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Bacteriology
(2) Anatomy of Bacterial cell
Essential structures which are present in all bacteria -include
: Cell wall Cytoplasmic membrane Cytoplasm Nuclear material
Cell wall Site : Outermost layer , common to all bacteria except
mycoplasma Function : - It is a complex rigid structure with some
elasticity which gives bacteria their definite shape -It is a
permeable to passage of liquid nutrient material into the cell and
to outward passage of substances produced within the cell
Gram positive bacteria –Composition of Cell wall -Cell wall of
gram +ve is thicker than gram –ve -Peptidoglycan : strength of cell
wall is due to its presence , constitutes 50-90% of the wall
-Teichoic acid : responsible for the pathogenecity (normal
bacterial flora is deficient in teichoic acid) -polysacchrides
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Gram negative bacteria -More complex than gram positive
-Peptidoglycan : consistutes 5-10% of the cell wall , thin layer
-Lipopolysacchride (LPS): -the outer most layer , it is called
endotoxin , released only after cell death and lyses . Responsible
for endotoxic shock (fever , hypotension , hypoglycaemia and DIC )
- All the toxicity of LPS is due to lipid A
Cytoplasmic membrane -Site : It lies just inside the cell wall
-composition: It is thin (0.5-10 nm) , elastic and consists mainly
of lipoprotein -Function : 1) it acts as semi-permeable membrane
controlling the inflow and outflow of metabolites to and from the
protoplasm 2)it permits the passive diffusion of water in and out
3)Selective transport of specific nutrients into the cell and that
of waste products out of it
Cytoplasm -It is a viscous watery solution of soft gel ,
containing a variety of organic and inorganic solutes and numerous
small granules called ribosomes -The cytoplasm of bacteria differ
from the eukaryote in not containing endoplasmic reticulum , Golgi
apparatus , mitochondria and lysosomes
Ribosomes -It is the site of protein synthesis -Composition :
ribosomal RNA (rRNA) and ribosomal protein -They are designated by
their sedimentation coefficient ( S or Svedberg unit) -They have
sedimentation coefficient of 70S -There are differences between
bacterial and host cell ribosomes (70S & 80S respectively)
-This allows us to use antibiotics such as streptomysin which
interferes with bacterial metabolism at ribosomal level without
upsetting human ribosomal function
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Mesosomes -They develop by invagination of cytoplasmic membrane
into the cytoplasm -Function:- 1-principal sites of respiratory
enzymes in bacteria 2-They are corresponding to mitochondria in
eukaryotic cells
Intracytoplasmic inclusions -they are not permanents or
essential structures and may be absent under certain conditions of
growth -they serve as storage of nutrients material e.g. Volutin
granules -They are present in large number when there is excess of
energy yielding nutrients and diminish or disappear when there is
energy source starvation
Bacterial Nucleus -The genetic information of a bacterial cell
is contained in a single , circular double –stranded molecule of
DNA -It is 1000 times the length of bacterial cell tightly coiled
-The nucloid contains no nuclear membrane , nucleolus and doesn’t
divide by mitosis
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Extracellular structure of bacterial cell →Capsule or Slime
layer → Flagella →Fimbriae or pilli →Spores
Capsule or Slime layer -Site :it is the outermost layer , lies
outside the cell wall in capsulated bacteria -It is gelatinous
covering layer of the entire surface -Nature : Polysacchride ,
polypeptide or protein -Slime layer : easily washed off -Capsule :
discrete thickened gel around the cell , cannot be stained by
ordinary stain but by India ink ( by ordinary stain it appears as
hallow around the bacteria) -It can be visualized by reaction with
specific antibody which cause a characteristic swelling of the
capsule . It is known as Quellin reaction Functions : 1- protect
the bacteria from antibacterial agents such as lytic enzymes found
in nature 2-They inhibit phagocytosis , thus contributing to the
virulence of the bacteria 3-Loss of the capsule render the bacteria
to be avirulent
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Flagella -Definition : long thread-like appendage -Nature : it
is protein in nature , formed of subunits known as flagellin
-Function :responsible for the motility of few coccal forms , half
of bacilli , and almost all of sprilla and vibrios -Flagella can be
visualized by ordinary light microscope by special stains or by
electron microscope -The flagellum consists of 3 parts : the
filament , the hook , and the basal body
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-Types : according to their number and distribution around the
cell , they may be : →Monotrichous : single polar flagellum
→Lophotrichous: they have a tuft of flagella at one pole
→Amphitrichous: they have single polar flagella or tuft of flagella
at both poles
→Peritrichous : flagella are distributed all round the cell
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Fimbriae or Pili -Definition : they are hair like filaments ,
they are shorter , thinner and much more numerous than flagella ,
present in flagellated and non flagellated bacteria -They can be
seen by electron microscope -Function : It is necessary for
initiation of infection , it mediates attachement of bacteria to
human cell through specific receptors -Sex pili : Certain bacteria
possess specialized pili which are linger than the common type
-These appear to be hollow and constitute tubes through which DNA
is transferred from one organism to another during conjugation
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Comparison between pili and sex pili
Pili Sex pili
Short . thin , numerous One long pilus Number
Allow attachment of bacterial cell to cell surfaces
Mediate conjugation Function
Spores -Definition : they are dormant forms of bacteria and
highly resistant (can remain for many years ) -They are formed in
response to unfavourable environmental condition -Sporulation of
bacteria is a method of preservation and not of reproduction . They
are formed outside human body -Formation of spores occurs after 4-8
hrs after stop of bacterial growth -They cannot be sterilized by
ordinary stains -The most common bacteria which form spores are
gram positive bacteria e.g. Bacillus sp , Clostridia sp -Their
killing requires moist heat at 100-120Oc form10 min , while other
vegetative cells can be killed by heating at 60 oC for 10 min
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SPORE FORMATION
1- Vegetative growth stops , DNA is duplicated 2-Septum forms ,
divide bacteria asymmetrically 3-The larger compartment engulfs the
smaller compartment , forming a fore spore within the mother cell
4-Peptidoglycan containing material is laid down between the 2
membranes that now surround the fore spore 5-The mother cell is
degraded and the endospore released
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continued –Spores -shape : round , oval or elongated -site :
occupying a terminal , subterminal , or central position -size
:they may be narrower than the width of the bacilli or broader and
bulging
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Key facts -All living cells are either prokaryotic or eukaryotic
-Prokaryotes such as bacteria are simple cells -Eukaryotes have a
nucleus , organelles such as mitochondria , and complex internal
membranes e.g. Fungi , human cells -Peptidoglycan is present in the
cell wall of both gram positive and gram negative but , it is
thicker in gram positive . It gives rigidity and shape of organism
-Lipopolysacchrides (LPS) is the outer most layer of outermembrane
of gram negative (not in gram positive ) bacteria -LPS is endotoxin
and therefore gram positive bacteria do not produce endotoxin
-Bacterial cytoplasm contains chromosomal , nuclear material ,
ribosomes , mesosomes , and inclusion / storage granules
-Structures external to the cell wall of bacteria are flagella ,
pili or fimbriae , capsule and slim layer -Flagella are used for
movement , pili for adhesion and capsule for protection of bacteria
against antibiotics and phagocytosis (virulence of bacteria)
-sporulation is a response to starvation in Bacillus spp and
Clostridium spp
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Important Questions 1- Enumerate the essential structures of
bacteria 2-What is the difference between gram positive and gram
negative cell wall 3-Mention the function of the following
structures a-cytoplasmic membrane b-ribosomes c-mesosomes d-nucleus
4-enumerate extra-structures of bacterial cells 5-Mention the
function of each of the following a-capsule b-flagella c-fimbriae
d-spores 6-choose : ... is not permanent structure ( cytoplasm –
DNA-RNA-intracytoplasmic inclusions) 7-complete a-The cell wall of
... contains about 90% peptidoglycan b-lipopolysacchride layer of
... can lead to endotoxic shock c-The size of prokaryote cell is ..
and cells in eukaryote cells is ... 8-State True or False 1-The
Capsule is the inner most layer of bacterial cell 2-Mobility of
bacteria is one of the functions of capsule 3-There are 4 types of
flagella according to its number and arrangement 4-Sporulation is a
method of reproduction 5-Fimbriae help bacteria to preserve itself
during unfavourable conditions 6-Spores can be killed by boiling
only
Answers 1- Cell wall Cytoplasmic membrane Cytoplasm Nuclear
material
2-Gram positive bacteria -Cell wall of gram +ve is thicker than
gram –ve -Peptidoglycan : strength of cell wall is due to its
presence , constitutes 50-90% of the wall
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-Teichoic acid : responsible for the pathogenecity (normal
bacterial flora is deficient in teichoic acid) -polysacchrides
Gram negative bacteria -More complex than gram positive
-Peptidoglycan : consistutes 5-10% of the cell wall , thin layer
-Lipopolysacchride (LPS): -the outer most layer , it is called
endotoxin , released only after cell death and lyses . Responsible
for endotoxic shock (fever , hypotension , hypoglycaemia and DIC )
- All the toxicity of LPS is due to lipid A 3-a-1) it acts as
semi-permeable membrane controlling the inflow and outflow of
metabolites to and from the protoplasm 2)it permits the passive
diffusion of water in and out 3)Selective transport of specific
nutrients into the cell and that of waste products out of it b) It
is the site of protein synthesis c) 1-principal sites of
respiratory enzymes in bacteria 2-They are corresponding to
mitochondria in eukaryotic cells d) The genetic information of a
bacterial cell is contained in a single , circular double –stranded
molecule of DNA 4) →Capsule or Slime layer → Flagella →Fimbriae or
pilli →Spores 5)a-1- protect the bacteria from antibacterial agents
such as lytic enzymes found in nature 2-They inhibit phagocytosis ,
thus contributing to the virulence of the bacteria 3-Loss of the
capsule render the bacteria to be avirulent b- responsible for the
motility of few coccal forms , half of bacilli , and almost all of
sprilla and vibrios c-: It is necessary for initiation of infection
, it mediates attachement of bacteria to human cell through
specific receptors d-a method of preservation 6-intracytoplamic
inclusions 7-a-gram +ve bacteria b-gram negative bacteria
c-70S-80S
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8-1-F 2-F 3-T 4-F 5-F 6-F