Cell Structure (1)

8/2/2019 Cell Structure (1)

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Bacterial Structure

Dr Laird


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Gross Morphology

Cell Wall

Cytoplasmic Membrane

Cytoplasm

Flagella, Pili & Fimbriae

Capsules

Endospores

Overview


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Be able to describe basic structure of a prokaryoticbacterial cell and how it differs from eukaryotic cell

Describe the bacterial cell wall of Gram positive andGram negative bacteria

Describe some features of Gram-positive and Gram-negative bacteria such as cytoplasmic membrane,

capsule and spore formation

Learning outcomes


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Morphology

Cocci Budding/

Appendaged

Rod

Filamentous

Spirochete


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Cell Size

Cell size varies from approx 2 Qm to 700 Ql diameter

Large cell size is unusual among the prokaryotic

organisms

An average size cell ofE. coliis between 1 - 2 Qm

Nanobacteria are of the lower cell size limit with a cell

diameter of around 0.1 Qm. The diameter required for

essential biomolecules in a cell is 0.15 Qm

Are nanobacteria really a living cell??

1 centimetre = 10 000 micrometers


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Cell Structure


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ProkaryotesProkaryotes

Cytoplasmic membrane

contains hopanoids,

lipopolysaccharides,teichoic acids

Energy metabolism is

associated with

cytoplasmic membrane

Flagella consist of one

protein flagellin

Ribosomes are 70S

Peptidoglycan cell walls

EukaryotesEukaryotes

Cytoplasmic membrane

contains sterols

Mitochondria present

Flagella complex structure

Ribosomes are 80S

Polysaccharide cell walls

either with chitin or

cellulose

Internal membranes,

endoplasmic reticulum andGolgi apparatus associated

with protein synthesis

Membrane vesicles such as

lysosomes and

peroxisomes present


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Cell Wall

Bacterial species can be divided into two groups depending on thestructure of there cell wall:

Gram-positives or Gram-negatives

The differences in the cell wall structure can be observed using aGram-staining technique.

Staining and classification

Two major classifications based on staining with crystal violet,

precipitation with iodine and washing with safranin:

Gram positive, turn purple

Gram negative, turn red.

Gram negative do not keep the stain because of a thin petidoglycanlayer


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Gram - negative Gram - positive


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Cell Wall

Gram - negative Gram - positive

Cytoplasmic

membrane

Peptidoglycan

Outer

membrane

Periplasm

cytoplasmicmembrane

C

E

L

L

W

A

L

L


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Cell Wall

Gram positive:

Thicker peptidoglycan wall

(90%), important for:Survivalof bacteria

Contains surface antigensthat evoke immune responseincluding teichonic acid and

lipoteichonic acid

Gram negative: Contain thin peptidoglycanlayer (10%)

No teichonic or lipoteichonicacid Contain an outer membrane(unique to Gram-negative) Contain a periplasmic spacewhich has enzymes, colegenase,

proteases and others that canbreakdown host cell components


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Not all bacteria can be identified by Gram stain:

Acid fast and related bacteria (mycobacteria):

They have a waxy coating and are nearlyimpermeable due to the presence of mycolic acidand large amounts of fatty acids, waxes, and

complex lipids. Thus acid-fast organisms requirea special staining technique involving heat to drivestain into the waxy cell wall.

Staining Technique:

The sample is heated and subjected to Ziehlscarbol fuschion, decolourised with alcohol andcounterstained with crystal Violet


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The barrier that separates the inside of the cell (cytoplasm) from

the outside environment.

Destruction of the cytoplasmic membrane disrupts the integrity of

the cell and leads to leakage of the cytoplamic contents and

ultimately cell death.

The cytoplasmic membrane consists of :

Phospholipid Bilayer

* Hydrophobic (fatty acid)

* Hydrophilic (glycerol-phosphate)

Membrane Proteins

* Integral (embedded in membrane)

* Peripheral (not emdeded but associated with membrane

surface)


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Functions of Cytoplasmic Membrane

Permeability Barrier(consists of a aqueous solution ofsalts, sugars, amino acids, nucleotides, vitamins and

coenzymes)

Protein Anchor(site of proteins involved in transport ofsubstance across the membrane)

Energy Conservation (site of generation and use ofproton motive force)


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The Cytoplasm

The cytoplasm is a liquid filled space enclosed by thecytoplasmic membrane, which contains intracellular

structures.

Intracellular Structures:

Nucleoid (Circular DNA)

Chromosomal DNA (non-membrane bound)

Small independent DNA (plasmids)


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The Cytoplasm

RibosomesMost numerous intracellular structure

Site of protein synthesis

70S ribosomes

30 s

50 s

70 s


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Flagella

http://www.cartage.org.lb/en/themes/sciences/Zoology/AnimalPhysiology/Anatomy/AnimalCellStructure/CiliaFlagella/flagella.jpg

Organelles of motility

Structure

A bundle of nine pairs microtubles

surrounding central pair called

axoneme.

Dynein (second protien) functions

as ATPase to drive the movement of

the flagella

Microtubules

consist of proteins called -tubulin

and- tubulin, that form filamentous

structures.

Are approx 25 nm in diameter(1 centimeter = 10 000 000 nanometers)

Contain a hollow core (15nm wide)

Propel cell using a whip like motion


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Pili & Fimbriae

Fimbriae and Pili are filamentous protein structures that extend from

the surface of the cell

Role of Fimbriae

Enable organisms to stick to surfaces including animal tissue To form biofilms (sheet of cells on a liquid surface)

A virulence factor that aids human pathogens in causing disease

Role ofPili

Are typically longer structures than fimbriae and only one or two are

present on the surface of the cell

Pili can be receptors for viruses

Facilitate genetic exchange

Twitching motility


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Capsules & Slime Layers

http://micro.magnet.fsu.edu/optics/olympusmicd/galleries/darkfield/images/bacterialcapsules.jpg

Capsules can be thick or thin, rigid or flexible and are

composed for polysaccharides.

If the matrix is tighly formed and excludes Indian ink it is

called a capsule, if matrix is permeable to Indian ink it is

called a slime layer

Capsules are firmly attached to the cell wall and slime

layers only have a loose affinity

Role:

Attachment to solid surfaces Encapsulated pathogenic

bacteria have greater resistance

to phagocytosis e.g.

Enterococcus sp.


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Bacteria produce structures called endosporesin a process called sporulation and are thedormant stage of bacterial life cycle

Spores are extremely resistant to heat, harshchemicals and radiation

Spores are survival structures

Spores allow for the dispersal of an organism

Spore bearing organisms are often found in soile.g. Bacillus sp. or Clostridium sp.

Endospores


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Spore Formation

Usually found in environmental strains suchas Bacillus sp. and Clostridium sp.


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Spores are not produced by growing cells butthose that have exhausted all essential nutrientse.g. carbon or nitrogen.

Spores can remain dormant for many years butcan germinate to form a vegetative cell quickly.

Three steps are involved:

Activation

Germination

Outgrowth

Spore Germination


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Activate with sublethal heating

1. Activation

http://staff.science.uva.nl/~rkort/The%20Bacterial%20Spore_files/image002

.gif

2.Germination

3.Outgrowth


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Prions

Consist entirely of protein, prions contain no

DNA or RNA

Scrapie in Sheep

BSE in cattle

CJD in humans


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Further Reading

Brock , Biology of Microorganisms, Chapter 4

Cell Structure (1)

Documents

Transcript of Cell Structure (1)