Figure 3.2
Scale animation
Units of Measurement
• 1 µm = 10–6 m = 10–3 mm
• 1 nm = 10–9 m = 10–6 mm
• 1000 nm = 1 µm
• 0.001 µm = 1 nm
• Generally, bacteria are
measured in um and can be
seen with a light microscope.
• Viruses are measured in nm and can only be seen with an
electron microscope.
Figure 3.2
• If a microbe measures 10 μm in length, how long is it in
nanometers? 3-1
Microscopy: The Instruments
• 3-2
Diagram the path of light through a compound
microscope.
• 3-3 Define total magnification and resolution.
•
Light Microscopy
• Use of any kind of microscope that uses visible light to
observe specimens
• Types of light microscopy
• Compound light microscopy
• Darkfield microscopy
• Phase-contrast microscopy
• Differential interference contrast microscopy
• Fluorescence microscopy
• Confocal microscopy
The Compound Light Microscope
Figure 3.1a
Compound Light Microscopy
• In a compound
microscope, the image
from the objective lens is
magnified again by the
ocular lens
• Total magnification =
objective lens  ocular lens
Figure 3.1b
Compound Light Microscopy
• Resolution is the ability of the lenses to distinguish two
points
• A microscope with a resolving power of 0.4 nm can
distinguish between two points ≥ 0.4 nm
• Shorter wavelengths of light provide greater resolution
Resolution
Effect of
Wavelength on
Resolution
Compound Light Microscopy
• The refractive index is a measure of the light-bending
ability of a medium
• The light may bend in air so much that it misses the small
high-magnification lens
• Immersion oil is used to keep light from bending
Refraction in the Compound Microscope
Figure 3.3
• Many things can happen to light as it passes
through a specimen on a slide.
• Reflection: If the light strikes an object and bounces back
(giving the object color)
• Transmission: The passage of light through an object
• Absorption: The light rays neither pass through nor bounce off
an object but are taken up by the object
• The more light that passes through the specimen
(vs. lost), the higher the resolution
Various Interactions of Light
Refraction
• The bending of light as it passes from one
medium to another of different density
• The bending of the light ray gives rise to an
angle of refraction, the degree of bending
• Index of refraction: A measure of the speed at
which light passes through the material
• When two substances have a different index of
refraction, the light is bent and is scattered
• When two substances have a similar index of
refraction (diamonds and oil) then the light is not
bent as it passes between the two substances
Microscopy: The Instruments
 Refractive index is the
light-bending ability of
a medium.
 The light may bend in
air so much that it
misses the small highmagnification lens.
 Immersion oil is used
to keep light from
bending.
Figure 3.3
• Through what lenses does light pass in a compound
microscope? 3-2
• What does it mean when a microscope has a resolution of
0.2 nm? 3-3
Electron Microscopy
• Uses electrons instead of light
• The shorter wavelength of electrons gives greater
resolution
ANIMATION Electron Microscopy
Transmission Electron Microscopy (TEM)
• Ultrathin sections of
specimens
• Light passes through
specimen, then an
electromagnetic lens,
to a screen or film
• Specimens may be
stained with heavy metal
salts
Figure 3.10a
Transmission Electron Microscopy (TEM)
• 10,000–100,000; resolution 2.5 nm
Figure 3.10a
Scanning Electron Microscopy (SEM)
• An electron gun
produces a beam of
electrons that scans the
surface of a whole
specimen
• Secondary electrons
emitted from the
specimen produce the
image
Figure 3.10b
Scanning Electron Microscopy (SEM)
• 1,000–10,000; resolution 20 nm
Figure 3.10b
Scanned-Probe Microscopy
• Scanning tunneling microscopy (STM) uses a metal
probe to scan a specimen
• Resolution 1/100 of an atom
Figure 3.11a
Scanned-Probe Microscopy
• Atomic force microscopy (AFM) uses a metal- and-
diamond probe inserted into the specimen.
• Produces three-dimensional images.
Figure 3.11b
• Why do electron microscopes have greater resolution
than light microscopes? 3-5
• For what is TEM used? SEM? Scanned-probe
microscopy? 3-6
Gram Stain
• Classifies bacteria into gram-positive
or gram-negative
• Gram-positive bacteria tend to be killed by penicillin and detergents
• Gram-negative bacteria are more resistant to antibiotics