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Prokaryotes
Chapter 28
The First Cells
Microfossils are fossilized forms of
microscopic life
-Oldest are 3.5 billion years old
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The First Cells
Stromatolites are mats of cyanobacterial
cells that trap mineral deposits
-Oldest are 2.7 billion years old
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The First Cells
Isotopic analysis of carbon-12 in fossils
suggests that carbon fixation was active
as much as 3.8 BYA
Biomarkers are organic molecules of
biological origin
-Lipids were found in ancient rocks
-This indicates that cyanobacteria are
at least 2.7 billion years old
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Prokaryotic Diversity
Prokaryotes are the oldest, and structurally
simplest forms of life
Prokaryotes are ubiquitous
Less than 10% of species are known
Bacteria (also called eubacteria)
Archaea (formerly called archaebacteria)
-Many archaeans are extremophiles
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Part 1: Similarities and Differences
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Similarities: Prokaryotic Features
1. Unicellularity
-Most are single-celled
-Some can form complex biofilms
2. Cell size
-Most are less than 1 mm in diameter
3. Chromosome
-Single circular double-stranded DNA
-Found in the nucleoid
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Prokaryotic Features
4. Internal compartmentalization
-No membrane-bounded organelles
5. Flagella
-Simple in structure; spin like propellers
6. Cell division
-Most divide by binary fission (asexual)
7. Genetic recombination
-Occurs through horizontal gene transfer
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Prokaryotic Features
8. Metabolic diversity
-Two types of photosynthesis
-Oxygenic = Produces oxygen
-Anoxygenic = Nonoxygen producing
- E.g: Sulfur and sulfate
-Chemolithotrophic prokaryotes derive
energy from inorganic molecules
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Differences: Bacteria vs. Archaea
1. Plasma membrane
-Bacterial lipids are unbranched
-Connected to glycerol by ester linkages
-Archaeal lipids are branched
-Connected to glycerol by ether linkages
-Tetraethers form a monolayer
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Bacteria vs. Archaea
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Bacteria vs. Archaea
2. Cell wall
-Bacteria have peptidoglycan
-Archaea lack peptidoglycan
3. DNA replication
-Archaeal DNA replication is more similar to
that of eukaryotes
4. Gene Expression
-Archaeal transcription and translation are
more similar to those of eukaryotes
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Early Classification Characteristics
1. Photosynthetic or nonphotosynthetic
2. Motile or nonmotile
3. Unicellular or filamentous
4. Formation of spores or division by
transverse binary fission
5. Importance as human pathogens or not
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Molecular Classification
1. Amino acid sequences of key proteins
2. Percent guanine-cytosine content
3. Nucleic acid hybridization
4. Ribosomal RNA sequencing
5. Whole-genome sequencing
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Molecular Classification
Based on these molecular data, several
prokaryotic groupings have been proposed
-Bergey’s Manual of Systematic
Bacteriology
-Contains about 7,000 bacterial and
archaeal species
The three-domain (Woese) system of
phylogeny is based on rRNA sequences
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Molecular Classification
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Part 2: Prokaryotic Cell Structure
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Prokaryotic Shapes
Most prokaryotes have one of 3 basic shapes
-Bacillus = Rod-shaped
-Coccus = Spherical
-Spirillum = Helical-shaped
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The Bacterial Cell Wall
Maintains shape and protects the cell from
swelling and rupturing
Consists of peptidoglycan
-Polysaccharides cross-linked with peptides
Archaea do not possess peptidoglycan
Cell wall is the basis of the Gram stain
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The Bacterial Cell Wall
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The Bacterial Cell Wall
Two main types
-Gram-positive bacteria (purple)
-Thick peptidoglycan
-Gram-negative bacteria (red/pink)
-Thin peptidoglycan
-Have an outer membrane
-Contains lipopolysaccharide
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The Bacterial Cell Wall
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External Layers
S-layer
-A rigid layer found in some bacteria and
archaea
-Aids in attachment
Capsule
-A gelatinous layer found in some bacteria
-Aids in attachment
-Protects from the immune system
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Bacterial Appendages
Pili
-Short, hairlike structures
-Found in Gram-negative bacteria
-Aid in attachment and conjugation
Flagella
-Long, helical structures
-Composed of the protein flagellin
-Involved in locomotion
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Bacterial Appendages
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Internal Structure
Nucleoid region
-Contains the single, circular chromosome
-May also contain plasmids
Ribosomes
-Smaller than those of eukaryotes and differ
in protein and RNA content
-Targeted by antibacterial antibiotics
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Internal Structure
Internal membranes
-Invaginated cell
membrane
-For respiration or
photosynthesis
Endospores
-Highly-resistant structures
-Released upon cell lysis
-Can germinate back to normal cell
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Part 3: Prokaryotic Genetics
Prokaryotes do not reproduce sexually
However, they undergo horizontal gene
transfer, which is of three types
-Conjugation = Cell-to-cell contact
-Transduction = By bacteriophages (virus)
-Transformation = From the environment
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Conjugation
In E. coli, conjugation is based on
the presence of the F plasmid
F+ cells contain the plasmid
F cells do not
The F+ cell produce an F pilus
that connects it to an F cell
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Conjugation
Transfer of the F plasmid occurs through the
conjugation bridge
The end result is two F+ cells
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Conjugation
The F plasmid can integrate into the bacterial
chromosome
-Hfr cell (high frequency of recombination)
The F plasmid can also excise itself by
reversing the integration process
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Conjugation
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Conjugation
An inaccurate excision may occur
-F’ cell
Conjugation can occur between an F’ and an
F cell
-The result is a partial diploid, or
merodiploid
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Transduction
Generalized transduction
-Occurs via accidents in the lytic cycle
-Viruses package bacterial DNA and
transfer it in a subsequent infection
-Virtually any gene can be transferred
Specialized transduction
-Occurs via accidents in the lysogenic cycle
-Imprecise excision of prophage DNA
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-Only a few genes can be transferred
Transduction
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Transformation
Natural transformation
-Occurs in many bacterial species, including
Streptococcus which was studied by Griffith
-DNA that is released from a dead cell is
picked up by another live cell
Artificial transformation
-Accomplished in the lab
-Used to transform E. coli for molecular
cloning
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Transformation
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Conjugative Plasmids
Conjugative plasmids may pick up additional
genes
-R (resistance) plasmids
-Encode antibiotic resistance genes
-Staphylococcus aureus
-Virulence plasmids
-Encode genes for pathogenic traits
-Enterobacteriaceae
-E. coli O157:H7 strain
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Mutation
Mutations can arise spontaneously in bacteria
-Also caused by radiation and chemicals
Mutations (and plasmids) can spread rapidly
in a population
-Negative consequences for humans
-For example:
-Methicillin-resistance
Staphylococcus aureus (MRSA)
-Vancomycin-resistant
Staphylococcus aureus (VRSA)
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Part 5: Prokaryotic Metabolism
Acquisition of Carbon
-Autotrophs = From inorganic CO2
-Heterotrophs = From organic molecules
Acquisition of Energy
-Chemolithotrophs = From inorganic
chemicals
-Phototrophs = From sunlight
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Prokaryotic Metabolism
Photoautotrophs
-Cyanobacteria
Chemolithoautotrophs
-Nitrifiers
Photoheterotrophs
-Purple and green nonsulfur bacteria
Chemoheterotrophs
-Majority of prokaryotes
-Use organic molecules for C and energy
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Prokaryotic Metabolism
Type III secretion system
-Found in many Gram-negative bacteria
-Used to transfer virulence proteins directly
into host cells
-Yersinia pestis – Bubonic plague
-Pseudomonads – Plant pathogens
-Blights, soft rot, wilts
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Human Bacterial Disease
In the early 20th century, infectious diseases
killed 20% of children before the age of five
-Sanitation and antibiotics considerably
improved the situation
In recent years, however, many bacterial
diseases have appeared and reappeared
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Human Bacterial Disease
Tuberculosis
-Mycobacterium tuberculosis
-A scourge for
thousands of years
-Afflicts the
respiratory system
-Mutidrug-resistant
(MDR) strains are
very alarming
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Human Bacterial Disease
Dental caries (tooth decay)
-Plaque consists of bacterial biofilms
-Streptococcus ferments sugar to lactic acid
-Tooth enamel degenerates
Peptic ulcers
-Helicobacter pylori is the main cause
-Treated with antibiotics
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Sexually transmitted diseases (STDs)
Gonorrhea
-Neisseria gonorrhoeae
-Can pass from mom to baby via birth canal
-Can cause pelvic inflammatory disease (PID)
Chlamydia
-Chlamydia trachomatis
-“Silent STD”
-Can cause PID and heart disease
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Sexually transmitted diseases (STDs)
Syphilis
-Treponema pallidum
-Can pass from mom to baby via birth canal
-Four distinct stages
-Primary - Chancre
-Secondary - Rash
-Tertiary - Latency
-Quaternary - Heart and nerve damage
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Sexually transmitted diseases (STDs)
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Beneficial Prokaryotes
Prokaryotes are crucial to chemical cycles
-Decomposers release a dead organism’s
atoms to the environment
-Photosynthesizers fix carbon into sugars
-Nitrogen fixers reduce N2 to NH3
(ammonia)
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Beneficial Prokaryotes
Prokaryotes may live in symbiotic
relationships with eukaryotes
-Mutualism = Both parties benefit
-Nitrogen-fixing bacteria on plant roots
-Cellulase-producing bacteria in animals
-Commensalism = One organism benefits
and the other is unaffected
-Parasitism = One organism benefits and
the other is harmed
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Beneficial Prokaryotes
Bacteria are used in genetic engineering
- “Biofactories” that produce various
chemicals, including insulin and antibiotics
Bacteria are used for bioremediation
-Remove pollutants from water, air and soil
-Exxon Valdez oil spill
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Beneficial Prokaryotes
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