Prokaryotes Chapter 28

The First Cells Microfossils are fossilized forms of microscopic life -Oldest are 3.5 billion years old

The First Cells Stromatolites are mats of cyanobacterial cells that trap mineral deposits -Oldest are 2.7 billion years old

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

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

Prokaryotic Features Unicellularity -Most are single-celled -Some can form complex biofilms Cell size -Most are less than 1 mm in diameter Chromosome -Single circular double-stranded DNA -Found in the nucleoid

Prokaryotic Features Internal compartmentalization -No membrane-bounded organelles Flagella -Simple in structure; spin like propellers Cell division -Most divide by binary fission Genetic recombination -Occurs through horizontal gene transfer

Prokaryotic Features Metabolic diversity -Two types of photosynthesis -Oxygenic = Produces oxygen -Anoxygenic = Nonoxygen producing - E.g: Sulfur and sulfate -Chemolithotrophic prokaryotes derive energy from inorganic molecules

Bacteria vs. Archaea 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

Bacteria vs. Archaea

Bacteria vs. Archaea Cell wall -Bacteria have peptidoglycan -Archaea lack peptidoglycan DNA replication -Archaeal DNA replication is more similar to that of eukaryotes Gene Expression -Archaeal transcription and translation are more similar to those of eukaryotes

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

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

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

Molecular Classification

Prokaryotic Shapes Most prokaryotes have one of 3 basic shapes -Bacillus = Rod-shaped -Coccus = Spherical -Spirillum = Helical-shaped

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 -Some have pseudopeptidoglycan Cell wall is the basis of the Gram stain

The Bacterial Cell Wall

The Bacterial Cell Wall Two main types -Gram-positive bacteria -Thick peptidoglycan -Teichoic and lipoteichoic acids -Gram-negative bacteria -Thin peptidoglycan -Have an outer membrane -Contains lipopolysaccharide

The Bacterial Cell Wall

External Layers S-layer -A rigid paracrystalline layer found in some bacteria and archaea -Aids in attachment Capsule -A gelatinous layer found in some bacteria -Protects from the immune system

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

Bacterial Appendages

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

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

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 -Transformation = From the environment

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

Conjugation Transfer of the F plasmid occurs through the conjugation bridge The end result is two F+ cells

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

Conjugation

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

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

Transduction

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

Transformation

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

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)

Prokaryotic Metabolism Acquisition of Carbon -Autotrophs = From inorganic CO2 -Heterotrophs = From organic molecules Acquisition of Energy -Chemolithotrophs = From inorganic chemicals -Phototrophs = From sunlight

Prokaryotic Metabolism Photoautotrophs -Cyanobacteria Chemolithoautotrophs -Nitrifiers Photoheterotrophs -Purple and green nonsulfur bacteria Chemoheterotrophs -Majority of prokaryotes -Use organic molecules for C and energy

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

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

Human Bacterial Disease Tuberculosis -Mycobacterium tuberculosis -A scourge for thousands of years -Afflicts the respiratory system -Mutidrug-resistant (MDR) strains are very alarming

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

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

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

Sexually transmitted diseases (STDs)

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)

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

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

Beneficial Prokaryotes