Download Microbiology: A Systems Approach

LECTURES IN
MICROBIOLOGY
The Prokaryotes
LESSON 4
Sofronio Agustin
Professor
Lesson 4 Topics
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External Structures
Cell Envelope
Internal Structures
Cell Shapes, Arrangement, and Sizes
Classification
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External Structures
 Flagella
 Pili and fimbriae
 Glycocalyx
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Flagella
 Composed of protein subunits called flagellin.
 “H” antigens used in serotyping of bacterial
strains.
 Example: Escherichia coli O157: H7
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Flagellar Structure
Three components of a flagellum: filament, hook and basal body
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Flagellar Arrangement
(a) Monotrichous (b) Lophotrichous (c) Amphitrichous (d) Peritrichous
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Bacterial Motility
The rotation of the flagella enables bacteria to be motile.
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Chemotaxis
Chemotaxis is the movement of bacteria in response to chemical signals. It
consists of a series of tumbles and runs toward or away from source of stimuli.
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Endoflagella
Spirochetes have their
flagella embedded in the
membrane = endoflagella
Also called axial filament
Example: T. pallidum
(corkscrew motility)
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Pili and Fimbriae
 Attachment
 Mating (Conjugation)
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Fimbriae
Fimbriae are smaller than flagella and are important for attachment.
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Pili
Pili enable conjugation to occur, which is the transfer of DNA
from one bacterial cell to another (“mating”).
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Glycocalyx
 Capsule
Protects bacteria from phagocytic cells
 Slime layer
Enable attachment and aggregation of
bacterial cells
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Capsule
The capsule is covalently
bound to the cell wall.
Associated with virulence
in bacteria.
Example:
Streptococcus pneumoniae
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Slime Layer
The slime layer is
loosely bound to the
cell.
Carbohydrate rich
material enhances
adherence of cells on
surfaces
Example:
Streptococcus mutans
and “plaque formation”
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Biofilms
The slime layer is
associated with cell
aggregation and the
formation of biofilms
Example:
Staphylococcus
epidermidis biofilms
on catheter tips
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Cell Envelope
 Cell wall
Gram-positive
Gram-negative
 Cytoplasmic membrane
 Cell wall-less bacteria
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Cell Wall
 Gram positive cell wall
 Thick peptidoglycan (PG) layer
 Acidic polysaccharides
 Teichoic acid and lipoteichoic acid
 Gram-negative cell wall
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Thin peptidoglycan (PG) layer
Lipopolysaccharide layer
Porins
Periplasmic space
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Peptidoglycan Layer
PG is a complex sugar and peptide structure important for cell wall stability and shape.
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Cell Wall Structures
Structures associated with gram-positive and gram-negative cell walls.
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Cytoplasmic Membrane
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Phospholipid bilayer
“Fluid mosaic” model
Embedded proteins for active transport
Enzymes for energy generation
Photosynthetic pigments
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L Forms
Mutations can cause some bacteria to lose the ability to synthesize the cell wall and are called L forms.
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Cell Wall Less Bacteria
 No peptidoglycan layer
 Cell membrane contains sterols for stability
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The Mycoplasma
Mycoplasma bacteria
have no cell wall, which
contributes to their
pleomorphic shapes
Smallest bacteria
(0.2 um)
Example:
Mycoplasma pneumoniae
(SEM on right)
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Internal Structures
 Cytoplasm
 Genome
 Inclusion bodies
 Actin
 Endospore
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Cytoplasm
 Gelatinous solution containing water,
nutrients, proteins, and genetic material
 Site for cell metabolism
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Genetic Structures
 Deoxyribonucleic acid (DNA)
 Ribonucleic acid (RNA)
 Ribosomes
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Bacterial Genome
Most bacteria contain a single circular double strand of DNA called a nucleoid.
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Prokaryotic Ribosome
A ribosome is a
combination of RNA and
protein, and is the site
for protein synthesis
Composed of large
(50S) and small (30S)
subunits
S = Svedverg unit,
measures molecular size
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Inclusion Bodies
Inclusion bodies enable a cell to store nutrients and to survive in
nutrient depleted environments
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Bacterial Cytoskeleton
Actin is a protein fiber present in some bacteria, which is involved in
maintaining cell shape.
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Endospores
Nutrient depletion induces
some bacteria (vegetative
cell) to form endospores in
order to survive
Dehydrated gel state due
to calcium-protein complex
Dipicolinic acid (found
only in spores) hardens the
spore
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Endospore Formation
Some pathogenic bacteria that produce toxins during the vegetative stage are
capable of forming spores. (e.g. Bacillus and Clostridium species)
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Bacterial Morphology
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Coccus
Rod or bacillus
Curved or spiral
Cell arrangements
Pleomorphism
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Typical Shapes and Arrangements
Scanning electron micrographs of different bacterial shapes and arrangements.
(a) Streptococcus
(b) Bacillus
(c) Spirochete
(d) Spirillum
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Pleomorphism
Some bacteria show varied shapes and arrangements called pleomorphism.
Ex: Corynebacterium diphtheriae’s “Chinese letter” arrangement.
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Bacterial Shapes and Arrangements
Cellular shapes and arrangements are useful in bacterial identification.
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The Dimension of Bacteria
Relative size of a bacterial cell compared to other cells including viruses.
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Classification
 Phenotypic methods
 Molecular methods
 Taxonomic scheme
 Unique groups
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Phenotypic Methods
 Cell morphology - staining
 Biochemical test – enzyme test
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Molecular Methods
 DNA sequence
 16S RNA
 Protein sequence
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Major Taxonomic Groups of Bacteria
The methods of
classification have
allowed bacteria to be
classified into different
taxonomic groups
Re: Bergey’s Manual
of Determinative
Bacteriology
(Table on right)
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Unique Bacterial Types
 Intracellular bacteria
 Photosynthetic bacteria
 Sulfur bacteria
 Gliding and fruiting bacteria
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Intracellular Bacteria
Intracellular bacteria
must live in host cells for
them to metabolize and
reproduce
Examples:
Rickettsiae
and Chlamydiae
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Cyanobacteria
Cyanobacteria are important photosynthetic bacteria associated with oxygen production.
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Sulfur Bacteria
Green and
purple sulfur
bacteria are
photosynthetic,
do not give off
oxygen, and are
found in sulfur
springs,
freshwater, and
swamps.
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Myxobacteria
An example of a fruiting body bacteria in which reproductive spores are produced.
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Archaea
 Associated with extreme environments
 Contain unique cell walls
 Contain unique internal structures
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Archaea
Archaea are found in:
hot springs
(thermophiles)
high salt content
areas (halophiles)
Example:
Halobacterium
salinarium
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