Prokaryotes

Chapter 28

2

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 m 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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10

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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1. Amino acid sequences of key proteins

2. Percent guanine-cytosine content

3. Nucleic acid hybridization

4. Ribosomal RNA sequencing

5. Whole-genome sequencing

Molecular Classification

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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

Molecular Classification

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Molecular Classification

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Part 2: Prokaryotic Cell Structure

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Most prokaryotes have one of 3 basic shapes

-Bacillus = Rod-shaped

-Coccus = Spherical

-Spirillum = Helical-shaped

Prokaryotic Shapes

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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

The Bacterial Cell Wall

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The Bacterial Cell Wall

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Two main types

-Gram-positive bacteria (purple)

-Thick peptidoglycan

-Gram-negative bacteria (red/pink)

-Thin peptidoglycan

-Have an outer membrane

-Contains lipopolysaccharide

The Bacterial Cell Wall

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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 StructureInternal 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

-Only a few genes can be transferred

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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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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

Sexually transmitted diseases (STDs)

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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

Sexually transmitted diseases (STDs)

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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