Bacterial anatomy

Bacterial Anatomy

A. Rigid cell wall

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B. Cytoplasmic components

- ribosomes

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- inclusion granules

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- mesosomes and

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- single, circular chromosome of DNA

Prokaryotic Cell (Bacillus megaterium)

Additionally some bacteria possess

1. Capsule – a gelatinous covering {too thin (microcapsule)}

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• Filamentous appendages (produce from cell surface)

- Flagella

- Fimbria

- Pili

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3. Spore

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The Bacterial Cell Wall

• A unique structure which surrounds the cell membrane.

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• Although not present in every bacterial species, the cell wall is very important as a cellular component.

Electron Micrograph of a Gram-Positive Cell Wall/Cell

Cell wall (tough and rigid structure)

• Confers protection and rigidity

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• 10-20nm in thickness

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• 20-25% dry weight of bacterial cell wall

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• Elastic and porous

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• Permits to enter the molecules less than 10,000 mol.wt

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• Takes part in cell division by forming in growth from cell wall.

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• Mucopeptide components possess target site for antibiotic, lysozyme, and bacteriophage

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Structurally, the wall is necessary for

* Maintaining the cell's characteristic shape- the rigid wall compensates for the flexibility of the phospholipid membrane and keeps the cell from assuming a spherical shape

* Countering the effects of osmotic pressure- the strength of the wall is responsible for keeping the cell from bursting when the intracellular osmolarity is much greater than the extracellular teichoic osmolarity

* Providing attachment sites for bacteriophages- teichoic acids attached to the outer surface of the wall are like landing pads for viruses that infect bacteria

* Providing a rigid platform for surface appendages- flagella, fimbriae, and pili all emanate from the wall and extend beyond it

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Cell wall structures�(Considerably difference between Gm +ve and –ve)

* The cell wall of Gram-positive bacteria consists of many polymer layers of peptidoglycan connected by amino acid bridges.

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* The peptidoglycan polymer is composed of an alternating sequence of N-acetylglucosamine (NAG) and N-acetyl-muraminic acid (NAMA).

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* Each peptidoglycan layer is connected, or crosslinked, to the other by a bridge made of amino acids and amino acid derivatives.

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* The particular amino acids vary among different species, however. The crosslinked peptidoglycan molecules form a network which covers the cell like a grid.

* Also, 90% of the Gram-positive cell wall is comprised of peptidoglycan.

Gm +ve cell wall

* Peptidoglycan layer (Gm+ve 16-80nm and Gm –ve 2nm)

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* Special components

- teichoic acids 🡪 constitute major surface antigen

- teichuronic acids

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• Other components – some contain polysaccharide and proteins

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• Cytoplasmic membrane

Cytoplasmic Membrane

Gram negative bacteria

• incorporate the counter stain rather than the primary stain. Because the cell wall of Gram(-) bacteria is high in lipid content and low in peptidoglycan content,

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• cause a lot of problems because many species are pathogenic.

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• pathogenic capability usually associated with certain components of their cell walls, particularly the lipopolysaccharide (endotoxin) layer.

Gram negative bacteria�(complex)

A) Outer membrane with surface lipopolisaccharide

- phospolipid bilayer contains specific proteins form PORINS and hydrophilic molecules transfer through it.

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B) Periplasmic space

- consist of

i) imp. Proteins ( enzymes and binding proteins)

ii) oligosaccharides for osmoregulation

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C) Peptidoglycan

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D) Cytoplasmic membrane

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Electron Micrograph of a Gram-Negative Cell Wall

Structure of a Gram-Negative Cell Wall

Structure of an Acid-Fast Cell Wall

Bacteria with defective cell wall

• Defect in cell wall can be brought by
• Lysozyme

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• Bacterial autolysin

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• Antibiotics

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• Bacteriophages

Bacteria with defective cell wall can be survived and induced in lab

1. Mycoplasma – naturally occurring
2. L- forms – cell wall defective forms

- more stable than protoplast and spheroplast

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- develop either spontaneously or in the presence of penicillin or other agents that interferes with cell wall synthesis

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- difficult to cultivate, require a solid medium containing agar having high osmotic strength

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- sometime spontaneously formed in body of patients treated with penicillin

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Contd..�Bacteria with defective cell wall can be survived and induced in lab

c) spheroplast:

- derived from Gm-ve bacteria

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- retains remnants of cell wall

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- produced by growth with penicillin

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- osmotically fragile and must be maintained in hypertonic culture medium

Contd..�Bacteria with defective cell wall can be survived and induced in lab

d) Protoplast:

- derived from Gm+ve bacteria

- contains cytoplasmic membrane

- totally lack of cell wall

- unstable and osmotically fragile

- produce artificially by lysozyme and hypertonic medium

- hypertonic medium is essential for their maintenance

Pleomorphism and involution forms

• Pleomorphism: exhibit variation in size and shape

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• Involution: exhibit variation as swollen and aberrant forms in aging

Cytoplasmic components

• Cytoplasm: colloidal system of viscous watery solution containing DNA, Ribosomes and storage granules
• no nuclear membrane, genetic materials concentrated as nucleoid
• Nucleoid: thin fiber of dsDNA helix tightly coiled inside the cytoplasm. By ordinary staining under light microscope can’t be differentiated from cytoplasm, but under electron microscope can
• Plasmid: extranuclear genetic material in cytoplasm consisting of DNA. Circular but much smaller than chromosomes