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Microbial structure
(2)
Structural approach : Is
it a view of artifact?
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BC Yang
Shapes
Bacillus anthracis
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Bacillus
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Coccus
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Coccobacillus
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Fusiform bacillus
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Spirillum
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Spirochete
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Vibrio
Staphylococcus aureus
Pseudomonas aeruginosa
Fusiform bacilli
Spirillum volutans
Borrelia burgdorferi
Vibrio cholerae
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Structural view of bacteria (1)

Microscopic prokaryotes (no nucleus nor
membrane-bound organelles)
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Contain ribosomes
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Enfolding of the cell membrane carry on
photosynthesis & respiration
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Surrounded by protective cell wall containing
peptidoglycan (protein-carbohydrate)

Many are surrounded by a sticky, protective coating
of sugars called the capsule or glycocalyx
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Structural view of bacteria (2)

One circular chromosome and some small DNA
called plasmids
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May have short, hairlike projections called pili on
cell wall to attach to host or another bacteria when
transferring genetic material

Some can move by flagella, gliding over slime they
secrete ( e.g. Myxobacteria)
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Some can form protective endospores around the
DNA when conditions become unfavorable
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Detail Structure
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Structure and function
STRUCTURE
FUNCTION
Cell Wall
protects the cell and gives shape (5-20 atm resistant)
Outer Membrane
protects the cell against some antibiotics (only present in
Gram negative cells)
Cell Membrane
regulates movement of materials into and out of the cell;
enzymes of respiration
Cytoplasm
contains DNA, ribosomes, and organic compounds
Chromosome
carries genetic information inherited from past
generations
Plasmid
contains some genes obtain through genetic
recombination
Capsule, and slime
layer
protects the cell (immune attack) and assist in attaching
the cell to other surfaces
Endospore
protects the cell against harsh environmental conditions,
(heat or drought)
Pilus (Pili)
attaching to other surfaces (for genetic recombination)
Flagellum
moves the cell
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Differences
Eukaryote
Prokaryote
Major groups
Algae, fungi, protozoa, plant,
animal
bacteria
Size
>5 mm
0.5-3 mm
Nucleus
Classic membrane
No nuclear membrane
Chromosome
Diploid genome; multiple DNA Haploid, single DNA
Nuclear structures
Cytoplamic structures
Mitochondria
Present
Absent
Golgi bodies
Present
Absent
ER
Present
Absent
Ribosome
80S(60S+40S)
70S(50S+30S)
Cytoplamic membrane
Contains sterols
No sterols
Cell wall
Absent or with chitin, cellulose
+ lipid, peptidoglycans
Reproduction
Sexual and asexual
Fission
Movement
Complex flagellum (9+2)
Simple flagellum
Respiration
Via mitochondria
On cytoplamic membrane
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Cell envelope: Cytoplamic membrane+ cell
wall+ capsules + loss slime etc….
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Gram-positive cells:

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Cytoplamic membrane+
thick peptidoglycan layer
+ capsules
Gram-negative cells:

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Cytoplamic inner
membrane+ thin
peptidoglycan layer +
periplamic space + outer
membrane + + capsules
/loss slime
high magnification AFM
image of the surface of
a single Pseudomonas
putida
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• The main component of the bacterial cell wall is peptidoglycan, a
hydrated, semi-rigid polymer of two sugar derivatives: NActeylglucosamine (G) and N-Acetylmuramic acid (M). Bound to
the sugar M are amino acids: Alanine -- Glutamic acid -- Mesodiaminopimelic acid (DAP) (Gm-) or Lysine (Gm+) -- Alanine.
Glucosamine?
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Teichoic acid
Are found exclusively in gram positive organisms. Are formed as
polymers of glycerol or ribitol through phosphodiester linkages.
http://www.cvm.uiuc.edu/courses/vp331/Structures_in_pathogenesi1.html
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A very complex net
http://www.arches.uga.edu/~kristenc/cellwall.html
Synthesis of cell wall:
The assembly of the wall components begins with the synthesis of precursors in
the cytoplasm, their transport across the cell membrane, and their final
polymerization. Eventually, penicillin-binding proteins catalyze covalent reactions
that result in the extension, cross-linking between glycan strand, morphogenessis
and eventual separation of the murein sacculus.
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透過骨架來改變形狀
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1.
2.
3.
4.
Crescentin : creS
430-residues protein
Distinct 7-residue repetitive pattern
coiled-coils: many fibrous proteins in eukaryotes
Fluorescent image of the CreS-GFP/CreS hybrid strain
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Look into the detail
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Gram positive
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in Gm+ organisms, an interbridge exists between the Lysine
molecule extending from one sugar backbone to the terminal
Alanine of the second.
Gm+ organisms have cell walls that may contain as much as 90%
peptidoglycan, with membrane associated proteins and Teichoic
acids comprising the remaining components.
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Gram negative

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cross-linking of peptidoglycan by bond between the DAP molecule
extending from one sugar backbone and the terminal Alanine of another.
typically composed of only about 10% peptidoglycan and possess an outer
membrane that houses the membrane associated proteins and LPS. The
peptidoglycan lies in-between the periplasm.
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The acid-fast cell wall of Mycobacterium contains
peptidoglycan, and a large amount of glycolipids such as
mycolic acid, arabinogalactan-lipid comlex, and
lipoarabinomannan.
Lipopolysaccharides is an amphiphile composed of three regions:
O-polysaccharide (the O- or somatic-antigen), the core
polysaccharide and lipid A. Lipid A is anchored in the outer
membrane. LPS is also known as endotoxin.
saccharide
Large-scale molecular dynamic simulation of
a lipopolysaccharide membrane solvated in a
4.2 nm water box.
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lipid
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鱟

Endotoxins are thermostable, lipopolysaccharide components
from the cell walls of viable or nonviable gram-negative
microorganisms.
 Measured endotoxin will include endotoxin that was derived from:
1) Any living cells that are present, 2) Cell wall debris from dead or
dying cells, and 3) Outer membrane fragments that are released
during cell growth.
 An endotoxin unit, EU, is equivalent to approximately 200 picograms of endotoxin.
 The LAL (Limulus Amebocyte Lysate) test for endotoxins can be
sensitive down to 0.03 EU/ml
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Not confuse with exotoxin
Tetanus toxin:
(a) The x-ray crystal structure for the tetanus toxin showing
how the amino acid chain is folded and (b) its calculated
molecular surface showing sites 1 and 2, predicted binding
sites for ligands.
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Cytoplasmic membrane

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5-10 nm unit membrane
Absence of sterols, except mycoplasma
Semifluid state, form mesosomes
Selective permeability and transport
Electron transport /oxidative ATP generation
 Bioxynthesis
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Phospholipids

Major components of the cell
membrane
 Consists of two fatty acids and the
third hydroxyl group of glycerol is
joined to a phosphate group
 being amphipathic, having both a
hydrophobic (hydrocarbons tails)
and a hydrophilic region
(phosphate head)
 In water, they self-assemble into
aggregates so that the phosphate
heads make contact with the water
and the hydrophobic hydrocarbon
tails are restricted to water-free
areas (micelle or phospholipid
bilayer)
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How to get across the polar barrel
Bacterial
membrane
proteins OmpA (left)
and GlpF
(right).
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ATP-dependent secretion systems
 There are five major secretion systems.

Type I and type III secrete proteins across both the inner membrane
and the cell envelope (outer membrane) in one step; secreted proteins
do not make an intermediate stop in the periplasm. Type I systems are
composed of far fewer components than type III systems.
 Type II and type III systems share a similar cell envelope component.
 The type III secretion system of Gram-negative bacterial pathogens
injects virulence factors into host cells. The needle complex is drawn
on the basis of its appearance in electron-microscopy images (see
inset), with several ring structures that span the inner and outer
membranes. Very similar to flagella system
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http://www.cdc.gov/ncidod/eid/vol2no4/mecsas.htm
Bacterial secretion systems: type I, type II, and type III.
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The known type IV systems differ with respect to the route of substrate translocation.
The A. tumefaciens T-DNA transfer system and the H. pylori CagA system are thought
to export substrates in one step across the membrane directly to the eukaryotic
cytosol. The B. pertussis Ptl system is thought to export PT in two steps across the
cell envelope to the extracellular milieu. Secreted holotoxin then binds to the
mammalian cell membrane.
http://www.blackwell-synergy.com/links/doi/10.1046/j.1365-2958.2001.02302.x/full/
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Bacterial chromosome
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No nuclear membrane
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Small polyamines and
magnesium, histone-like
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Lack of mitotic apparatus;
fission
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~3 x 109bp (1mm long),
supercoiled state, single haploid
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Circular (most bacteria) or
linear (Streptomyes sp.)
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Associated with mesosome
DNA spread of E. coli
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plasmid
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Spores
Ca+2
Calcium bound to dipicolinic acid
Keratin-like protein coat
may stay inactive several years, then re-activate
when conditions favorable
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1. the bacterium senses that its home or habitat is turning bad
2. it makes a copy of its chromosome
3. the rubbery cell membrane that surrounds the bacterial cell fluid
begins pinching inward around this chromosome copy.
4. the membrane of the mother cell surrounds and swallows up the
daughter cell.
5. between these two membranes a thick wall forms made out of
stuff called peptidoglycan.
6. a tough outer coating made up of a bunch of proteins forms
around all this, closing off the entire daughter cell.
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Capsule
 Bacterial capsules outlined by India
ink viewed by light microscopy.
 A discrete layer of polysaccharide
surrounding the cells.
 Sometimes bacterial cells are
embedded more randomly in a
polysaccharide matrix called a slime
layer or biofilm.
 Polysaccharide films that may
inevitably be present on the surfaces
of bacterial cells, but which cannot be
detected visually, are called
glycocalyx.
Capsule Stain of B. anthracis:
Capsule appears pink,
bacterial cells purple
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Capsule
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Composed of poly-(D-glutamic acid), single antigenic
type + proteins
Nontoxic, serves as an impedin in establishment of
infection
Production enhanced in the presence of Na+bicarbonate
may be plasmid-borne

prevents desiccation of the bacteria
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prevent phagocytosis by larger microorganisms and the
white blood cells of invaded host organisms
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help the bacteria adhere to the host substrate

assist in warding off attacks by phages
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Pili (Fimbriae)
 singular pillus
 helical filaments, with ~1000 copies of
the major pilin, plus one or a few
copies of ~5 minor pilins. The minor
proteins provide binding specificity,
membrane anchoring, and adapter
functions.
Mu, Egelman, Bullitt, 2002. J. Bact.
284(17):4868-4874
Bullitt, & Makowski, 1995. Nature
373(6510):164-167
a) Adhesion
b) In pathogenesis
c) In environment
d) Sex pili
Sex pili also called as type IV secretion system!
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Flagella
singular flagellum
A tiny motor machine
Share similarity with type III secretion system
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Swing and tumble
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Each flagellum is a rigid structure
The basal body causes rotation of the structure -- like a
propeller. The rate is about 12000 rpm
Energy is derived from a proton gradient. Power has been
calculate to be about 10-15 amperes of current.
Average velocities are in the range of 20 - 80 µM/s. About 1/2
foot per hour. A cheetah is 4 feet long and moves about 70
mph. 25 body lengths/second. A microbe that is 2 µM long,
moves 10 to 40 body lengths per second. (some bacteria are as
mobile as higher animals)
Movement is a bias random walk during chemotaxis
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