Post on 14-Apr-2018
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Major components of
prokaryotic cells
Dr. Thomas Seviour
twseviour@ntu.edu.sg
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Members of the Microbial
Worldtwo types of cells prokaryotic cellrelatively simple morphologylacks a true membrane-delimited nucleusBacteria and ArchaeaTerm prokaryote blurred
eukaryotic cellmorphologically complex
has a true membrane-delimited nucleusComplex cytoskeletonprotozoa, algae, fungi, plants and animals
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Now clear that PROKARYOTE CELLSare possessed by two differentphylogenetic groups
THE BACTERIATHE ARCHAEAThese differ from each other as
profoundly as eukaryotic cells differ
from prokaryotic cellsTerm prokaryote becoming blurred
Prokaryotes
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Cell Organization Bacteriaand Archaea
Common Features
Cell envelope 3 layers
Cytoplasm
External structures
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Bacterial cell morphology
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Bacterial Cell Envelope
Plasma membrane
Cell wall
Layers outside the cell wall
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Plasma membrane Absolute requirement for all living organisms separation of cell from its environment selectively permeable barrier
some molecules are allowed to pass into or out of the cell transport systems aid in movement of molecules
location of crucial metabolic processes detection of and response to chemicals in
surroundings with the aid of special receptormolecules in the membrane
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Fluid Mosaic Model of
Membrane Structure lipid bilayers with floating
proteins
amphipathic lipidspolar ends (hydrophilic interact with water)
non-polar tails(hydrophobic
insoluble in water) membrane proteins
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The fluid mosaic of bacterialmembranes
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Membrane Proteins peripheral loosely connected to
membrane
easily removed integral amphipathic
embedded withinmembrane
carry out importantfunctions
may exist asmicrodomains
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The fluid mosaic of bacterialmembranes
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Asymmetric Membrane
Lipids
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Phospholipids, such as phosphatidylethanolamine
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Bacterial Lipids saturation levels of membrane lipids reflect the
environmental conditions such as temperature
e.g. at low temperatures, more unsaturated lipids tomaintain fluidity during growth
bacterial membranes lack sterols but do containsterol-like molecules, hopanoids stabilize membrane found in petroleum
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Hopanoids
Sterol like (e.g. cholesterol) Natural pentacyclic compounds Hydrophobic tail, hydrophilic head Natural Membrane InsertionMolecules (MIM) Increase plasma membrane strength
and rigidity
Adjust membrane permeability Adaptation to extreme environmental
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Antimicrobial peptides Act by inserting themselves in the plasma
membrane of cells
Destabilizing MIM Potent, broad spectrum antibiotics Part of the innate immune response The amino acid composition, charge and size of
some AMPs allows them to attach to and insert
themselves into membrane bilayers, thus killingbacteria by membrane disruption
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Different models for membrane
lysis by antimicrobial peptides
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Synthetic MIMs Biocides, e.g phenylene
ethynylene polyelectrolyteoligomers (OPEs)
Size and charge mimiclipid bilayers
Insertion into membranecauses structural damagethat allows leakage of
water through membrane
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Synthetic MIMs Transmembrane electron transfer molecules (TETMs) E.g. Polyvinylene stilbene Designed to increase electron transfer across
membrane
Thus enhance performance of microbial fuel cells orbioelectrochemical systems
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TETMs
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But TETMs also perturb the
membrane and are biocidal
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Ultimate conformation of the POPE/POPG bilayer predicted by the
molecular dynamics simulation after
200 ns following the intercalation of
one and four molecules respectively of
DSSN+ (A, B), DSBN+ (C, D), and 4F-
DSBN+ (E, F) at 300 K. A, C and Erepresent the low concentrations of
TETMs, and B, D and F represent high
concentrations. The TETMs are shown
in blue, the phosphate head group inyellow, the water molecules in red-
white, and POPE/POPG acyl chains as
green lines
Thus need to understand membraneperturbation for intelligent design ofenhancement MIMs
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Cell walls of Bacteria
peptidoglycan (murein) rigid structure that lies just outside the plasma
membrane
Bacteria are divided into two major groups basedon the response to Gram-stain procedure. gram-positive bacteria stain purple; thick
peptidoglycan
gram-negative bacteria stain pink; thinpeptidoglycan and outer membrane
staining reaction due to cell wall structure Without a cell wall = protoplast 24
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Cell Wall Functions
maintains shape of the bacteriumalmost all bacteria have one
helps protect cell from osmotic lysis helps protect from toxic materials may contribute to pathogenicity
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Gram positive and Gramnegative cell wall organization
differs
Both possess a PEPTIDOGLYCAN or MUREINlayer
In GRAM VES we see an OUTERLIPOPOLYSACCHARIDE MEMBRANEcontaining LIPID A
In GRAM+VES we see TEICHOIC ACIDS
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Peptidoglycan Structure important component of both gram-
positive and gram-negative bacteria
polysaccharide formed from peptidoglycansubunits
two (1-4)- -linked alternating sugarsform backbone N-acetylglucosamine (NAG) N-acetylmuramic acid (NAM) In some bacteria no acetyl group or glycolyl substitution
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Peptidoglycan subunits and linkers
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Attached to NAM are short peptide chains, usually 4amino acid residues Serve to link together the glycan chains in several ways Amino acids there are often unusual Include D-isomers and an amino acid found nowhere
else in biological world
form backbone N-acetylglucosamine (NAG) N-acetylmuramic acid (NAM) In some bacteria no acetyl group or glycolyl substitution
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Peptidoglycan subunit
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Strands Are Crosslinked
peptidoglycan strands have a helicalshape
Amino acids serve to link together theglycan chains in several ways
Together called GLYCANTETRAPEPTIDE = BUILDING BLOCKOF PEPTIDOGLYCANS
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Helical structure enables
360 crosslinking
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Linkage of glycan chains
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Varied and involves thetetrapeptides of adjacent chains
In many Gram ve bacteriainvolves a direct link between3rd amino acid residue of one
chain (dibasic DAP or L-lysine)and 4th amino acid of otherchain (D-Alanine) eg E.coli
In many Gram +ves involves aninterbridge of several amino
acid residues Peptide bonds involved in
linkage (i.e. CO-NH)
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GENERAL COMMENTS
PEPTIDOGLYCAN found ONLY in BACTERIA Diaminopimelic acid (DAP) and muramic
acid unique to BACTERIA
DAP common in Gram ve bacteria, butreplaced in many Gram+ve bacteria by L-Lysine
D-isomers of amino acids not found inproteins
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Gram-Positive Cell Walls composedprimarily ofpeptidoglycan
also contain largeamounts ofteichoic acids
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Teichoic acids Found in nearly all GRAM +ve bacteria May help maintain structure, attachment,
protect cell from harmful substances
Polyphosphate sugar alcohols Chemically diverse, with glycerol, ribitol,
mannitol teichoic acids now known
Most bacteria have >1 kind in cell wall Some glycerol teichoic acids bound to
cytoplasmic membrane lipids (i.e.LIPOTEICHOIC ACIDS)
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Teichoic acid structure
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Gram-Positive cells
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Periplasmic Space of Gram +Bacteria
lies between plasma membrane and cellwall and is smaller than that of gram-negative bacteria
periplasm has relatively few proteins Peptidoglycan is ~5-10% of cell wall weight enzymes secreted by gram-positive
bacteria are called exoenzymes aid in degradation of large nutrients
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Gram-Negative Cell Walls more complex than gram positive consist of a thin layer of peptidoglycan
surrounded by an outer membrane
outer membrane composed of lipids,lipoproteins, and lipopolysaccharide (LPS) no teichoic acids periplasmic space differs from G+
may constitute 2040% of cell volume many enzymes present in periplasm
hydrolytic enzymes, transport proteins and otherproteins 41
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Gram-Negative outer layers
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outer membrane liesoutside the thinpeptidoglycan layer
Brauns lipoproteinsconnect outer membraneto peptidoglycan
Adhesion sites direct contact between
plasma membrane and
outer membrane substances may move
directly into cellthrough adhesion sites
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Outer membrane Lipopolysaccharides (LPS) in outer half ofouter membrane (phospholipids other) i.e.
asymmetric
Lipopolysaccharides consist of three parts
lipid A
core polysaccharide with ketodeoxyoctanoic acid(KDO) and range of sugars
O side chain (O somatic antigen) containsunusual dideoxy sugars
Buried in outer membrane
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Lipolysaccharide
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Lipid Aburied inmembrane
Lipid Aand corearestraight
O-sidechain bent
at an angle
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Other characteristics of
outer membrane more permeable than plasma membrane due
to presence of porin proteins and transporterproteins
porin proteins form channels through which smallmolecules (600-700 daltons) can pass
-barrel
structure
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Importance of LPS contributes to negative charge on cell
surface
helps stabilize outer membrane structure may contribute to attachment to surfacesand biofilm formation creates a permeability barrier protection from host defenses (O antigen) can act as an endotoxin (lipid A)
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Osmotic Protection (cell wall) hypotonic environments solute concentration outside the cell is less than
inside the cell
water moves into cell and cell swells cell wall protects from lysis
hypertonic environments solute concentration outside the cell is greater than
inside
water leaves the cell plasmolysis occurs
Penicillin and lysozyme studies provideevidence of role of cell wall, i.e. cells treatedwith both lyse in hypotonic solution
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Protoplast formation and
lysis
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Components Outside of the CellWall
outermost layer in the cell envelope glycocalyxCapsules mainly polysaccharides,
provide protection against predators,chemicals and dessication
Slime layers more diffuse thancapsules, may aid mobility
S layers aid in attachment to solid surfacese.g., biofilms in plants and animals 51
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Bacterial capsules
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Bacterial glycocalyx
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e.g. Aerobic granular sludge
Romain Lemaire
Glycocalyx in biofilms
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Glycocalyx in biofilms
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S Layers
regularly structured layers of protein orglycoprotein that self-assemble
Outside cell wall in G+ and membrane in G- protect from ion and pH fluctuations, osmotic
stress, enzymes, and predation
maintains shape and rigidity promotes adhesion to surfaces protects from host defenses potential use in nanotechnology S layer spontaneously associates
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S layer with distinct floor-
tile pattern
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Archaeal Cell Envelopes differ from bacterial envelopes in the molecular
makeup and organization
S layer may be only component outside plasmamembrane
some lack cell wall capsules and slime layers are rare
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Archaeal Membranes composed of unique
lipids
isoprene units (fivecarbon, branched)
ether linkages ratherthan ester linkages
to glycerol
some have amonolayer structure
instead of a bilayerstructure
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Archaeal membranes
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Archaeal Cell Walls Differ fromBacterial Cell Walls
lack peptidoglycanmost common cell wall is S layermay have protein sheath external to
S layer
S layer may be outside membrane andseparated by pseudomurein
pseudomurein may be outermost layer similar to gram-positive microorganisms
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Cell envelopes ofArchaea
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Methanococcus, Halobacterium, Pyrodictium,
Sulfolobus and Thermoproteus
Methanospirillum
Methanosarcina Methanothermus and methanopyrus
Metahnobacterium, Methanospaera, Methanobrevibacter, Halococcus and Natronococcus
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Pseudomurein
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NOTE: L-amino acids inlinkers instead of D-amino acids
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Bacterial and Archaeal
Cytoplasmic Structures
Cytoskeleton
Intracytoplasmic membranes
Inclusions
RibosomesNucleoid and plasmids
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Protoplast and Cytoplasm
protoplast is plasma membrane andeverything within
cytoplasm - material bounded by theplasma membrane
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The Cytoskeleton
Internal architecture or scaffold of cell Not previously considered to be a part of
prokaryotes, but
homologs of all 3 eukaryotic cytoskeletalelements have been identified in bacteriaand 2 in archaea
functions are similar as in eukaryotes Role in cell division, protein localization, and
determination of cell shape66
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Bacterial cytoskeleton
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e.g. FtsZ role in cell division
e.g. Mbl maintains cell shapein rods, segregates
chromosomes
e.g. Crescentin inducescurvature
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Organic inclusion bodies glycogen polymer of glucose units
poly--hydroxybutyrate (PHB) polymers of -hydroxybutyrate Only found in prokaryotes Osmotically inert
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Inorganic inclusion bodies polyphosphate granules also called volutin granules and metachromatic
granules
linear polymers of phosphates
sulfur granules produced by sulphur bacteria usingH2S as energy source
magnetosomes contain iron in the form of magnetite used to orient cells in magnetic fields Present in aquatic magnetotactic bacteria that want
to find nutrient rich waters!
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Magnetosomes
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Microcompartments
not bound by membranes butcompartmentalized for a specificfunction
carboxysomes - CO2 fixing bacteria contain the enzyme ribulose-1,5,-
bisphosphate carboxylase (Rubisco),enzyme used for CO2 fixation
Carboxysomes consist of polyhedralshell
Shell prevents CO2 from escaping,thus concentrating CO2
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Gas vacuoles
found in cyanobacteria and some other aquaticprocaryotes
provide buoyancy aggregates of hollow cylindrical structures called gas
vesicles Floating allows efficient capture of light for ATP
production
Vacuoles are aggregates of vesicles Vesicle walls are formed from proteins, which form a
rigid cylinder impermeable to water but not gases Proteins can be collapsed to sink, assembled to float
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Gas vacuoles and gas
vesicles
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Ribosomes complex structures consisting of protein and RNA sites of protein synthesis
entire ribosome bacterial and archaea ribosome = 70S eukaryotic (80S) S = Svedburg unit
bacterial and archaeal ribosomal RNA 16S molecule in small(i.e. 30S) subunit 23S and 5S in large subunit archaea has additional 5.8S in large one (also seen
in eukaryotic large subunit)
proteins vary archaea (i.e. 68), more similar to eukarya (i.e. 78)than to bacteria (i.e. 55)
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Bacterial ribosomes
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The Nucleoid
irregularly shaped region in bacteria and archaea usually not membrane bound (few exceptions) location of chromosome and associated proteins usually
a closed circular, double-stranded DNA molecule One copy of chromosome per cell
supercoiling andnucleoid proteins (HU) probablyaid in folding nucleoid proteins differ from histones
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E.Colinucleoids and
chromosomes
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Plasmidsextrachromosomal DNA found in bacteria, archaea, some fungi usually small, closed circular DNA
molecules
exist and replicate independently ofchromosome episomes may integrate into chromosome
contain few genes that are non-essential
confer selective advantage to host (e.g.,drug resistance)80
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Motility Bacteria
andArchaea
have directed movement chemotaxis move toward chemical attractants such as
nutrients, away from harmful substances
move in response to temperature, light, oxygen,osmotic pressure, and gravity
Flagellar movement - flagellum rotates like apropeller
Spirochete motility axial filaments flex and spin
Twitching motility Gliding motility cells coast along solid surface 81
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Chemotaxis movement towards a chemical attractant or awayfrom a chemical repellant concentrations of chemoattractants and
chemorepellants detected by chemoreceptors onsurfaces of cells
External structures: Pili
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External structures: Piliand Fimbriae
fimbriae (s., fimbria); pili (s., pillus)short, thin, hairlike, proteinaceous
appendages (up to 1,000/cell)mediate attachment to surfaces
some (type IV fimbriae) required formotility or DNA uptake
sex pili (s., pilus)similar to fimbriae except longer, thicker,
and less numerous (1-10/cell)genes for formation found on plasmidsrequired for conjugation
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External structures: Flagella
threadlike, locomotor appendages extendingoutward from plasma membrane and cell wall functions motility and swarming behavior
attachment to surfaces may be virulence factors
thin, rigid protein structures ultrastructure composed of three parts
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Patterns of Flagella
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Patterns of Flagella
distribution
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Also, amphitrichous(one flagellum at each
end of cell) and
lophotrichous (cluster offlagella at one or both
ends)
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Three Parts of Flagella
filament extends from cell surface to the tip hollow, rigid cylinder composed of the protein flagellin some bacteria have a sheath aroundfilament
hook links filament to basal body
basal body series of rings that drive flagellar motor
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Ultrastr ct re of Bacterial
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Ultrastructure of Bacterial
flagella
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Flagellar motor
Stator
Rotor
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Archaeal Flagella
thinnermore than one type of flagellin proteinflagellum are not hollowhook and basal body difficult to
distinguish
more related to Type IV secretionssystems
growth occurs at the base, not the end90
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The Bacterial Endospore
complex, dormant structure formed by somebacteria
resistant to numerous environmental conditions heat
radiation chemicals Desiccation
Endospore resistance due to Calcium complexed with dipicolinic acid
Small acid-soluble DNA binding proteins Dydrated core Spore coat and exosporium protect
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Mature endospore
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Life cycle of an endospore
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Formation of Vegetative Cell
activation prepares spores for germination often results from treatments like heating
germination environmental nutrients are detected spore swelling and rupture of absorption of
spore coat loss of resistance increased metabolic activity
outgrowth - emergence of vegetativecell
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Germinationtransformation of
endospore into
vegetative cellcomplex,
multistage
process
Comparison of Prokaryotic
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Comparison of Prokaryoticand Eukaryotic Cells
Take home messages:
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Take home messages:
Structural-conformation relationships key toelucidating roles of intracellular molecules,and thus intracellular processes
Similar approach to elucidate extracellularprocesses (e.g. in biofilm systems, currentarea of research at SCELSE here at NTU!)