Transcript Lect-2

LECTURE 2:
An Overview of Bacterial
Cell Structure
Viro102:
Bacteriophages & Phage Therapy
3 Credit hours
NUST Centre of Virology & Immunology
Cell types
Prokaryotic Cell
Bacteria &
Archaea
Eukaryotic Cell
animals, plants,
fungi
Bacterial Cell Morphology
Bacterial Cell
no membrane bound organelles
Capsule
Cell Wall
Cell membrane
Ribosome
Plasmids
Chromosome
Flagellum
Pilli
Capsule
Outer most covering of some bacteria
Well organized layer, not easily washed off
Helps to protect bacteria against
o Phagocytosis, A capsule specific antibody may be
required for phagocytosis to occur
o Bacteriophages
o They also exclude bacterial viruses and
o Hydrophobic toxic materials such as detergents
Capsule
 Capsules also contain water which protects
bacteria against desiccation
 Allow bacteria to adhere to surfaces and cells.
 Composed of polysaccharides
Bacillus Anthracis has protinaceous capsule
composed of poly-D glutamic acid
Slime Layer
Glycocalyx
Bacterial Cell
no membrane bound organelles
Capsule
Cell Wall
Cell membrane
Ribosome
Plasmids
Chromosome
Flagellum
Pilli
A comparison b/w G(+ve) & G(-ve)
GRAM POSITIVE
Lipoteichoic acid
Peptidoglycan-teichoic acid
Cytoplasmic membrane
Cytoplasm
GRAM NEGATIVE
Porin
Outer Membrane
Braun lipoprotein
Inner (cytoplasmic) membrane
Cytoplasm
Lipopolysaccharide
Classification of Bacteria
In 1884, Hans Christian Gram developed a
staining technique for the identification of
bacteria Gram staining
Based on this technique, Bacteria is
characterized into
o Gram positive
o Gram negative
The difference is due to the cell wall
differences in these bacteria
Gram Staining Technique
What happens in Gram staining?
 Crystal Violet (purple)
o Primary stain; positive stain.
o Stains all cell walls purple.
 Iodine
o Mordant (Any substance used to facilitate the fixing of a dye to a fibre).
o Combines with CV to form an insoluble complex that gets trapped in
thicker peptidoglycan layers.
 Ethanol
o Decolorizer.
o CV complex washed out of Gram negative organisms because it cannot
be trapped by outer layer.
 Safranin (pink)
o Counterstain.
o Stains all cells, but only the negative ones actually appear pink.
Prepare a thin
smear of fresh
bacteria
Apply primary
stain & then wash
Apply mordant &
then wash
Visualize
Apply secondary
stain & then wash
Apply ethanol
Results under a Microscope
Yogurt: (Lactobacillus {G+} and Enterobacteria{G-})
G -ve
Bacillus
G +ve
Gram stain of cerebrospinal fluid
showing B. anthracis
Bacterial Cell
No membrane bound organelles
Capsule
Cell Wall
Cell membrane
Ribosome
Plasmids
Chromosome
Flagellum
Pilli
Cell Membrane
 Composition:
o Phospholipid bilayer just like eukaryotic PM (Fluid Mosaic)
o Lipids are not homogeneously distributed in membrane
o Contain fatty acids with additional methyl hydroxyl or even
cyclic groups
o The relative proportions of these fatty acids can be
modulated by the bacterium to maintain the optimum
fluidity of the membrane (e.g. following temperature
change)
Cell Membrane
Functions:
o Acting as a permeability barrier for most molecules
o Serving as the location for the transport of
molecules into the cell
o Function in energy production as the location about
which a proton motive force is generated
o Act as site for photosynthesis in Cyanobacteria
Bacterial Cell
No membrane bound organelles
Capsule
Cell Wall
Cell membrane
Ribosome
Plasmids
Chromosome
Flagellum
Pilli
Ribosomes
Site of protein synthesis
Made up of protein and RNA
The 70S ribosome is made up of a 50S and 30S
subunits
 The 50S subunit contains the 23S and 5S rRNA
 The 30S subunit contains the 16S rRNA
Bacterial Cell
No membrane bound organelles
Capsule
Cell Wall
Cell membrane
Ribosome
Plasmids
Chromosome
Flagellum
Pilli
Bacterial Chromosome
 Present in the cytoplasm
 The bacterial chromosome is not packaged using
histones to form chromatin as in eukaryotes but
instead exists as a highly compact supercoiled
structure, the precise nature of which remains
unclear.
Most bacterial chromosomes are circular although
some examples of linear chromosomes exist (e.g.
Borrelia sp.)
Bacterial Cell
No membrane bound organelles
Capsule
Cell Wall
Cell membrane
Ribosome
Plasmids
Chromosome
Flagellum
Pilli
Plasmids
Most bacteria also contain small independent pieces
of DNA called plasmids that often encode for traits
that are advantageous but not essential to their
bacterial host
Bacterial Cell
No membrane bound organelles
Capsule
Cell Wall
Cell membrane
Ribosome
Plasmids
Chromosome
Fimbriae
Pili
Flagellum
Fimbriae
Many prokaryotes have short, fine, hair like
appendages that are thinner than flegella
known as Fimbriae
Interchangeably term fimbraie and pili.
Cell may covered by 1000s of fimbraie, visible
under electron microscope
Cont’d
Slender tubes composed of helical
arrangement of protein subunits
3-10 nm in diameter
Several millimeter long
Usually facilitate the attachment of a
bacterium to a surface (e.g. to form a biofilm)
or to other cells (e.g. animal cells during
pathogenesis)
Cont’d
Moreover help in motility
Pseudomonas aeroginosa, Neisseria
gonorrhoeae, some strain of E.coli
Bacterial Cell
No membrane bound organelles
Capsule
Cell Wall
Cell membrane
Ribosome
Plasmids
Chromosome
Fimbriae
Pili
Flagellum
Pili
Hair like appendages
Different from fimbraie
Pili are larger
1-10 / cell
Genetically determined by conjugative
plasmid
Phages specifically bind on sex pili, hence
infect bacteria
Pili
During bacterial conjugation, a sex pilus
emerging from one bacterium ensnares the
recipient bacterium
This is direct contact merging the cytoplasms
of two bacteria via a controlled pore which
allows for the transfer of bacterial DNA from
the bacteria with the pilus (donor) to the
recipient bacteria
Usually the fertility factor is required to
produce sex pili
Bacterial Cell
No membrane bound organelles
Capsule
Cell Wall
Cell membrane
Ribosome
Plasmids
Chromosome
Fimbriae
Pili
Flagellum
Flagellum
Flagella are whip like structures protruding
from the bacterial cell wall and are
responsible for bacterial motility
Composed of different proteins including
flagellin
The arrangement of flagella about the
bacterial cell is unique to the species
observed. Common forms include:
 Peritrichous Multiple flagella found at several locations
on the cell.
 Polar Single flagella found at one of the cell poles.
 Lophotrichous A clump of flagella found at one cell pole.
Flagellum
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