Bacterial Cell Structures & Functions
Bacterial Cell Structure
Bacterial Cell Structure
• Surface layers -Cell wall, cell membrane,
capsule
• Appendages - Flagella, pili or fimbriae
• Cytoplasm - Nuclear material, ribosome,
mesosome, inclusions etc.
• Special structure - Endospore
Cell envelope• Various layers-Collectively cell envelope
• Gram positive- plasma memebrane, cell wall sometimes capsule
• Gram negative- plasma memebrane, cell wall , outer membrane sometimes capsule
• Plasma membrane in gram negative bacteria is sometimes called inner membrane
• Space between inner membrane and outer membrane is called Periplasmic space.
Bacterial Cell Wall:
•10-25nm in thickness, Neg 10-15nm, Pos 20-25nm
Functions
•Accounts shape of the cells
•Provides protection of the cells against Osmotic damage
•Confers rigidity
•Takes part in cell division
•Target site for antibiotic
•Carries bacterial antigens- virulence & immunity
• General structure:
• Chemically made up of Peptidoglycan.
• It is made by two hexose sugars
N- acetylglucosamine [NAG] and
N- acetylmuramic acid [NAM]
in alternating chains interconnected by tri, tetra or penta pedtide chains.
Gram positive cell walls:
a. Peptidoglycan-Thicker in gram positive
b.Polysachharides –Teichoic acids- polymer of glycerol and ribitol phosphates
• Some gram positive bacteria eg Mycobacteria contain lipid- Mycolic acids
Gram negative cell walls• Complex structure
A. Lipoprotein layer- connects the peptidoglycan to outer membrane.
B. Outer membrane- Outer membrane proteins-target site for antibiotics.
C. Lipopolysachharides- This layer consists of lipid A to which is attached a polysachharide .
D. Periplasmic space- Inner and outer membrane.
E. Peptidoglycan
Cytoplasmic membrane• 5-10nm thick, elastic semipermeable layer
which lies beneath cell wall
• Chemically consists of phospholipids and protein molecules
• Acts as osmotic barrier
• Consists of enzymes permease, oxidase and polymerase
• Contains enzymes of tricarboxylic acid cycle and enzyme necessary for cell wall synthesis.
• Bacterial electron transport system
Cytoplasm
• Organic and inorganic solutes, water
• Lacks mitochondria and endoplasmic reticulum etc
• Contains ribosomes, mesosomes, vacuoles and inclusions.
Ribosomes• Centre of protein synthesis
• Composed of ribosomal RNA and robosomal proteins
• Two subunits 50s and 30s - 70s
Mesosomes• Centre for respiratory enzymes
• Septal and lateral
• Septal attached to bacterial chromosome involved in DNA segregation and formation of cross wall during binary fission.
Inclusions• Sources of stored energy.
• May be present as polymetaphosphate,lipids and polysachharides and granules of sulphur.
Nucleus• No nuclear membrane and nucleolus.
• Dna doesn’t contain any basic proteins.
• Genomic DNA is double stranded in the form of circle.
Plasmids• Small circular covalently closed double
stranded DNA molecules found in cytoplasm.
• Not essential for life , confer on certain properties like drug resistance and toxigenecity.
• Can be transmitted from one bacteria to another by conjugation or by bacteriophage.
CAPSULE AND SLIME LAYER
• Amorphous viscid bacterial secretion surrounding the bacteria
• Loose undemarcated secretion-slime layer• Sharply defined structure – capsule• Very thin- microcapsules• Protects bacteria against
phagocytes,adherence promote virulence, reservoir of food,
• Demonstrated by negative staining and capsule swelling reaction [ Quellung reaction].
Flagella• Cytoplasmic appendages protruding through cell wall.• Thread or hair like structure- protein flagellin• Organ of locomotion• All motile bacteria except spirochaetes
• Parts:1.Basal body: Embedded in cell envelope & consists of
small,central rod surrounded by a series of rings2. Hook :Connects basal body with the filaments3.Filament or shaft: External to cell surface Composed of protein molecule flagellin
Organ of bacterial locomotion
• Structure
• Gram negative- 2 pair of rings-
• M -Plasma membrane
• S -periplasmic space
• P- peptidoglycan
• L- lps
• Gram positive- 1 pair-
• M- Plasma membrane
• S - peptidoglycan
Structure of the flagellum
• Parts:
1. Basal body: Embedded in cell envelope & consists of small , central rod surrounded by a series of rings
2. Hook :Connects basal body with the filaments
3. Filament or shaft:
External to cell surface
Composed of protein molecule - flagellin
• Arrangements/Types
Demonstration:
• Electron microscopy
• Silver impregnation methods
• Dark field microscopy
• Special stains eg Leifsons stain
Fimbriae• Hair like appendages projecting from cell
surface as straight filaments.
• Also called pili
• 0.1-1um length and 10nm thick
• Gram negative bacteria
• Protein pilin
• Best seen in liquid cultures
• Antigenic
• E. coli fimbriae
• Types
1. Common pili- Adhesion to host cells
2. Sex pili or F fertility pili-
Found on male or donor or + strains help in attachment to female or recipient or – strains through conjugation tubes and aid in gene transfer.
Functions
Adhesion, Transfer of genetic materials
Demonstration
Electron microscopy, Haemagglutination
Bacterial spores• Highly resistant resting stages formed in
unfavourable condition
• Formed inside the cells so called endospores
• Each form one spore, which on germination form a single vegetative cell
• Non metabolising and non reproducing
• Highly resistant to heat, UV radiation, mechanical disruption, chemical disinfectants etc
• Structure
• The core of the fully developed spore has homogenous protoplasm, containing chromosome, enzymes of glycolysis and protein synthesis.
• Core is surrounded by spore walls or inner membrane.
• Outside this spore wall is thick layer the cortex enclosed by outer membrane.
• Spore coat surrounds this spore wall.
• Some bacteria has additional loose outer covering Exosporium.
• Demonstration
• Gram stain- unstained.
• AFB stain- 0.25-0.5% H2So4- Red colour.
• Use
• Spores of bacillus stearothermophilus are employed as indicator of proper sterilisation.
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