Cell Structure[1]

8/3/2019 Cell Structure[1]

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Bacterial cell structureBacterial cell structureBacterial cell structureBacterial cell structure

Prof. Dr. Suzan El-fikyProf. Dr. Suzan El-fiky


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Bacteria are divided according toGram stain into Gram positive &

Gram negative .


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CELL ENVELOPE:1. Cell Wall:

Gram Positive Cell Wall Gram Negative Cell Wall

2. Cytoplasmic Membrane:


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Cell WallPeptidoglycan. It is a rigid layer,

which surrounds the cytoplasmicmembrane of all bacteria Chemicallythe peptidoglycan is composed

of alternating molecules of N-

acetylglucose amine and N-acetylmuramic acid


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Gram Positive Cell Wall

1. The peptidoglycan layer is the major

constituent of the cell wall of gram-positive bacteria (50%-80% of thewall).


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2.Special components of Gram positive

cell wall include:a.Teichoic acids,They are water soluble polymers of ribitol or

glycerol phosphate. Teichoic acids are theTeichoic acids are themajor surface antigen of gram positivemajor surface antigen of gram positivebacteria.bacteria.

b.Polysacharides,

which may be neutral sugars as mannose oracidic sugars as glucoronic acid.


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Gram Negative Cell Wall1-Peptidoglycan layer is thinner (only 5-10% of cell wall).


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2-Special components of Gramnegative cell wall (external to

peptidoglycan):


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2-Special components of Gram negative

cell wall (external to peptidoglycan):a-Lipoprotein which stabilizes the outer

membrane and fixes it to thepeptidoglycan.

b-Outer membrane:


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b-Outer membrane:

It is a bilayer structure, composed ofan inner and an outer leaflet. The

inner leaflet is similar in compositionto the cytoplasmic membrane(phospholipid) While the outer leafletis replaced by lipopolysacharide.


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Lipopolysaccharide (LPS):

It consists of: Lipid A, which is

responsible for toxicity of gramnegative bacteria (endotoxin).


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b-Outer membrane: It also contains protein molecules(porins), that form pores in the outer

membrane, allowing diffusion of lowermolecular weight compounds.


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Functions of Cell Wall:

1-It is responsible for the shape of thebacterial cell.

2-It can stand high internal pressure.3-The LPS is the site of O antigen and

endotoxin in gram-negative bacteria.


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4-In Gram-positive bacteria, teichoicacid is the major surface antigen.

5-The site of action of many antibiotics.

6-Contains specific receptors forbacterial viruses.

7-Plays a role in cell division.


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Cytoplasmic Membrane:

It is a thin, elastic, semipermeablemembrane consisting of a phospholipid

bilayer.


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Functions of The CytoplasmicMembrane:

1-It is the osmotic barrier of the cellcontrolling the passage of nutrients

into the cytoplasm and the endproducts of metabolism out of it.


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2-It is responsible for the electron-transport and oxidative phosphorylationbecause the cytochromes and otherrespiratory enzymes are located on it.

3-It is responsible for the active transportof nutrients due to the presence of the

permeases on it.


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4-It secretes hydrolytic enzymeswhich degrade large organic

polymers into smaller moleculesenough to penetrate the cytoplasmicmembrane.


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THE CYTOPLASM AND ITS

COMPONENTS:


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1. Ribosomes:

2. Bacterial nucleoid:

3. Plasmid: 4. Inclusion granules:


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EXTERNAL STRUCTURE:

1. Capsule:

2. Flagella:

3. Fimbriae or Pili:


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1. Capsule: Capsule is a mucogelatinous layersurrounding the cell wall.

It is not a constant structureandits presence is not linked with theviability of bacteria.

It is not visible in all bacteria.


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The capsule may be demonstratedby staining a bacterial suspension

withIndia inkIndia ink, also by adding tothe bacteria specific capsularantibodies.


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The capsular substance is usuallyantigenic and is made of

polysaccharides,


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In pathogenic bacteria, the capsuleplays an important part in the

determination of virulence and inprotecting them against phagocytosisand protects the cell wall againstattack by antibacterial agents


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2. Flagella: Long filamentous, wavy appendages,.

peritrichous arrangement,

polararrangement


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Function; Flagella are organs of locomotion that

carry flagellar or H antigens made ofa protein (flagellin).


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3. Fimbriae or Pili:

Hair-like projections .

found in gram-negative bacilli.

They differ from flagella in being;shorter, straight, and thinner.


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a. Common pili: They are structures by which bacteria can

attachattachthemselves to host cells (organs of

adhesions) they are formed from an antigenicprotein called pilin.b. Sex (F) pili: Exist only in bacteria containing the F factor,

which plays a part during conjugation.


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Bacterial Spores

(Endospores): Spores are a highly resistant restingphase of bacteria.

Spores are met within certain gram-positive bacteria like clostridiumclostridiumandbacillus,bacillus,


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Spores are in a state of dormancy, theyare non-metabolizing, they play a veryimportant part in the transmission of

certain diseases like Anthrax, Tetanus, Gas gangrene Botulism.


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Process of sporulation


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Endospore location The spores may be situated in thecenter of the bacillus (equatorial),

near one end (subterminal) protruding from one end of the cell

(terminal)


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Sporulation: The process of endospore formationwithin a vegetative (parent) cell is

known as sporulation.


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structural layers (1)spore wall which is the inner mostlayer surrounding the spore

membrane, it contains normalpeptidoglycan which will become thecell wall of the germinating vegetativecell.


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(2)Spore cortex is the thickest layerof the spore envelope. It contains an

unusual type of peptidoglycan and itsautolysis plays a role in sporegermination


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(3)Spore coat is composed of akeratin-like protein which confers on

spores their relative resistance toantibacterial chemical agents


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(4)Exosporiumis composed oflipoprotein.


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Chemical composition of spores:

New constituents appear as forexample calcium dipicolinatewhich is

responsible for stabilization ofresponsible for stabilization ofenzymes against dessiccation andenzymes against dessiccation andheatheatby forming calcium dipicolinateenzyme-complex.


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Germination:

Endospores can remain dormant forthousands of years.

An endospore returns to itsvegetative state by a process calledgermination (which is triggered by

physical or chemical damage of thespore coat).


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The enzymes of the endospore thenbreak down the extra layersbreak down the extra layerssurrounding the endospore,

water enters, loss of calcium dipicolinate,loss of calcium dipicolinate, and metabolism resumes and a normal

cell reappears.


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Cell Structure[1]

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