Prokaryotes Chapter 28

The First Cells Microfossils are fossilized forms of microscopic life –Oldest are 3.5 billion years old –Seem to resemble present-day prokaryotes 2

Stromatolites are mats of cyanobacterial cells that trap mineral deposits –Oldest are 2.7 billion years old –Modern forms are also known 3

Unicellularity –Most are single-celled –Colony-forming –Some can form complex biofilms Cell size –Most are less than 1  m in diameter Chromosome –Single circular double-stranded DNA –Found in the nucleoid region of cell –Often have plasmids Cell division –Most divide by binary fission 4

Genetic recombination –Occurs through horizontal gene transfer Internal compartmentalization –No membrane-bounded organelles Flagella –Different from eukaryotic flagella Metabolic diversity –Oxygenic and anoxygenic photosynthesis –Chemolithotrophic 5

Prokaryotic Cell Structure 3 basic shapes –Bacillus – Rod-shaped –Coccus – Spherical –Spirillum – Helical-shaped 6

Cell wall –Peptidoglycan forms a rigid network Maintains shape Withstands hypotonic environments Archaea have a similar molecule –Gram stain Gram-positive bacteria have a thicker peptidoglycan wall and stain a purple color Gram-negative bacteria contain less peptidoglycan and do not retain the purple-colored dye – retain counterstain and look pink 7

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Gram positive bacteria –Thick, complex network of peptidoglycan –Also contains lipoteichoic and teichoic acid Gram negative bacteria –Thin layer of peptidoglycan –Second outer membrane with lipopolysaccharide –Resistant to many antibiotics 9

S-layer –Outside of peptidoglycan or outer membrane layers in gram-negative and gram-positive bacteria –Diverse functions – often involves adhesion Capsule –Gelatinous layer found in some bacteria –Aids in attachment –Protects from the immune system 10

Flagella –Slender, rigid, helical structures –Composed of the protein flagellin –Involved in locomotion – spins like propeller Pili –Short, hairlike structures –Found in gram-negative bacteria –Aid in attachment and conjugation 11

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Endospores –Develop a thick wall around their genome and a small portion of the cytoplasm –When conditions improve can germinate and return to normal cell division –Bacteria causing tetanus, botulism, and anthrax 13

Nucleoid region –Contains the single, circular chromosome –May also contain plasmids Ribosomes –Smaller than those of eukaryotes –Differ in protein and RNA content –Targeted by some antibiotics 14

Prokaryotic Genetics Prokaryotes do not reproduce sexually 3 types of horizontal gene transfer –Conjugation – cell-to-cell contact –Transduction – by bacteriophages –Transformation – from the environment All 3 processes also observed in archaea 15

Conjugation Plasmids may encode advantageous info –Are not required for normal function In E. coli, conjugation is based on the presence of the F plasmid (fertility factor) –F + cells contain the plasmid –F - cells do not 16

F + cell produces F pilus that connects it to F - cell Transfer of F plasmid occurs through conjugation bridge F plasmid copied through rolling circle replication The end result is two F + cells 17

The F plasmid can integrate into the bacterial chromosome –Events similar to crossing over in eukaryotes –Homologous recombination Hfr cell (high frequency of recombination) –F plasmid integrated into chromosome –Replicated every time host divides The F plasmid can also excise itself by reversing the integration process –An inaccurate excision may occur picking up some chromosomal DNA – F ′ plasmid 18

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Transduction Generalized transduction –Virtually any gene can be transferred –Occurs via accidents in the lytic cycle –Viruses package bacterial DNA and transfer it in a subsequent infection Specialized transduction –Occurs via accidents in the lysogenic cycle –Imprecise excision of prophage DNA –Only a few host genes can be transferred 21

22 Infection with Phage Phage adheres to cell.Phage DNA is injected in to cell.Phage DNA is replicated and host DNA is degraded. Phage particles are packaged with DNA and are released. Infection with Transducing Phage Cell contains DNA from donor. Transducing phage adheres to cell. Phage injects a piece of chromosomal DNA. DNA is incorporated by homologous recombination. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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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 –Encoded by bacterial chromosome Not an accident of plasmid or phage biology 24

Artificial transformation –Some species do not naturally undergo transformation –Accomplished in the lab –Used to transform E. coli for molecular cloning 25

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R (resistance) plasmids –Encode antibiotic resistance genes –Acquire genes through transposable elements –Important factor in appearance of antibiotic resistant strains of Staphylococcus aureus Virulence plasmids or transduction –Encode genes for pathogenic traits –Enterobacteriaceae –E. coli O157:H7 strain 27

Mutations can arise spontaneously in bacteria as with any organism –Radiation and chemicals increase likelihood Auxotrophs are nutritional mutants –Studied using replica plating Mutations (and plasmids) can spread rapidly in a population –Methicilin-resistant Staphylococcus aureus (MRSA) –Vancomycin-resistant Staphylococcus aureus (VRSA) 28

Prokaryotic Metabolism Acquisition of Carbon –Autotrophs – from inorganic CO 2 Photoautotrophs – energy from Sun Chemolithoautotrophs – energy from oxidizing inorganic substances –Heterotrophs – from organic molecules Photoheterotrophs – light as energy source but obtain organic carbon made by other organisms Chemoheterotroph – both carbon atoms and energy from organic molecules –Humans are also an example 29

Type III secretion system –Found in many gram-negative bacteria –Molecular syringe to inject virulence proteins into host cell cytoplasm –Yersinia pestis (bubonic plague), Salmonella, Shigella 30

Human Bacterial Disease In the early 20 th 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 31

Tuberculosis (TB) Scourge for thousands of years Mycobacterium tuberculosis Afflicts the respiratory system Thwarts immune system Easily transferred from person to person through the air Multidrug-resistant (MDR) strains are very alarming 32

Dental caries (tooth decay) –Plaque consists of bacterial biofilms –Streptococcus sobrinus ferments sugar to lactic acid –Tooth enamel degenerates Peptic ulcers –Helicobacter pylori is the main cause –Treated with antibiotics 33

Sexually transmitted diseases (STDs) Gonorrhea –One of the most prevalent communicable diseases in North America. –Neisseria gonorrhoeae –Transmitted through exchange of body fluids –Can pass from mom to baby via birth canal Chlamydia –Chlamydia trachomatis –“Silent STD” – incidence has skyrocketed –Can cause PID and heart disease 34

Syphilis –Treponema pallidum –Transmitted through sex or contact with open chancre –Can pass from mom to baby via birth canal –Four distinct stages Primary – Chancre – highly infectious Secondary – Rash – infectious Tertiary – Latency – no longer infectious but attacking internal organs Quaternary – Damage now evident 35