Members of the microbial world

7/27/2019 Members of the microbial world

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Major components of

prokaryotic cells

Dr. Thomas Seviour

[email protected]

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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!)

Members of the microbial world

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