Click here to Visit - www.thedentalhub.org.in
Section III
Microbiology
Part I
General Microbiology
Topic 1
Historical Introduction
SHORT NOTES
Q. 1. Louis Pasteur Q. 2. Robert Koch
Or, Or,
Louis Pasteur’s contributions to microbiology Koch’s postulates
Ans. Ans.
l Louis Pasteur (1822–1895) was a French chemist. He l Robert Koch (1843–1910) was a German practitioner
propounded microbial theory of fermentation. who is known as the father of medical microbiology.
l It was he who coined the term vaccine. l His contributions to the field of microbiology are as
l The various contributions by Pasteur in 30 years follows:
(1860–1890) of long, brilliant and active research are 1. He introduced staining techniques for bacteria.
as follows: 2. He described various methods of obtaining bacteria
1. Microbial theory of fermentation: Conclusive evidence in pure culture using solid media.
of role of microorganisms in disease production 3. He isolated first bacteria (Bacillus anthracis) in pure
2. Development of methods and technique for cultivation culture.
of microorganisms 4. He discovered the bacillus of tuberculosis and the
3. Studies on silkworm disease, anthrax, chicken cholera cholera vibrio.
and hydrophobia 5. He demonstrated Koch phenomenon, which is a hyper-
4. Control of diseases of silkworm sensitivity reaction seen in a guinea pig already infected
5. Principles and practice of sterilization: Introduction with the tuberculous bacilli when the tubercle bacillus
of sterilization technique and development of steam or its protein is injected into it.
sterilizer, hot air oven and autoclave 6. He proposed the Koch postulates, which are the criteria
6. Development of live attenuated vaccine against laid down by Koch.
a. chicken cholera, According to these a microorganism can be accepted as the
b. anthrax and causative agent of an infectious disease only if the following
c. rabies virus. conditions are satisfied:
These works of Pasteur were acclaimed throughout the a. The bacterium should be constantly associated with
world. In 1889, the Pasteur Institute, Paris was built by the lesions of the disease.
public contribution and Louis Pasteur served as its first b. It should be possible to isolate the bacterium in pure
director. culture from the lesions.
241
,Click here to Visit - www.thedentalhub.org.in
242 Quick Review Series: BDS 2nd Year
c. Inoculation of such pure culture into suitable laboratory l He won Nobel Prize for medicine in 1905 for investiga-
animals should reproduce the lesions of the disease. tion and discoveries related to tuberculosis.
d. It should be possible to reisolate the bacterium in
pure culture from the lesions produced in the
experimental animals.
Topic 2
Morphology and Physiology of Bacteria
Q. 1. Describe the morphology of bacterial cell with a 4. Chemically the cell wall is composed of mucopeptide
neat labelled diagram and explain briefly. (peptidoglycan or murein) scaffolding formed by
N-acetyl-glucosamine and N-acetyl-muramic acid mol-
Or
ecules alternating in chains, which are cross-linked by
Describe the various structures of a bacterial cell with a peptide chains.
neat diagram. 5. The interstices of this scaffolding contain other chemicals
varying in different species.
Ans. Bacteria are unicellular prokaryotic microorganisms.
6. The mucopeptide component of cell wall possesses target
The morphology of ideal bacterial cell is as follows (Fig. 2.1): sites for antibodies, lysozymes and bacteriophages.
1. The outer layer or cell envelope of bacteria consists of 7. The chemical structure of cell wall of Gram-positive and
two components: (a) a rigid cell wall and (b) cytoplas- Gram-negative bacteria differs considerably. In general
mic membrane or plasma membrane. the cell wall of Gram-positive bacteria has simpler
chemical nature than that of Gram-negative bacteria.
Capsule
Mesosomes
Ribosomes
Gram-Positive Bacterial Cell Wall
Cell wall
Flagellum
Cytoplasmic a. The peptidoglycan layer is much thicker (16–80 nm)
membrane
than the Gram-negative cell wall (2 nm).
b. The periplasmic space is absent and the peptidoglycan
Pili DNA Granular
inclusion
is closely associated with the cytoplasmic membrane.
c. Special components: Most Gram-positive cell walls
FIGURE 2.1 Structure of bacterial cell.
contain significant amounts of teichoic and teichuronic
2. The cell envelope encloses the protoplasm comprising of acids. The teichoic acids constitute major surface anti-
cytoplasm, cytoplasmic inclusions or organelles such as nu- gens of Gram-positive bacteria.
clear body, ribosomes, granules, vacuoles, mesosomes, etc.
3. In addition to these essential structural components,
some bacteria also possess some additional appendages
Cell Wall of a Gram-Negative Bacteria
like capsule or loose slime layer, flagella, fimbriae, etc. It is complex and contains three components outside the
peptidoglycan layer.
a. Lipoprotein layer: It connects the outer membrane to
CELL WALL
peptidoglycans.
1. Bacterial cell wall is about 10–25 nm in thickness and b. Outer membrane: It is a phospholipid bilayer and con-
accounts for the shape of the bacterial cell and confers tains various proteins known as outer membrane proteins.
on its rigidity and ductility. Among these are porins which form transmembrane
2. It is a tough and rigid structure surrounding the bacte- pores that serve as diffusion channels for small mole-
rium like a shell. The cell wall carries bacterial antigens cules. Hydrophilic molecules are transported through
that are important in virulence and immunity. these porins. They also serve as specific receptors for
3. It may be demonstrated by plasmolysis, microdissection, some bacteriophages.
reaction with specific antibody, mechanical rupture of c. Lipopolysaccharides (LPS): They are present on the
the cell, differential staining procedures or by electron cell walls of Gram-negative bacteria and account for
microscopy. their endotoxic activity and the O antigen specificity.
, Click here to Visit - www.thedentalhub.org.in
Section | III Microbiology 243
The LPS consists of three regions as follows: 3. Function: They are the principal sites of respiratory
1. Region I is the polysaccharide portion determining the enzymes and the site of synthesis of cross wall septa.
O antigen specificity.
2. Region II is the core polysaccharide. INTRACYTOPLASMIC INCLUSIONS
3. Region III is the glycolipid portion (lipid A), which is
responsible for the endotoxic activities. 1. These are of various types, the chief types of which are
volutin, polysaccharides, lipids and crystals.
2. Volutin granules or metachromatic or Babes-Ernst gran-
Periplasmic Space ules are highly refractive, strongly basophilic bodies
1. It is the space in between the inner and outer mem- containing of polymetaphosphate and are characteristi-
branes and contains a number of important proteins and cally present in diphtheria bacilli.
oligosaccharides. 3. Special staining techniques like Albert’s or Neisser’s
demonstrate the granules more clearly.
4. Polysaccharide granules are demonstrated by iodine and
CYTOPLASMIC MEMBRANE (PLASMA
lipid inclusions with fat soluble dyes like Sudan Black.
MEMBRANES) 5. Function: They act as the source of stored energy.
1. Cytoplasmic membrane is a thin (5–10 nm) layer lining
the inner surface of the cell wall and separating it from VACUOLES
the cytoplasm. It consists of three layers constituting a
1. These are fluid containing cavities separated from the
‘unit membrane’ structure.
cytoplasm by a membrane.
2. The central layer is of protein molecules and on its
either side there are lipid molecules. It also contains
small amounts of carbohydrates. NUCLEUS
1. They appear as oval or elongated bodies generally one
Functions of Cell Membrane per cell. The bacterial nucleus is a thin fibre of double-
a. It acts as a semipermeable membrane controlling the inflow stranded DNA helix tightly coiled in the form of a circle
and outflow of metabolites to and from the protoplasm. inside the cytoplasm.
b. This flow is not dependant on molecular size but 2. Bacterial nuclei have no nuclear membrane or nucleolus.
in many cases depends upon the presence of specific 3. Some bacteria may possess extranuclear genetic elements
enzymes known as permeases. consisting of DNA, which are called plasmids or episomes.
c. It also contains cytochrome oxides enzymes of TCA 4. They are not important for life of the bacteria but confer
cycles and polymerizing enzymes necessary for synthe- certain properties like toxigenicity and drug resistance
sis of cell wall. on the bacteria.
CYTOPLASM SLIME LAYER AND CAPSULE
1. It is a colloidal system of a variety of organic and inor- 1. Many bacteria secrete a viscid material around the cell sur-
ganic solutes in a viscous watery solution. face. When this secretion is organized into a sharply defined
2. It does not exhibit internal motility or protoplasmic stream- structure, it is called as capsule, e.g. Pneumococcus and
ing and lacks endoplasmic reticulum or mitochondria. when it is loose undemarcated secretion, it is called as slime
3. Function: It contains ribosomes, mesosomes, inclusions layer, e.g. Leuconostoc.
and vacuoles. 2. Capsules are usually 98% water and 2% solid.
3. Capsules too thin to be demonstrated under microscope
are called as microcapsules, e.g. Haemophilus influenzae.
RIBOSOMES 4. This amorphous viscid material is generally but not in-
1. Ribosomes are slightly smaller (70 S) and are seen inte- variably polysaccharide or polypeptide or hyaluronic
grated in the linear strands of mRNA to form polysomes. acid in nature.
2. Function: They are the centres for protein synthesis. 5. Some bacteria may have both the capsule and slime
layer, e.g. Streptococcus salivarius.
MESOSOMES (CHONDROIDS) 6. Demonstration (Gram stain): Slime and capsule have
little affinity for basic dyes and is not visible in gram-
1. These are vesicular, convoluted, multilaminated struc- stained smears. They can be demonstrated by special
tures formed as invaginations of the plasma membrane staining technique employing copper salts.
into the cytoplasm. 7. Capsule may be readily demonstrated by negative strain-
2. They are often seen in relation to the nuclear body and ing in wet films with India ink where they are seen as clear
are more prominent in Gram-positive bacteria. haloes around the bacteria against a black background.
Section III
Microbiology
Part I
General Microbiology
Topic 1
Historical Introduction
SHORT NOTES
Q. 1. Louis Pasteur Q. 2. Robert Koch
Or, Or,
Louis Pasteur’s contributions to microbiology Koch’s postulates
Ans. Ans.
l Louis Pasteur (1822–1895) was a French chemist. He l Robert Koch (1843–1910) was a German practitioner
propounded microbial theory of fermentation. who is known as the father of medical microbiology.
l It was he who coined the term vaccine. l His contributions to the field of microbiology are as
l The various contributions by Pasteur in 30 years follows:
(1860–1890) of long, brilliant and active research are 1. He introduced staining techniques for bacteria.
as follows: 2. He described various methods of obtaining bacteria
1. Microbial theory of fermentation: Conclusive evidence in pure culture using solid media.
of role of microorganisms in disease production 3. He isolated first bacteria (Bacillus anthracis) in pure
2. Development of methods and technique for cultivation culture.
of microorganisms 4. He discovered the bacillus of tuberculosis and the
3. Studies on silkworm disease, anthrax, chicken cholera cholera vibrio.
and hydrophobia 5. He demonstrated Koch phenomenon, which is a hyper-
4. Control of diseases of silkworm sensitivity reaction seen in a guinea pig already infected
5. Principles and practice of sterilization: Introduction with the tuberculous bacilli when the tubercle bacillus
of sterilization technique and development of steam or its protein is injected into it.
sterilizer, hot air oven and autoclave 6. He proposed the Koch postulates, which are the criteria
6. Development of live attenuated vaccine against laid down by Koch.
a. chicken cholera, According to these a microorganism can be accepted as the
b. anthrax and causative agent of an infectious disease only if the following
c. rabies virus. conditions are satisfied:
These works of Pasteur were acclaimed throughout the a. The bacterium should be constantly associated with
world. In 1889, the Pasteur Institute, Paris was built by the lesions of the disease.
public contribution and Louis Pasteur served as its first b. It should be possible to isolate the bacterium in pure
director. culture from the lesions.
241
,Click here to Visit - www.thedentalhub.org.in
242 Quick Review Series: BDS 2nd Year
c. Inoculation of such pure culture into suitable laboratory l He won Nobel Prize for medicine in 1905 for investiga-
animals should reproduce the lesions of the disease. tion and discoveries related to tuberculosis.
d. It should be possible to reisolate the bacterium in
pure culture from the lesions produced in the
experimental animals.
Topic 2
Morphology and Physiology of Bacteria
Q. 1. Describe the morphology of bacterial cell with a 4. Chemically the cell wall is composed of mucopeptide
neat labelled diagram and explain briefly. (peptidoglycan or murein) scaffolding formed by
N-acetyl-glucosamine and N-acetyl-muramic acid mol-
Or
ecules alternating in chains, which are cross-linked by
Describe the various structures of a bacterial cell with a peptide chains.
neat diagram. 5. The interstices of this scaffolding contain other chemicals
varying in different species.
Ans. Bacteria are unicellular prokaryotic microorganisms.
6. The mucopeptide component of cell wall possesses target
The morphology of ideal bacterial cell is as follows (Fig. 2.1): sites for antibodies, lysozymes and bacteriophages.
1. The outer layer or cell envelope of bacteria consists of 7. The chemical structure of cell wall of Gram-positive and
two components: (a) a rigid cell wall and (b) cytoplas- Gram-negative bacteria differs considerably. In general
mic membrane or plasma membrane. the cell wall of Gram-positive bacteria has simpler
chemical nature than that of Gram-negative bacteria.
Capsule
Mesosomes
Ribosomes
Gram-Positive Bacterial Cell Wall
Cell wall
Flagellum
Cytoplasmic a. The peptidoglycan layer is much thicker (16–80 nm)
membrane
than the Gram-negative cell wall (2 nm).
b. The periplasmic space is absent and the peptidoglycan
Pili DNA Granular
inclusion
is closely associated with the cytoplasmic membrane.
c. Special components: Most Gram-positive cell walls
FIGURE 2.1 Structure of bacterial cell.
contain significant amounts of teichoic and teichuronic
2. The cell envelope encloses the protoplasm comprising of acids. The teichoic acids constitute major surface anti-
cytoplasm, cytoplasmic inclusions or organelles such as nu- gens of Gram-positive bacteria.
clear body, ribosomes, granules, vacuoles, mesosomes, etc.
3. In addition to these essential structural components,
some bacteria also possess some additional appendages
Cell Wall of a Gram-Negative Bacteria
like capsule or loose slime layer, flagella, fimbriae, etc. It is complex and contains three components outside the
peptidoglycan layer.
a. Lipoprotein layer: It connects the outer membrane to
CELL WALL
peptidoglycans.
1. Bacterial cell wall is about 10–25 nm in thickness and b. Outer membrane: It is a phospholipid bilayer and con-
accounts for the shape of the bacterial cell and confers tains various proteins known as outer membrane proteins.
on its rigidity and ductility. Among these are porins which form transmembrane
2. It is a tough and rigid structure surrounding the bacte- pores that serve as diffusion channels for small mole-
rium like a shell. The cell wall carries bacterial antigens cules. Hydrophilic molecules are transported through
that are important in virulence and immunity. these porins. They also serve as specific receptors for
3. It may be demonstrated by plasmolysis, microdissection, some bacteriophages.
reaction with specific antibody, mechanical rupture of c. Lipopolysaccharides (LPS): They are present on the
the cell, differential staining procedures or by electron cell walls of Gram-negative bacteria and account for
microscopy. their endotoxic activity and the O antigen specificity.
, Click here to Visit - www.thedentalhub.org.in
Section | III Microbiology 243
The LPS consists of three regions as follows: 3. Function: They are the principal sites of respiratory
1. Region I is the polysaccharide portion determining the enzymes and the site of synthesis of cross wall septa.
O antigen specificity.
2. Region II is the core polysaccharide. INTRACYTOPLASMIC INCLUSIONS
3. Region III is the glycolipid portion (lipid A), which is
responsible for the endotoxic activities. 1. These are of various types, the chief types of which are
volutin, polysaccharides, lipids and crystals.
2. Volutin granules or metachromatic or Babes-Ernst gran-
Periplasmic Space ules are highly refractive, strongly basophilic bodies
1. It is the space in between the inner and outer mem- containing of polymetaphosphate and are characteristi-
branes and contains a number of important proteins and cally present in diphtheria bacilli.
oligosaccharides. 3. Special staining techniques like Albert’s or Neisser’s
demonstrate the granules more clearly.
4. Polysaccharide granules are demonstrated by iodine and
CYTOPLASMIC MEMBRANE (PLASMA
lipid inclusions with fat soluble dyes like Sudan Black.
MEMBRANES) 5. Function: They act as the source of stored energy.
1. Cytoplasmic membrane is a thin (5–10 nm) layer lining
the inner surface of the cell wall and separating it from VACUOLES
the cytoplasm. It consists of three layers constituting a
1. These are fluid containing cavities separated from the
‘unit membrane’ structure.
cytoplasm by a membrane.
2. The central layer is of protein molecules and on its
either side there are lipid molecules. It also contains
small amounts of carbohydrates. NUCLEUS
1. They appear as oval or elongated bodies generally one
Functions of Cell Membrane per cell. The bacterial nucleus is a thin fibre of double-
a. It acts as a semipermeable membrane controlling the inflow stranded DNA helix tightly coiled in the form of a circle
and outflow of metabolites to and from the protoplasm. inside the cytoplasm.
b. This flow is not dependant on molecular size but 2. Bacterial nuclei have no nuclear membrane or nucleolus.
in many cases depends upon the presence of specific 3. Some bacteria may possess extranuclear genetic elements
enzymes known as permeases. consisting of DNA, which are called plasmids or episomes.
c. It also contains cytochrome oxides enzymes of TCA 4. They are not important for life of the bacteria but confer
cycles and polymerizing enzymes necessary for synthe- certain properties like toxigenicity and drug resistance
sis of cell wall. on the bacteria.
CYTOPLASM SLIME LAYER AND CAPSULE
1. It is a colloidal system of a variety of organic and inor- 1. Many bacteria secrete a viscid material around the cell sur-
ganic solutes in a viscous watery solution. face. When this secretion is organized into a sharply defined
2. It does not exhibit internal motility or protoplasmic stream- structure, it is called as capsule, e.g. Pneumococcus and
ing and lacks endoplasmic reticulum or mitochondria. when it is loose undemarcated secretion, it is called as slime
3. Function: It contains ribosomes, mesosomes, inclusions layer, e.g. Leuconostoc.
and vacuoles. 2. Capsules are usually 98% water and 2% solid.
3. Capsules too thin to be demonstrated under microscope
are called as microcapsules, e.g. Haemophilus influenzae.
RIBOSOMES 4. This amorphous viscid material is generally but not in-
1. Ribosomes are slightly smaller (70 S) and are seen inte- variably polysaccharide or polypeptide or hyaluronic
grated in the linear strands of mRNA to form polysomes. acid in nature.
2. Function: They are the centres for protein synthesis. 5. Some bacteria may have both the capsule and slime
layer, e.g. Streptococcus salivarius.
MESOSOMES (CHONDROIDS) 6. Demonstration (Gram stain): Slime and capsule have
little affinity for basic dyes and is not visible in gram-
1. These are vesicular, convoluted, multilaminated struc- stained smears. They can be demonstrated by special
tures formed as invaginations of the plasma membrane staining technique employing copper salts.
into the cytoplasm. 7. Capsule may be readily demonstrated by negative strain-
2. They are often seen in relation to the nuclear body and ing in wet films with India ink where they are seen as clear
are more prominent in Gram-positive bacteria. haloes around the bacteria against a black background.
