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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
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
12/30/13 5Dr. Shyamal Kr Paul, Bacterial structure
Capsule/ glycocalyx
• Capsule is gelatenous layer covering the entirebacterium. It is composed of mostlypolysaccharideexceptBacillus anthracis whichpolysaccharideexceptBacillus anthracis whichcontains Glutamic acid.
12/30/13 6Dr. Shyamal Kr Paul, Bacterial structure
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.
12/30/13 8Dr. 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.
12/30/13 10Dr. Shyamal Kr Paul, Bacterial structure
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.
12/30/13 11Dr. Shyamal Kr Paul, Bacterial structure
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
12/30/13 12Dr. Shyamal Kr Paul, Bacterial structure
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
12/30/13 15Dr. Shyamal Kr Paul, Bacterial structure
Motility
• Almost all Spiral bacteria aremotile
• About 1/2 ofBacilli aremotile• About 1/2 ofBacilli aremotile
• Almost allCocciare non-motile
12/30/13 18Dr. Shyamal Kr Paul, Bacterial structure
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
12/30/13 19Dr. Shyamal Kr Paul, Bacterial structure
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.
12/30/13 20Dr. Shyamal Kr Paul, Bacterial structure
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)
12/30/13 21Dr. Shyamal Kr Paul, Bacterial structure
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,
12/30/13 22Dr. Shyamal Kr Paul, Bacterial structure
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)
12/30/13 23Dr. Shyamal Kr Paul, Bacterial structure
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
B. subtilisB. subtilis spore anatomyspore anatomy
outer coatouter coatinner coatinner coatcortexcortexmembranesmembranes
ococ
icicmembranesmembranescorecorenucleoidnucleoid
12/30/13 28Dr. Shyamal Kr Paul, Bacterial structure
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.)
12/30/13 29Dr. 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)
12/30/13 31Dr. Shyamal Kr Paul, Bacterial structure
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
12/30/13 32Dr. Shyamal Kr Paul, Bacterial structure
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.
12/30/13 35Dr. Shyamal Kr Paul, Bacterial structure
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.
12/30/13 36Dr. Shyamal Kr Paul, Bacterial structure