SURVEY OF PROKARYOTIC CELLSChapter 4

TYPICAL PROKARYOTIC CELL

Figure 4.7a

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 4.5a

Unusually Shaped Bacteria

THE MOST COMMON BACTERIAL SHAPES

Starting from the outside

Cell Extensions and Surface Structures

Provide Motility

Flagella Axial filaments

Attachments/Channels

Fimbriae Pili

Coverings/ Glycocalyx

Slime layer Capsule

MOBILITYFlagella & 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

• animationFigure 4.8

Pseudomonas (3,300X)Polar, monotrichous flagellum

Spirillum (694X)

Polar, amphitrichous (am- fit-tri-chous) flagellum

Lophotrichous (le-fa-tri-kes) flagella

Salmonella (1200X)

Peritrichous (per-rit-tri-chous) flagella

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 th

e cause Lyme disease

Figure 4.10a

ATTACHMENTS/CHANNELSPili & Fimbriae

Figure 4.11

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).

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 COATINGThe 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

Cell Wall• 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. N-acetylglucosamine (NAG)2. N-acetylmuramic acid (NAM)

The sugars are joined by short peptide chains that consist of four amino acids (tetrapeptides)

Figure 4.12

Peptidoglycan• Polymer of disaccharide

(“glyco” portion):• N-acetylglucosamine (NAG) • N-acetylmuramic acid (NAM)

3D VIEW

Peptidoglyccan Structure

Figures 4.1a, 4.1d, 4.2b, 4.2c

Arrangements• Pairs: Diplococci,

diplobacilli

• Clusters: Staphylococci

• Chains: Streptococci, streptobacilli

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 SLIDESChapter 3

Observing Microorganisms

Figure 3.2

Scale animation

Figure 3.2

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.

• 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

Figure 3.1a

The Compound Light Microscope

Figure 3.1b

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

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

Figure 3.3

Refraction in the Compound Microscope

• 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

• 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

Refraction

• 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

Refractive index is 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.

Microscopy: The Instruments

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

• Uses electrons instead of light• The shorter wavelength of electrons gives greater

resolution

ANIMATION Electron Microscopy

Electron Microscopy

Figure 3.10a

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 3.10b

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 3.11a

Scanned-Probe Microscopy• Scanning tunneling microscopy (STM) uses a metal

probe to scan a specimen• Resolution 1/100 of an atom

Figure 3.11b

Scanned-Probe Microscopy• Atomic force microscopy (AFM) uses a metal- and-

diamond probe inserted into the specimen.• Produces three-dimensional images.

• 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