2. Bacteria (singular: bacterium) form a large group of
unicellular prokaryotes that do not i contain a nucleus
and other membrane-bound organelles.
They were first observed by Antony Van Leeuwenhoek
in 1676 using a single-lens microscope of his own
design.
He called them "animalcules" and published his
observations in a series of letters to the Royal Society
of London
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3. The name bacterium was introduced much
later by Christian Gotfried Ehrenberg in 1838.
Typically a few micrometers in length,
bacteria have a wide range of shapes, ranging
from spheres to rods and spirals.
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4. SIZE OF BACTERIA: Bacterial cells are typically
0.5-5.0 micrometers in length.
Medically important bacteria generally measure
0.2-1.5 micrometers in diameter and 3-5
micrometers in length.
Among the smallest bacteria are members of the
genus Mycoplasma which measure only 0.3
micrometers, as small as the largest viruses.
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5. Bacteria show great variation in their
shapes as discussed below:
Cocci
These are spherical or oval cells. On
the basis of arrangement of
individual organisms they may be;
Micrococci-cocci that occur singly.
.Diplococci-cocci that occur in pairs
(diplo meaning pair).
Streptococcci-cocci that occur in
chains (strepto meaning chain).
.Staphylocacci-cocci that occur in
bunches (staphylo meaning bunch).
• Tetrad- cocci that occur in groups of
four (tetra meaning four).
• Sarcinae(octad)-cocci that occur in
group of eight(octa meaning eight.
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7. Bacilli
These are rod or stick-shaped cells.
On the basis of arrangement of an individual organism they may be;
Microbacilli- bacilli that occur singly.
Diplobacilli- bacilli that occur in pairs. Streptobacilli-bacilli that
occur in chains.
Palisade arrangement: Bacilli lined side by side like matchsticks & at
angles to one another e.g. Corynebacterium diphtheriae causing
diphtheria.
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8. Comma-shaped - bacilli that are curved and look like a comma e.g.
Vibrio cholera causing cholera.
Spirillum-bacilli that are coiled like a cork-screw through 1-5 complete
turns eg. Spirillum mims causing rat bite fever.
3. Coccobacili: These are the bacteria which are intermediate to
coccus and bacilus e.g,Brucella.
4. Spirochetes (speir meaning coil; chaite meaning hair): They are
relatively longer, slender, flexous, non-branched microorganisms of
spiral shape having several cols e.g, Treponemna pallidum causing
syphilis (Figure 1.4).
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9. STRUCTURES EXTERNAL TO CELL WALL
FLAGELLA:
Flagella (sing: flagellum) are long, hollow, helical
filamentous organs of locomotion that arise from
cytoplasmic membrane & pass out through cell
wall. They are 1020 nm in diameter, 3-20 Lm in
length & are found on Gram +ve & Gram-ve
bacteria.
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11. A flagellum consists of three distinct parts;
Basal body: It constitutes the extreme basal part of
the flagellum attached with plasma membrane.
Hook: It represents a broader & thicker basal region
of flagellum & passes out through the cell wall.
Filament: It is the thinner, elongated & terminal part
of flagellum.
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13. Flagellar types of bacteria: On the basis of flagellation
(pattern of flagelar arrangement) bacteria can e grouped
as: .
Monotrichous - bacteria with single polar flagellum.
Amphitrichous - bacteria with single polar flagellum or
tuft of flagella at both poles.
Lophotrichous - bacteria with tuft of flagella at one pole.
Peritrichous - bacteria with flagella distributed all round
the cell. Functions: Flagella help in motility of bacteria.
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15. PILI (FIMBRIAE: Ill Sing: pilus) are hollow, non-
helical, filamentous appendages that i are
thinner, shorter & more numerous than flagella.
Pili occur in both flagellated & non - flagellated
bacteria
Each bacterium possesses 100-200
peritrichously-borne (present al l around a cell
pll originating from cell membrane. Pili are 1-1.5
um in length & 4-8 nm in diameter & are made
up of protein pilin.
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16. Functions:
Pili do not function in motility, since they are
found in both motile and non - motile bacteria.
There are however several functions associated
with different types of pili
One type of pilus is known as F -pilus (fertility
pilus) or sex pilus as it is involved in bateral
conjugation where it establishes a bridge
between a donor & a recipient cell.
Some pili allow the pathogenic bacteria to attach
to epithelial cells lining the respiratory, intestinal
or genitourinary tract & thus establish an
infection
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17. CAPSULE OR SLIME LAYER :
Many bacteria produce substances of high
molecular weight at the time of their active
growth. These substances collect on surface of
cells & form a gelatinous covering around these
cells.
When the gelatinous covering does not form a
persistent layer but is present more diffusely
forming a loose mass or matrix around the
bacterial cell, it is called slime layer.
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18. When the gelatinous covering forms a wel-
defined persistent (discrete) layer, it is called
capsule.
Most bacterial capsules are composed of
polysaccharide eg. capsule of Klebsiella
pneumoniae, but few capsules are composed
of polypeptide eg. i Capsule of Bacillus
anthracis.
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19. It provides protection against temporary drying
by binding water molecules.
It may block attachment of bacteriophages.
It may inhibit the engulfment of pathogenic
bacteria by phagocytes (WBC's) & thus
contributes to virulence (infective ability) of
bacteria.
It may promote attachment of bacteria to smooth
surfaces
It may protect the bacteria from antibacterial
agents such as lytic enzymes found in nature.
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20. It is a rigid structure surrounding the plasma
membrane & is present in all prokaryotes
except Mycoplasma & Methanoplasma.
Structure & chemical composition: The main
constituent or back-bone of bacterial cell wall
is peptidoglycan (also known as murein,
muramic acid or mucopeptide) which is an
insoluble, porous, cross-linked polymer of
enormous strength & rigidity.
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21. Peptidoglycan is found only in prokaryotes,
where it occurs in the form of a bag shaped
macromolecule surrounding the cytoplasmic
membrane.
Peptidoglycan is present in Gram +ve & Gram
-ve bacterial cell walls. Hlowever, the cell wal
of Gram +ve bacteria differs markedly from
that of Gram -ve bacteria in the following
perspectives:
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24. Functions:
1. The cell wall being rigid structure, gives
shape to the cell.
2. Cell wall prevents the cell from expanding
& bursting, when osmotic pressures are
exerted on it.
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25. TO BE CONTINUED……….
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28. CYTOPLASMIC MEMBRANE/PLASMA MEMBRANE:
Immediately beneath the cell wall is plasma
membrane, approximately 7.5 nm thick & is
composed of phospholipids (about 20-30%) &
proteins (about 60-70%).
The phospholipids form a bilayer in which
most of the proteins are firmly held called
integral or intrinsic proteins
Other proteins called peripheral or Extrinsic
proteins are loosely attached.
29. Functions
1. The plasma membrane acts as a differentially
permeable barrier, regulating the flow of
materials in and out of the cell. Specific proteins
in the membrane
allow/facilitate the passage of small molecules
(nutrients & waste products) across the
membrane.
2. It contains various enzymes involved in
respiratory metabolism & in synthesis of cell
wall, septum formation, membrane synthesis &
DNA replication.
30. CYTOPLASM:
It is a homogenous aqueous solution bounded by
cell membrane & is divided into three distinct
areas.
i. Cytoplasmic area - Granular in appearance &
rich in ribosomes (70S - 50S & 30S) on which
proteins are synthesized.
ii. Chromatin area - Rich in DNA.
iii. Fluid portion - Consisting of dissolved
substances (cell solutes, metabolites, inorganic
ions).
31. Unlike higher eukaryotic cell (animals &
plants), the bacterial cytoplasm lacks
Endoplasmic reticulum, Golgi apparatus,
Mitochondria & a true membrane bound
nucleus.
INCLUSIONS: Often contained in the
cytoplasm of prokaryotic cells is one or
another type of inclusion granule. Inclusions
are distinct granules that may occupy a
substantial part of the cytoplasm.
35. MESOSOMES:
Many bacteria especially Gram positive
bacteria possess characteristic
membrane invaginations or infoldings in the
form of system of convoluted tubules &
vesicles known as mesosomes. On the basis
of their location in a cell they may be;
37. Central:
Central mesosomes penetrate deeply into the
cytoplasm, are located near the middle of cell &
appear to be attached to nuclear material of cell.
They are thought to be involved in DNA replication
& septa formation at the time of cell division.
Peripheral:
Peripheral mesosomes do not penetrate into
cytoplasm & are located near periphery. They are
thought to be involved in export of exocellular
enzymes like penicillinase.
38. NUCLEAR MATERIAL:
Bacterial cells contain neither a distinct
membrane-bound nucleus nor a mitotic
apparatus.
However, they do certain an area near the
center of the cell that is regarded as a nuclear
structure.
Because it is not discrete nucleus, it has been
designated by many names like nucleoid, the
chromatin body, the nuclear equivalent &
even the bacterial chromosome.
39. The nucleoid is typically one large circular
molecule of DNA, more or less free in the
cytoplasm although coiled & super coiled
anchored by proteins.
Sometimes a smaller extra chromosomal piece
of DNA is present in addition to nucleoid called
as plasmid.
40. ENDOSPORES:
certain bacterial species during their
unfavorable environmental condition (such as
starvation & desiccation) produce highly
resistant, metabolically dormant structures
called spores.
As these spores are formed within the parent
bacterial cell, they are called endospores.
They are thick walled, highly refractile bodies.
43. Endospores are produced (one per cell) by
certain species of bacillus (B.anthracis,
B.subtilis),
Clostridium (Cl. tetani, Cl.welchii & Cl.
botulinum), Sporosarcina,
Thermoactinomyces & few
other genera.
44. Formation of Spore (Sporulation): Spore
formation is initiated by the appearance of a
clear area in a portion of protoplasm near one
end of the bacterial cell which incorporates
part of the nuclear material equivalent to one
genome of the cell. This clear area with
nuclear
material becomes gradually more opaque
with condensation of nuclear chromatin
forming the forespore.
45. The cell membrane grows inwards &
undergoes enfolding forming a double
layered membrane structure (spore wall)
around the core (forespore). The innermost
layer of
spore wall forms the spore membrane. The
spore wall then synthesizes other layers -
spore
cortex, spore coats & exosporium (Fig. 1.14
46. Resistance: The endospo are resistant to ordinary
boiling, heating & disinfectants.
They can withstand boiling up to 3 hours, dry heat
at 150 °C for 1 hour. However, they can
be destroyed by autoclaving at 121 °C for 15-20
minutes.
All endospores contain large amount of dipicolinic
acid (DPA), a unique compound that
accounts for 10-15% dry weight of spore. It occurs
in combination with large amounts of
calcium & this Ca-DPA complex is thought to play
a major role in heat resistance of
endospores.
47. Germination of spore: The conversion of spore
into a vegetative cell under favorable
environmental conditions is known as
germination. It may occur in less than 2 hours
under
optimal conditions & consists of following three
stages:
a. Activation: The activation of spore is brought
about by one or another agent such as heat
(60°C for 1 hour), low pH (acidic), abrasion etc.
that damage the
48. Initiation: Once activated, the process of
initiation begins marked by binding of
effector
substances from a rich medium to the spore
coat. This binding activates autolysins (self-
produced lysing enzymes) that destroy the
peptidoglycan of cortex, allowing the uptake
of
water & release of calcium dipicolinate. The
number of hydrolytic enzymes produced
degrades the various constituents of spore
49. Outgrowth: With the disintegration of cortex
& swelling of spore, a single germ cell
emerges after breaking open the spore coat.
The new vegetative cell consists of the spore
protoplast with a surrounding wall. This is
followed by a period of active biosynthesis
producing an outgrowth that ultimately gets
transformed into a new vegetative cell.
