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Bacterial Structure
Aims and Learning outcomes:
To describe the structure and function of
bacterial flagellae, pili and fimbriae,
capsules, ribosomes and storage granules.
To describe the formation of endospores
and their subsequent germination.
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Flagella
Long thin appendages that arise at the level
of the cytoplasmic membrane and extend
through the wall into the surroundingmedium.
Enable bacteria to be mobile.
Very thin, 20nm, not visible by lightmicroscope unless stained.
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Flagella
Structure of flagella: not straight but helical,
constant distance between 2 adjacent curves
- wavelength, which is constant for a givenorganism.
Composed of subunits of flagellin.
Complex structure in Gram negativebacteria.
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Flagella
Three main parts of the structure:
the motor called the basal body,
the hook (consisting of a single type of
protein) at the base of the flagellum
connecting the filament to the motor.
The filament itself.
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Flagella: basal body.
Basal body is anchored in the cytoplasmic
membrane and cell wall.
Small rod passing through a system of
rings. In Gram negative bacteria outer ring
in LPS, one in peptidoglycan and third in
the cytoplasmic membrane. In Gram positive bacteria only the 2 inner
rings present.
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Flagella:Basal body
2 proteins calledMotare anchored either
side of the S-M ring and drive the flagellar
motor, causing a torque that rotates thefilament.
TheFliproteins act as a motor switch.
Energy supplied by the passage of protonsfrom outside the cell into the cytoplasm via
the basal body mot complex. 1000 protons
per single rotation.
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Flagella antigens and phase
variation. Salmonella spp. typed by their O somatic
and H flagellar antigens.
>2000 serotypes.
Have the ability to differentially express the
protein antigen as either phase 1 or phase 2
- phase variation.
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Motility and chemotaxis.
Normal movement is randomly composed
of runs and tumbles
Chemotaxis -Response to different chemicalgradients.
Compare the chemical state of environment
to that sensed seconds before and ifattracted move up the gradient. Movement
away if the chemical is repellant.
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Pili and fimbriae.
Fine hair like filaments on the surface of
many Gram negative bacteria. Not involved
in motility. Fimbriae composed of 20 Kda protein.
Pili generally longer.
Both involved in attachment of bacteria to
host cells. Bind to lectins on cell
surfaces,e.g mannose. Gives tissue
specificity.
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Pili and fimbriae.
Bacteria within a human host are constantly
losing and reforming fimbriae because
fragile structures. Aids evasion of the immune system as
bacteria, e.g.N. gonorrhoea change the
antigenic structure of their pili/fimbriae.
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Sex Pili and conjugation.
Used by conjugative plasmids to transfer
copies of itself to a new host. F+ and F-
Sometimes other genetic material ismobilised during conjugation.
After conjugation the previously negative
cell has a copy of the plasmid and is able topass it on to another cell. Efficient manner
for transfer of antimicrobial resistance.
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Sex Pili and conjugation.
The F plasmid ofE.coli can also mobilise
the chromosome to be transferred through
cell to cell contact. Episome -integrates into the chromosome
and leads to transfer of large regions of the
host genetic material and extensive geneticrecombination. F+ and F- strains.
Cells already containing a plasmid are poor
recipients for the same or similar plasmid.
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Capsules and Slime layers
Consist of polysaccharide or sometimes
protein, general term glycocalyx.
Capsule: thick viscous gel outside andattached to cell wall.
Slime: Capsule loosely attached to cell wall
and easily washed off.
Hydrophilic. Appear as halos around cells
in a Gram stain
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Capsules and Slime layers
Variable thickness e.g.Klebsiella spp and
Strep pneumoniae up to 10m thick.
Microcapsule ofE.coli and Salmonellaspp.too thin to be seen on light microscope,
detected by antisera.
Capsule not always expressed in vitro.
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Composition of Capsules
Mainly acidic polysaccharide. The acidic
groups being glucuronic acid or phosphate.
Most are immunogenic and stimulate hostantibody response.
Exceptions are Group B meningococci -
capsule contains N-acetylneuraminic acidwhich is found on host cells. Hyaluronic
acid ofS.pyogenes capsule can lead to host
cell damage because found on host cells.
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Capsules and Slime
layers:advantages to bacteria. Protect cells from dessication and toxic
material.
Attachment to host cells.
Soluble material released into solution
blocks opsonising antibodies.
Resist Complement alternative pathway.
Resist phagocytosis by neutrophils.
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Chromosome.
Single circular chromosome of double
stranded DNA,called a nucleoid, not
membrane bound, 300 to 400 m in length. Supercoiled like a rubber band. Genes
arranged linearly along the chromosome.
Size ranges from 600 to 9500 Kb.
Composed of nucleotides, adenine, guanine,
cytosine, thymine (uracil in RNA)
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Protein Synthesis
Takes place on ribosomes, 70s (30s +50s) in
the cytoplasm. 30s contains 16s RNA, 50s
contains 23s RNA Up to 10,000 ribosomes per cell. Up to 35%
of dry weight is RNA in growing cells.
Bacterial mRNA is polycistronic ie. can betranslated by several ribosomes
simultaneously.
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Protein Synthesis
Protein translation via tRNA, mRNA and
30s subunit initiation complex + 50s
subunit Location of protein encoding genes
determined by looking for Open reading
frames. Start codon, codons and stopsequence.
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Cytoplasm - carbon storage
polymers Amorphous gel containing enzymes and
ions and a variety of granules, accumulation
of food reserves (polysaccharides, lipids orpolyphosphates).
One of the most common inclusion bodies
consists of poly--hydroxybutyric acid(PHB). Used as storage for carbon and
energy storage polymers.
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Cytoplasmic inclusions
Glycogen also used as storage product,
starchlike polymer of glucose subunits.
Smaller than PHB granules.
Plasmids and transposons.
Transposons - jumping genes can insert into
genome. Can also be transmitted with
plasmids.
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Endospores
Two genera of medical importance produce
highly resistant endospores, which enable
them to survive under adverseenvironmental conditions for long periods
of time :Bacillusand Clostridia.
Spherical or oval structures, dormant orresting phase.
Survive in soil for up to 30 years e.g.
B.anthracis, C.tetanus, C.perfringens.
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Endospores
Resistant to drying, heat, pressure and many
chemical disinfectants.
Killed by heating to 120oC for 15-20 mins.
Size, shape and location of spores within
stationary phase of vegetative cells are
helpful for identification.
Appear as non-staining refractile areas
within the cell
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Endospores: Germination.
Activation: in vitro accomplished by
heating at sub-lethal but elevated
temperature. Germinate when placed in nutrients. Loss of
resistance to heat and chemicals, loss of
calcium dipicolinate and cortexcomponents. Spore acid-soluble spore
proteins degraded.
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Endospores: outgrowth
Visible swelling as a result of water uptake
and synthesis of new RNA, proteins and
DNA. Cell emerges from broken spore and begins
to divide.