dr shyamal kr. paul associate professor, … file/l structure of...dr shyamal kr. paul associate...

Structures of bacterial cell

Dr Shyamal Kr. Paul

Associate Professor, Microbiology, MMC

12/30/13 1Dr. Shyamal Kr Paul, Bacterial structure

Bacterial Structures• Flagella

• Pili

• Capsule

• Plasma Membrane• Plasma Membrane

• Cytoplasm

• Cell Wall

• Lipopolysaccharides

• Teichoic Acids

• Inclusions

• Spores12/30/13 2Dr. Shyamal Kr Paul, Bacterial structure

Gram Staining


Essentialstructures

• Cell wall

• Cell membrane

Non-essential structures

• Capsule / Glycocalyx/ Slime layer

• Cell membrane

• Nucleoid

• Ribosome

• Periplasmic space (GNB)

• Flagella

• Fimbria

• Plasmid

• Transposone

• Endospores12/30/13 4Dr. Shyamal Kr Paul, Bacterial

structure

Ribosomes:Ribosomes are complex aggregated globular structures

composed of several RNA molecules and many associated proteins

Characteristics of ribosomes: • Bacterial ribosomes coefficient of sedimentation is 70

Svedberg units (S). composed of two subunits, 50S, and 30S.

• They function as the active centers for protein synthesis.• Differs from Eukaryotic by size and chemical

composition– Bacterial ribosomes are 70 S in size, with 50S and 30 S

subunits.– Human ribosomes are 80 S in size, with 60S and 40 S subunits.

• This difference is the basis of selective toxicity for many antibiotics


Capsule/ glycocalyx

• Capsule is gelatenous layer covering the entirebacterium. It is composed of mostlypolysaccharideexceptBacillus anthracis whichpolysaccharideexceptBacillus anthracis whichcontains Glutamic acid.


Types of capsule

1. Capsuleif substance is organized and firmly attached to cell

wall

2. SlimeLayer2. SlimeLayerif substance is unorganized and loosely attached to

cell wall

3. Glycocalyx –

is a thin polysaccharide coating that is secreted bymany bacteria. It covers like a filmand helps inadherence to various surfaces.12/30/13 7Dr. Shyamal Kr Paul, Bacterial

structure

Functions & Clinical significance of Capsules

1. Contribute to Virulence of bacteria by preventingphagocytosis by WBC’s Prevents attack byantimicrobials

2. Prevents drying out or dessication

3. Allows bacteriato adhereto varioussurfaces3. Allows bacteriato adhereto varioussurfaces

4. Helps in identification of bacteria by using antisera.Capsular swelling test or quellung reaction.

5. Typing of bacteria is done on the basis antigenic varionof capsule.

6. Used for vaccine preparation.


Capsule

Klebsiella pneumoniae Bacillus anthracis12/30/13 9Dr. Shyamal Kr Paul, Bacterial

structure

BiofilmIn nature, and in many medical situations, colonies of bacteria

construct and live in abiofilm , made up principally ofcapsule material.

A biofilm usually consists of a consortium(mixture) ofbacterialiving in a matrix of slimewhich is secretedby oneof thebacterialmembers.bacterialiving in a matrix of slimewhich is secretedby oneof thebacterialmembers.

Dental plaque is an example of a natural biofilm, as is a slimymass of bacteria attached to a rock in a mountain stream.

In medical situations, bacteria in a biofilmmay have certainadvantages over planktonic counterparts. For example,biofilm bacteria may be less susceptible to phagocytes,drugs, or neutralizing antibodies.


Flagella and axial filaments

• Flagella and axial filaments are theorgans of motility.

• FlagellaAre filamentousproteinstructures• FlagellaAre filamentousproteinstructuresattached to the cell surface.

• The diameter of a bacterial flagellumis about20 nanometers, well-belowthe resolvingpower of the light microscope.


Structure of flagella

• Flagella is composed of manysubunits of single proteinflagellin.

• Flagellin arrangedin several• Flagellin arrangedin severalinterwined chains.

• It has a filament, hook andbasal body – embedded in cellmembrane


Movement of flagella


Arrangement of Flagella

• Arrangement basis for classification– Monotrichous; 1 flagella– Lophotrichous; tuft at – Lophotrichous; tuft at one end

– Amphitrichous; both ends

– Peritrichous; all around bacteria


FlagellaFlagella


Motility

• Almost all Spiral bacteria aremotile

• About 1/2 ofBacilli aremotile• About 1/2 ofBacilli aremotile

• Almost allCocciare non-motile


Functions / Clinical significance of flagella

• Flagella are important for– Motility (dispersal) –

• Virulence factor in case of V.cholerae, Esch coli, Proteus

– Antigenic determinant – Antigenic determinant • Interfere with phagocytosis, • Helps in identification and typing by antibody

detection• Also in diagnosis of disease by antibody detection

in serum (widal test


Movement of flagella

• The flagellar filament is rotated by a motor apparatusin the plasma membrane allowing the cell to swiminfluid environments.

• Bacterial flagella are powered by proton motive force(chemiosmotic potential) established on the bacterialmembrane.

• Bacteria are known to exhibit a variety of types of• Bacteria are known to exhibit a variety of types oftactic behavior, i.e., the ability to move (swim) inresponse to environmental stimuli.– For example, duringchemotaxisa bacteriumcan sense the

quality and quantity of certain chemicals in its environmentand swimtowards them(if they are useful nutrients) or awayfrom them(if they are harmful substances).

– Duringaerotaxis, bacteria swimtoward or away fromO2.


Axial Filament

Spirochaets move by using aflagellum like structure called axialfilament , which wraps round spiralshaped cell to produce an

Treponema pallidum

shaped cell to produce anundulating motion.- found only inspirochetes (flexible spirals)


Pilli

• Pilli are hair like filamentsthat that extend from cellsurface.

• They are shorter and straiterthan flagella & composedofthan flagella & composedofsubunits of protein pillinarranged in helical strands.

• Found maninly in GNB• A specialized type of pilus

(always called a pilus), the For sex pilus, mediates thetransfer of DNA betweenmating bacteria,


Functions of common Pilli

• Pili play roles in– Adhesion of bacteria to specific receptors in host cell

surface.– Exchange of genetic material by specialized kind of pilli

called sex pilli by conjugation.– Avoidance of immune response– Avoidance of immune response

– Pili of different bacteria are antigenically distinct and formation of antibody by the host.

– Ab of one bacteria cannot prevent the attachment of another species.

– Some species are able to make pili of different antigenic type (antigenic variation)


Pili and flagella

Salmonella


Pili in ConjugationPili in Conjugation


Bacterial endospores

• Metabolically inactive• Produced in response to nutrient

limitation or extreme environmentslimitation or extreme environments• Highly resistant• Stable for years• Not reproductive• Bacillus and Clostridium species

sporulate12/30/13 26Dr. Shyamal Kr Paul, Bacterial

structure

Endospore


B. subtilisB. subtilis spore anatomyspore anatomy

outer coatouter coatinner coatinner coatcortexcortexmembranesmembranes

ococ

icicmembranesmembranescorecorenucleoidnucleoid


Endospores are resistant to:Endospores are resistant to:

•• Heat (both wet and dry)Heat (both wet and dry)

•• Ultraviolet (UV) radiationUltraviolet (UV) radiation•• Extreme desiccation (including Extreme desiccation (including •• Extreme desiccation (including Extreme desiccation (including

vacuum)vacuum)

•• LysozymeLysozyme

•• Chemicals (organic solvents, oxidizing Chemicals (organic solvents, oxidizing agents, etc.)agents, etc.)


Endospores

Bacillus anthracis12/30/13 30Dr. Shyamal Kr Paul, Bacterial structure

Endospores

Endospores are dormant alternate life forms.

• Are resistant to chemicals, heat, radiation, desiccation, some disinfectants, and some ‘sterilizing’ disinfectants, and some ‘sterilizing’ procedures.

• Bacillus (an obligate aerobic) found in the soil, the genus Clostridium (an obligate anaerobe)


Bacillus subtilisBacillus subtilis differentiation differentiation cyclecycle

Germination and Outgrowth

Exponential Growth

Dormancy

*Repair*Repair

Postexponential PhaseSporulation

Dormancy

II

III

IV

V*Protection*Protection*Photochemistry*Photochemistry


EndosporesFormation of endospores:• Under conditions of starvation, especially the lack of carbon

and nitrogen sources, a single endospores formwithin someof the bacteria. The process is calledsporulation.

• the DNA replicates and a cytoplasmic membrane septumforms at one end of the cell.

• A second layer of cytoplasmic membrane then forms aroundone of the DNAmolecules to forma forespore. Both ofthese membrane layers then synthesize peptidoglycan in thespace between themto form the first protective coat, thecortex. Calcium dipocolinate is also incorporated into theforming endospore. Aspore coat composed of a keratin-like protein then forms around the cortex. Sporulationgenerally takes around 15 hours.12/30/13 33Dr. Shyamal Kr Paul, Bacterial

structure

EndosporesStructure:• The completed endospore consists of multiple layers of resistant

coats (including a cortex, a spore coat, and sometimes an exosporium) surrounding a nucleoid, some ribosomes, RNA molecules, and enzymes

• Bacterial endosporesare resistant to antibiotics, most disinfectants, and physical agents such as radiation, boiling, and drying. Because-

• Multiple layers of spore• The impermeability of the spore coat • The impermeability of the spore coat • Very low metabolic activity

The heat resistance of endospores is due to a variety of factors:– Calcium-dipicolinate, abundant within the endospore, may

stabilize and protect the endospore's DNA.– Dehydration that results resistance to heat and radiation.– Finally, DNA repair enzymescontained within the endospore

are able to repair damaged DNA during germination.12/30/13 34Dr. Shyamal Kr Paul, Bacterial

structure

Plasmid:• Extra-chromosomal (mostly circular) DNA

molecules that may encode from 40-50 genes and replicates independently of chromosome

• Contains useful but not indispensable genes (up to several 100s)

• Low and high-copy-number plasmids• Low and high-copy-number plasmids• R plasmids (R = resistance to antimicrobial drugs

and heavy metals)• Most common mode of transfer is conjugation• Often determining important characteristics such as

the ability to produce exotoxins or enzymes capable of inactivating antibacterial agents.


Plasmid:

• Two types of plasmid present in bacteria.• 1. Conjugative plasmids are transferred frombacterium to

bacterium(usually members of the same species or of very closelyrelated species) through conjugation.These plasmids are common inGram-negative bacilli and are relatively large(25-150 milliondaltons). Largeplasmidsareusuallypresentat 1 –2 copiesper cell;daltons). Largeplasmidsareusuallypresentat 1 –2 copiesper cell;their replica-tion is closely linked to the replication of the bacterialchromosome.

• 2. Nonconjugative plasmids,common in Gram-positive cocci aswell as in some Gram-negative organisms (e.g.,Haemophilusinfluenzae, Neisseria gonorrhoeae) are usually small (1 –10 milliondaltons). Small plasmids may be present at more than 30 copies percell; distribu-tion to progeny during cell division is ensured by thelarge number present.


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