.1. Bacterial cell
Structure and Function
Bacterial
are unicellular prokaryotic organism. Bacterial cell have simpler internal
structure. It lacks all membrane bound cell organelles such as mitochondria,
lysosome, golgi, endoplasmic reticulum, chloroplast, peroxisome, glyoxysome,
and true vacuole. Bacteria also lacks true membrane bound nucleus and
nucleolus. The bacterial nucleus is known as nucleoid.
A typical bacterial cell have following structure.
A. Structure Outside cell wall
1.
Capsule
2.
Flagella
3.
Pili
4.
Sheath
5.
Prostheca
6.
Stalks
B. Structure Inside cell wall
1.
Cell
wall
2.
Cytoplasmic
membrane
3.
Nucleoid
4.
Mesosome
5.
Ribosome
6.
cytoplasm
7.
Spore
1. Capsule:
§ Capsule is 0.2µm thick viscus layer
outer layer to the cell wall.
§ Capsule is 98% water and 2%
polysaccharide or glycoprotein/ polypeptide or both.
§ There are two types of capsule.
i. Macro-capsule: thickness
of 0.2µm or more, visible under light microscope
ii. Microcapsule: thickness
less than 0.2µm, visible under Electron microscope
§ Capsule is very delicate structure. It can be
removed by vigorous washing.Capsule is most important virulence factor of
bacteria.
§ Function:
- It helps in
attachments as well as it prevent the cell from desiccation and drying.
- Capsule resist
phagocytosis by WBCs
2. Flagella:
§ It is 15-20 nm hair like helical
structure emerges from cell wall.
§ Flagella is not straight but is
helical. It is composed of flagellin protein (globular protein) and known as H antigen.
§ Flagella has three parts. Basal
body, Hook and filament
Function:
§ It helps motility of the bacteria
3. Pili or fimbriae:
§ Pili are hollow filamentous and
non-helical structure.
§ They are numerous and shorter than
flagella
§ Pili is the characteristic feature
of gram –ve bacteria.
§ Pili is composed of pilin protein.
§ Bacteria containing pili: Shigella, Proteus, Neisseria gonorrhoae, E.
coli
Function:
§ Attachment: pili helps the
bacteria to attach the host cell surface. Most of the human pathogens of respiratory
tract, urinary tract are attached with the help of pili.
§ Pili (fimbrae) possess antigenic
property
§ Specialized function: some pili
are modified for specialized function. Eg. Sex pilus (F-pili) help in transfer
of DNA from donor to recipient cell during conjugation.
§ F-pili also act as receptor for
bacteriophage.
4. Sheath:
§ Some bacteria forming chain or
trichome are enclosed by a hollow tube like structure known as Sheath.
§ Aquatic bacteria mostly form
sheath
§ Some sheathed bacteria are;
Sphaerotilus, leptothrix, clonothrix etc
Function:
§ Mechanical support
§ Sometime sheath is impregnated
with ferric or manganese hydroxide which provide strength to sheath.
5. Prosthecae:
§ Prosthecae are semi-rigid
extension of cell wall and cell membrane
§ One bacteria may contains one or
many prosthecae.
§ Some prosthecae develop bud at the
tip and hence helps in reproduction.
§ Some prosthecate bacteria
are: Caulibacter, Stella,
Prosthecobacter, Hyphomicrobium
Function:
§ Prosthecae increase surface area
for nutrition absorption. It is usually formed in bacteria living in very
dilute environment where concentration of nutrition is low.
§ Helps in adhesion
§ Asexual reproduction by budding
6. Stalk:
§ It is non-living ribbon like
tubular structure.
§ It is formed by excretory product
of bacteria.
§ Some stalked bacteria are:
Gallionella, Planctomyces
Function:
§ Helps in attachment to solid
surface.
7. Cell wall:
§ It is an important structure of a
bacteria.
§ It give shape to the organism.
§ On the basis of cell wall composition,
bacteria are classified into two major group ie. Gram Positive and gram
negative.
Gram positive cell wall: Cell wall composition of
gram positive bacteria.
1.
Peptidoglycan
2.
Lipid
§ Teichoic acid
Gram negative cell wall : Cell wall composition of gram negative bacteria
1. Peptidoglycan
2. Outermembrane:
§ Lipid
§ Protein
§ Lipopopysaccharide (LPS)
Peptidoglycan:
§ It consists of glycan backbone formed by repeated
unit of NAG (n-acetyl Glucosamine) and NAM (N-acetyl muramic acid) and the glycan
backbone is cross linked by peptide bond.
§ Peptidoglycan layer is present in cell wall of both
gram positive as well as gram negative bacteria. However, gram positive have
thick layer of peptidoglycan.
Teichoic acid:
§ Teichoic acid is water soluble polymer of glycerol
or ribitol phosphate present in gram positive bacteria.
§ It constitutes about 50% of dry weight of cell
wall.
§ It is the major surface antigen of gram positive
bacteria
Outer membrane:
§ It is an additional layer present in gram negative
bacteria.
§ It is composed of lipid bilayer, protein and
lipo-polysaccharide(LPS)
LPS:
§ LPS is composed of lipid-A and
polysaccharide.
§ Lipid-A: it is phosphorylated
glucosamine disaccharide. It is antigenic
§ Polysaccharide: it consists of core-polysaccharide
and O-polysaccharide.
8. Cell membrane:
§ Cell membrane is the inner layer
that lies inside the cell wall and encloses the cytoplasm.
§ It is also known as cytoplasmic
membrane or plasma membrane.
§ It is about 80nm thick.
§ Cell membrane of bacteria is
composed of phospholipid and proteins.
Function:
§ It is selectively permeable as it
allows to pass selective substances such as sugar, aminoacids across it.
9. Nucleus:
§ Nucleus is the most important part
of the cell.
§ It controls and directs all the
cellular activities and stores hereditary information of cell
§ Bacterial nucleus is known as
nucleoid; it lacks nuclear membrane, nuceloplasm and nucleolus.
§ Bacterial DNA is naked (lacked
histone protein)
Function:
§ It contains and stores hereditary
information of the cell.
§ It controls all cell activities.
10. Ribosome:
§ Bacterial ribosome is of 70s type.
§ Ribosomes are rounded granules
found freely floating in the cytoplasm
§ Ribosomes are known as universal
cell organelle because it is found in both bacterial cell and eukaryotic cell.
§ Chemically the ribosomes are made
up of nucleic acids (particularly RNA and proteins).
Function:
§ It helps in protein synthesis
11. Mesosome:
§ Mesosome is a spherical or round
sac like structure found commonly in gram positive bacteria.
§ Function: It is the site for
respiration in bacterial cell
12. Cytoplasm:
§ It is colorless, viscus fluid
present inside cell membrane.
§ All the cell organelles and
inclusions are found floating in cytoplasmic fluid.
§ It contains proteins, lipid,
minerals, nucleic acids, glycogen, water etc.
Function:
§ It helps to distribute water,
oxygen as other substances throughout the cell.
§ Literally, all the cellular
content including nucleus, and other cell organelle are floating in cytoplasm.
13. Spores (endospore):
§ Spore is metabolically dormant
structure produced during unfavourable condition by the process called
sporulation
§ Sporulation occur during late log
phase or early stationary phase
§ Under favourable condition spores
germinate to give vegetative cell.
