bactstructureflagellaetc[1]

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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.

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