HISTORY OF MICROBIOLOGY Microbiology and Origin of Life: Many explanations have been offered for the origin of Life on the planet earth. One of the most acceptable ones suggests that the life originated in the sea. Following millions of years of a chemical evolutionary process. This hypothesis proposes that the inorganic compounds of the atmosphere subject to the influence of UV light, electrical discharges and /or high temperatures interacted, resulting in the formation of organic compounds which precipitated in the sea where they accumulated. These organic compounds subject to additional physical effects of the environment combined and formed peptides, polypeptides and other more complex organic substances which served as the precursors of the first form of life. One of the many unanswered questions is 'that of the process of duplication (multiplication) in the first forms of life. History of microbiology: History is the story of achievements of men and women but it records relatively few outstanding names and events. Similarly, it has been said that, in science, the credit of-one finding goes to one who convinces the world and hot to the one who first had the idea. Microbiology began when people learned to grind lenses from pieces of glass and combine them to produce magnifications great enough to see the microbes. The history of microbiology can be divided into following topics. • Discovery of microscope and developments in microscopy • Theory of spontaneous generation and its disproval (Biogenesis vs. abiogenesis) • Establishment of germs theory of fermentation
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HISTORY OF MICROBIOLOGY
Microbiology and Origin of Life:
Many explanations have been offered for the origin of Life on the planet earth. One
of the most acceptable ones suggests that the life originated in the sea. Following
millions of years of a chemical evolutionary process. This hypothesis proposes that
the inorganic compounds of the atmosphere subject to the influence of UV light,
electrical discharges and /or high temperatures interacted, resulting in the formation
of organic compounds which precipitated in the sea where they accumulated. These
organic compounds subject to additional physical effects of the environment
combined and formed peptides, polypeptides and other more complex organic
substances which served as the precursors of the first form of life. One of the many
unanswered questions is 'that of the process of duplication (multiplication) in the first
forms of life.
History of microbiology:
History is the story of achievements of men and women but it records relatively few
outstanding names and events. Similarly, it has been said that, in science, the credit
of-one finding goes to one who convinces the world and hot to the one who first had
the idea. Microbiology began when people learned to grind lenses from pieces of glass
and combine them to produce magnifications great enough to see the microbes. The
history of microbiology can be divided into following topics.
• Discovery of microscope and developments in microscopy
• Theory of spontaneous generation and its disproval (Biogenesis vs. abiogenesis)
• Establishment of germs theory of fermentation
• Establishment of germs theory of disease '
• Developments in medical microbiology
• Developments in Agricultural microbiology . .
A) Discovery of microscope and developments in microscopy;
Roger Bacon (1220-1292): He postulated that the disease is caused by invisible living
creatures.
Fracastpro (1483-1553) and Pienciz (1762): They also made again a similar
suggestion, but these people had no proof.
Kircher(1658): He referred to "worms" invisible to the naked eye in decaying
bodies, meat milk and diarrheal secretions. Although his descriptions lacked
accuracy, he was first to recognize the significance of bacteria and other microbes in
disease. ,
Robert Hooke (1665): Described cells in a piece of cork. Discovered compound
microscope. He was a member of secretaries of the Royal Philosophical society of
London. He published a book called “Micrographia ". In which he described many
molds and bacteria.
Antony Van Leeuwenhoek (1632-1723): He was the first to report his
observations of bacteria and protozoa with accurate descriptions and drawings. He
observed living creatures in a drop of rain water and called them as little
"animalcules". He was a lens grinder and made more than 250 microscopes
consisting of home ground lenses mounted in brass and silver having magnification up
to 200-300 times. He made drawings of bacteria in rainwater, saliva, vinegar and
other substances and described them with pictures; He related his exciting
discoveries in a series of more than 300 letters to his friends In the "Royal Society of
London and French Academy of Sciences. The significance of his discoveries,
however, went unrecognized as there was little awareness that microbes cause
diseases. .
Joblet (France) in 1754 wrote extensively on microscopic-objects. In Muller (Danish
Scientist) 1773, examined many forms and made attempts to describe and classify
them into different groups. He introduced terms like Bacillus, Vibrio and Spirillum.
During the next 5O years better microscopes where developed and in 1808, a
German scientist Ehrenberg made surveys of microscopic forms and published it in
two volumes. In 1844, Dolland demonstrated the usefulness of oil immersion lens
it observe small objects more clearly. In 1870, Abbe developed sub stage condense
for better illumination of objects. By using better microscopes, German botanist Chon
and his students published series of papers on bacteria during 1872 - 76.
The different techniques in microscopy such as bright field, phase contrast
fluorescent and ultraviolet microscopy made observation of minute objects more easy
The development of Electron Microscope by Zwophin in 1963 has made it possible
to magnify objects up to 2,00,000 times and some of the minute objects like
"viruses" which are not easily visible under tight microscope can be seen and
studied.
B) Theory of spontaneous generation (abiogenesis) vs Biogenesis:
Spontaneous generation (abiogenesis): Origin of living things spontaneously from
the non-living ones i.e. non-living origin or inanimate origin.
Biogenesis: Origin of living things from the living things only i.e. life from life.
The discovery of microbes-spurred interest in the origin of living things. As far as the
human beings were concerned, the Greek explanation that the Goddess Gaea was
able to create people from stones and other inanimate objects had been largely
discarded. The idea of spontaneous generation. dates back at least to the ancient
Greeks who believed that decaying meat produced maggots and that the flies and
frogs arose from the mud under appropriate climatic conditions.
Aristotle (384-322 BC): He taught that animals might originate spontaneously
from the soil, plants or others unlike animals.
Francesco Redii (1626): He doubted the spontaneous generation of maggots from
the meat. He performed an experiment by placing meat in a jar covered with wire
gauze. Attracted by the odor of meat, the flies laid eggs on the covering, and from
these eggs, the maggots developed. Thus, he concluded that the origin of maggots
was from the flies and not from the meat.
Thus, the matter was settled for the forms of life such as maggots, mice and
scorpions, but the origin of microbes was yet doubtful. There appeared champions
for and challengers of spontaneous generation, each with a new, and sometimes
fantastic explanation or bit of experimental evidence.
Louis Joblet (1710): He observed that hay, when infused in water and allowed to
stand for some days gave rise to countless microbes. He boiled this infusion and
placed one portion in a closed vessel and the other in an open vessel. The infusion in
the open vessel was full of microbes after incubation whereas no life (microbes)
appeared in the closed vessel. Thus, he proved that infusion alone was incapable of
generating a new life spontaneously. .
John Needham (1713-1781): He conducted a similar experiment as that of Joblet
and got conflicting results. The microbes developed in the heated closed vessel as
well as in the unheated ones. He, therefore,-believed in the spontaneous generation.
The conflicting results were due to insufficient heating, which failed to kill the heat
resistant, "spores", and nothing was known about spores at that time.
Lazaro Spailanzani (1729-1799): He boiled the beef broth for an hour and then
sealed the flasks and incubated. No microbes appeared after incubation. He
confirmed the results by repeated experiments. However, he failed to convince
Needham, who insisted that air was essential for the spontaneous generation of
microbes and that it had been excluded from the flasks by sealing.
Two workers answered this argument some 60-70 years later independently.
Franz Schulze (18TB-1873): He passed the air in to the boiled infusions through
strong acid solutions. The microbes did riot appear even after a long period of
Incubation.
Theodor Schwann (1810-1882): He passed the air into his flasks containing
boiled beef broth through red hot tubes. In this case also, the microbes did not
appear.
The die heard advocates of spontaneous generation were still not convinced. They
said, "acid and heat altered the air so that it would not support the growth of
microbes".
Schroder and Dusch (1850): They performed a more convincing experiment by
passing air through cotton into the flasks containing heated broth. Thus, the
microbes were filtered out of the air by the cotton fibers so that microbial growth did
not occur. Thus a basic technique of plugging bacterial culture tubes with cotton a
stopper was initiated, (cotton plugging technique),
The concept of spontaneous generation was revived for the last time, by Pouchet
(1859) who published an extensive report proving the occurrence of spontaneous
generation. ,
Louis Pasteur (1822-1895): He performed experiments that ended the argument of
spontaneous generation forever. He prepared a special flask with a long, narrow,
goose neck opening. The nutrient solutions were heated in the flasks and the air-
untreated and unfiltered could pass in and out of the flask. The germs (microbes)
settled in the goose neck area; and no microbes appeared in the solution even after a
long period of incubation.
John Tyndall (1820-1893): Finally, Tyndall conducted experiments in a specially
designed box (dust free box) to prove that the dust particles carried the germs. He
demonstrated that, if no dust was present, the sterile broth remained free of microbial
growth for indefinite periods."
C) Establishment of Germs Theory of Fermentations
Louis Pasteur was a Professor of Chemistry at France. He was studying the methods
and processes of making consistently good wines and beer. He found that the
fermentation of fruits and grains resulting in the production of alcohol was
brought .about by the microbes. By examining many batches of "ferment” he f6und
microbes of different kinds. One type of microbes predominated in good lots whereas
the poor quality wines and beer contained the other type of microbes. He suggested
that these undesirable microbes might be removed by heating, not enough to hurt
the original flavor of fruit juice, but enough to destroy a very high percentage of
microbial population. He found that, holding the fruit juices at 62.8°C or 145bF
temperature for 30 minutes did the" job Today, this process known as
"pasteurization" is widely used in fermentation and the dairy industries.
The methods of pasteurization used in the dairy industry are as follows.
a) Low Temperature Holding (LTH) Method: Heating every particle of milk or
milk product to at least 145°F (62.8°C) for 30 minutes.
b) High Temperature Short Time (HTST) Method: Heating every particle of
milk or milk product to at least 161°F (71.7°C) for 15 seconds. ' -
D) Establishment of Germs Theory of Disease:
Before Pasteur had proved by experiment that bacteria are the cause of some
diseases, many students had expressed strong arguments for the germs theory of
disease.
Fracastoro (1483-1553) : He suggested that the diseases might be due
invisible organisms transmitted from one person to the other.
Plenciz (1762): He proposed that the living;-agents are the cause of diseases
and different germs may be responsible for different diseases,
Holmes (1809-1884) Stated that Puerperal fever, a disease of childbirth was
contagious, and was probably caused by germs carried from one mother to
another by midwives and physicians.
Joseph Lister (1827-1912) : He used a dilute solution of phenol (carbolic acid)
to" soak the surgical dressings and spray the operating room. The wounds
became rarely infected and healed rapidly. This technique of "antiseptic surgery"
was quickly accepted by the surgeons.
Louis Pasteur (1822-1895) :After a great success in solving the problem of
undesirable microbes in the French wine industry, the French Government
requested Pasteur to investigate the pebrine disease of silkworm which was
ruining the French silk industry. Pasteur isolated the parasite (protozoan) causing
the pebrine disease. After investigation he showed that the farmers could
eliminate the disease –by using only healthy, disease-free caterpillars for the
breeding stock. Pasteur also studied the problem of anthrax, a disease of cattle,
sheep and sometimes human beings. H e isolated the organism causing anthrax
disease (Bacillus anthracis) and grows it in the laboratory flasks. He isolated
these bacteria from the blood of animate that had died of anthrax. .
Robert Koch (1843-1910): Koch was busy with the anthrax problem in Germany;
He isolated the typical bacilli with squarish ends from the blood of the cattle died
of anthrax. He grows these bacteria in laboratory pure, cultures and then injected
them into other healthy, susceptible animals, where the disease was produced.
From these experimentally infected animals he isolated the bacterium similarly to
'the previously inoculated one.. This was the first time a bacterium had seen proved
to be the cause of an animal disease (pebrine is caused by a protozoan). This led to
the establishment of following Koch's Postulates, which provided guidelines to
identify the-causative agent of an infectious disease.
Kochs Postulates:
1) Association: A specific organism can always be found in association with a
given disease.
2) Isolation : the organism can be isolated and grown in pure culture in
laboratory.
3) Inoculation: the pure culture will produce the same disease when inoculated
into a healthy susceptible animals
4) Re isolation: It is possible to recover the same organism in pure culture from
the experimentally infected animal.
ROLE OF MICROBES IN FERMENTATION:
The souring of milk and the production of alcoholic beverages has been
throughout recorded history, but the fermentation process concerned has
understood for only a century. Berzelius. Liebig, Wohler, and other distinguished
influential organic chemists of the last century interpreted the transformation of
sugar in lactic acid or in to ethyl alcohol and carbon dioxide as purely chemical
phenomena. When was pointed out that yeast or other microbes are always present,
they devised explanation other than one which eventually proved top be true. Liebig
for example,, noting how yeast is destroyed and decomposes, was communicated in
some way to sugar and substances to contact with it. Berzelius interpreted
fermentation as a contact phenomenon and Mitscherlich saw in yeast globules a
catalyst similar In behavior to spongy platinum contact with hydrogen peroxide,
None of these investigators accepted the thesis, first proposed by Cagniard
de Latour, Schwann, and Kutzing between -1835 and 1638, that yeasts actively
transform sugar in to alcohol and carbon dioxide. They postulated that in other
fermentations, various microbes formed characteristic end products during their
growth.
Pasteur in 1857: Observed the formation of lactic acid from sugar by
several kinds of bacteria. He noted that a gray deposit in fermentation vessels
consisted of microscopic, very short globules, occurring either singly or in small,
irregular masseses. These globules were much smaller than those of beer yeast.
When they were transferred to a fresh nutrient solution containing sugar, yeast
extract and chalk, lactic acid was produced and the globules increases greatly in
number. Pasteur demonstrated that the presence of globules was a necessary
prerequisite to lactic acid formation. He later showed that in alcoholic, acetic,
butyric and other, fermentations, the typical end product appeared only when a
specific microorganism was present.
A further discovery-in connection with butyric fermentation was that, the
organism responsible grows only in the absence of air. It was when found that
alcoholic fermentation also occurs only in the absence of air, but yeast can grow in
presence of air and, in fact, grows more rapidly arid abundantly with than without
air. However, oxygen is toxic to the butyric bacterium. This was apparently the first
indication that organisms could exist in the complete absence of oxygen - a
revolutionary concept.
The germ theory of fermentation, stating that the microorganisms bring
about specific changes in their substrates laid the foundation of important
industrial developments. The research necessary to prove the germ theory of
fermentation also demonstrated the necessity for strict control of the various
factors associated with the fermentation process: the composition of the
fermenting solution, the identity-and purity of the microbial population, and
incubation conditions such as temperature and aeration.
Fermentation:
Fermentation is defined as the incomplete, oxidation produced by
microorganisms acting on compounds, which for most art- are carbohydrates
or carbohydrate like in nature.
It is a anaerobic .processes, final products being H2O and CO2. Bacteria,
fungi and yeasts can carry out the fermentation.
Bacteria: 1) Lactobacillus 2)
Streptococcus
3) Pseudomonas linineri 4) Sarcina
yehtriculi
Fungi: 1) Aspergillus , 2) Rhiropus 3} Mucor
4) Yeasts: Saccharomyces cervislae
Saccharomyces ellipsoideus
METABOLISM IN BACTERIA
Term metabolism refers to the sum total of all biochemical transformations that occur
in the cells. It is in fact the chemistry of life. This chemistry is generally divided
into two sections:
Anabolism and Catabolism.
1. Anabolism or Biosynthesis: It includes all such transformations that are
involved in the synthesis of organic macromolecules. Making the major portion or
cellular mass. From the simpler compounds, present in the extra cellular
environment. Anabolism is normally energy utilizing process. Energy is used in the
form of ATP.
2. Catabolism: The mechanism of generating ATP is diverse among organisms. The
ATP required for anabolism, may be produced by the process of photosynthesis.
However, chemical energy in the form of inorganic or organic compound (that are
degraded) can also be used to drive biosynthesis. Such process involving the
direct use of chemical energy (obtained from breakdown of chemical compounds-
inorganic or organic) is termed catabolism or degradative metabolism .Thus
catabolism is characterized by the release of energy.
The three important groups of organic compounds involved in metabolism are
carbohydrates, lipids and proteins. The fourth group, the nucleic acids remains
unchanged for long periods of time -in the cell.
Chemotrophs are those organisms that obtain energy by the oxidation of
electron donors in their environments.
These molecules can be organic (chemo organo trophs) or inorganic
(chemolithotrophs). The chemotroph designation is in contrast to
phototrophs, which utilize solar energy.
Chemotrophs can be either autotrophic or heterotrophic.
Chemoautotrophs (Gr: Chemo = chemical, auto = self, troph =
nourishment).
They deriving energy from chemical reactions, synthesize all necessary
organic compounds from carbon dioxide.
Chemoautotrophs use inorganic energy sources, such as hydrogen sulfide,
elemental sulfur, ferrous iron, molecular hydrogen, and ammonia.
Most are bacteria or archaea that live in environments such as deep sea
vents and are the primary producers in such ecosystems.
Chemoautotrophs generally fall into several groups: methanogens,
halophiles, sulfur oxidizers and reducers, nitrifiers, and thermoacidophiles.
chemical, hetero = (an)other, troph = nourishment) are unable to fix
carbon and form their own organic compounds.
Chemoheterotrophs can be chemolithoheterotrophs, utilizing inorganic
energy sources such as sulfur or chemoorganoheterotrophs, utilizing
organic energy sources such as carbohydrates, lipids, and proteins
Phototrophs (Gr: = light, = nourishment) are the organisms that carry
out photon capture to acquire energy.
They use the energy from light to carry out various cellular
metabolic processes.
It is a common misconception that phototrophs are obligatorily
photosynthetic.
Many, but not all, phototrophs often photosynthesize: they
anabolically convert carbon dioxide into organic material to be
utilized structurally, functionally, or as a source for later catabolic
processes (e.g. in the form of starches, sugars and fats).
All phototrophs either use electron transport chains or direct proton
pumping to establish an electro-chemical gradient which is utilized
by ATP synthase, to provide the molecular energy currency for the
cell.
Most of the well-recognized phototrophs are autotrophs, also known
as photoautotrophs, and can fix carbon.
They can be contrasted with chemotrophs that obtain their energy
by the oxidation of electron donors in their environments.
Photoheterotrophs produce ATP through photophosphorylation but
use environmentally obtained organic compounds to build structures
and other bio-molecules.
Photoautotrophic organisms are sometimes referred to as holophytic
Fermentation versus Respiration:
Heterotrophs exhibit two basic strategies, the fermentation and respiration for
oxidizing organic compounds to synthesis ATP from ADP. In fermentation the
organic substrate acts as an. electron acceptor {oxidizing agent). Therefore
both, the electron donor and the acceptor are internal to the organic
substrate. There is no net change in the oxidation state of the products
relative to the starting substrate molecule. The oxidized products are exactly
counterbalanced by the reduced products, and thus the required oxidation-
reduction balance is achieved. The coenzymes,, that are reduced are
reoxidized by its end, so that they are in fact not consumed in the process.
There is no. requirement of oxygen or other electron acceptor.
In contrast to fermentation, respiration requires an external electron
acceptor; that is a molecule other than the one derived from" the electron
donor must act as electron acceptor "(oxidizing agent) to achieve a balance of
oxidation-reduction reactions. These balance of Oxidation Reduction-is thus
also achieved without consumption of co-enzymes. The most common external
electron acceptor in respiration is molecular oxygen-thus called aerobic
respiration. When another molecule as nitrate or sulphate serves as the
terminal electron acceptor, the pathway is called anaerobic respiration.
Fermentation yields far less ATP per substrate molecule than respiration since
same substrate serves both donor and acceptor of electrons. There is no
complete oxidation! The AG° for complete oxidation of glucose to carbon
dioxide and water is 686 kcal/mole, compared to only 58 kcal/mole when
glucose is partially oxidized to two molecules of lactic acid in fermentation.
Depending upon the conditions of growth, an organism used other respiration
or-fermentation catabolic pathways (i:e, respiration-anaerobic or aerobic, OR
fermentation). Therefore three general methods exist by which a carbon arid
energy source can be broken down to provide energy.
1. Aerobic respiration. Respiration can be defined as an ATP generating
metabolic process in which either organic or inorganic compounds serve as
electron donors (become oxidized) and inorganic compounds serve as 'the
ultimate acceptors (become reduced). Usually the ultimate electron acceptor- is
molecular oxygen and this is called aerobic respiration. Thus carbon and energy
source is broken-down by a series of reactions, the oxidation stages occurring at
the expense of oxygen as the terminal electron acceptor. Aerobic respiration is
also simply termed respiration.
In aerobic respiration, sugars are first converted to the key metabolic
intermediate, pyruvic acid. Certain catabolic reaction pathways are common to
both respiration and fermentation. Among these are the three pathways of
conversion of sugars, to pyruvic acid. They are: i) the-EMP (also called the
glycolytic pathway), (ii) the..Pentose phosphate. Pathway, also called .the hexose.
Mono phosphate shunt and (iii) the Entner-Doudoroff pathway. The first two occur
in "many organisms (pro-as well as eukaryotes),.whereas the third is restricted to
some prokaryotes. We shall consider the glycolytic pathway, which is most
common not only in microorganisms but also in plants and animals. Glucose is first
converted to pyruvic acid through glycolysis and this acid is then oxidized to CO2
through the TCA or Krebs' cycle...
BACTERIAL VIRUSES
Viruses are infections agents so small that they can only be seen at
magnifications-provided by the electron microscope.
They are 10 to 100 times smaller than most bacteria. With an approximate
size range 20 to 300 nm.
Thus they pass through, the pores of filters which do not permit the passage of
most bacteria.
Viruses are incapable of independent growth in artificial media.
They can grow only in animal or plant cells or in microorganisms.
They reproduce in these cells by replication (a process In which many copies
or replicas are made of each-viral component and are then assemble to
produce progeny virus),Thus viruses are referred to as obligate intracellular
parasites. (If the least requirement for life is that an organism duplicates
itself, then-viruses may be viewed as microorganisms.
Viruses largely lack metabolic machinery of their own to generate energy or
to synthesize, proteins. They depend on the host cells to carry out these viral
functions. However, like the host cells, viruses have the genetic information for
replication and viruses have information in their genes for usurping the host cell's
energy-generating and protein-synthesizing systems.
Actually, viruses in transit from one host cell to another are small packers of
genes.
The viral genetic material is either DNA or RNA but the virus does not have
both. (Host cells have both DNA and RNA.).
The nucleic acid is enclosed in a highly specialize protein coat of varying
design.
The coat protects the genetic material when the virus is outside of any host
cell and serves as a vehicle for entry into another specific host cell. The
structurally complete mature and infectious virus is called the Virion.
During reproduction in the host cells viruses may cause disease.
In fact, viruses incite the most common acute infectious diseases of humans
{like the "cold" of flu").
And there is growing evidence that they any cause many chronic diseases
as well.
Significantly, all viruses' are generally insensitive to the broad range of available
antibiotics such as penicillin, streptomycin, and others.
From the above discussion of what does or does not constitute a virus,- we
may now attempt a definition for this group of infectious agents. We can define
Defination
Viruses as noncellular infectious entities whose genomes are a nucleic acid either
DNA or RNA; which reproduce, only in living cells: and which use the cells biosynthetic
machinery to direct the/synthesis of specialized particles (virions), which contain
the viral genomes and transfer them efficiently to other cells.
Bacterial viruses, or bacteriophage (or simply phages) have provided the
microbiologist with a model for virology (the study of viruses) and molecular
biology (a discipline which examines the structure, function and organization of
the macromolecules in which biological specificity is encoded);
BACTERIPHAGES: DISCOVERY AND SIGNIFICANCE:
Bacteriophages, viruses that infect bacteria, were discovered independently
by Frederick W. Twort in England in 1915 and by Felix d’Herelle at the Pasteur
Institute in Paris in 1917.
Twort, Observed that bacterial: colonies sometimes underwent lysis (dissolved
and disappeared) and that this lytic effect could be transmitted from colony to
colony. Even high dilutions of material from a lysed colony that had been passed
through a bacterial filter could transmit the lytic effect. However, heating the
filtrate destroyed its lytic property. From these observations Twort cautiously
suggested that the lytic agent might be a virus.
D'Herelle rediscovered this phenomenon in 1947 (hence the term Towrt-
d'Herellle phenomenon) and coined the word bacteriophage, which means
"bacteria eater." He considered the filterable agent to be an invisible microbe for
example a virus that was parasitic for bacteria.
Since the bacterial hosts of phages are easily cultivated under controlled
conditions, demanding relatively little in terms of time, labor and space compared
with the maintenance of plant and animal hosts, Bacteriophages have received
considerable attention in viral research, Furthermore, since Bacteriophages are
the smallest and simplest biological entities known which are capable of self-
replication (making copies of themselves), they have been used widely in genetic
research. Of importance too have. been studies on the bacterium bacteriophage
interaction. Much has been learned about host-parasite ; relationships from these
studies, which have provided a better understanding of plant and animal
infections with viral pathogens. Thus the bacterium-bacteriophage interaction has
become the model system for the study of viral pathogenicity.
GENERAL CHARACTERISTICS
Bacterial viruses are widely distributed in nature.
Phages exist for most, if not all, bacteria.
With the proper techniques, these phages can be isolated quite easily in the
laboratory.
Bacteriophages, like all viruses, are composed of a nucleic acid core
surrounded by a protein coat.
Bacterial viruses occur in different shapes, although many have a tail through
which they inoculate the host cell with viral nucleic acid.
There are two main types of bacterial viruses: lytic, or virulent, and lysogenic.
When lytic phages infect cell, the cells respond, by producing large numbers of
lyses, releasing new phages infect other host cells. This is called a lytic cycle.
In the lysogenic type of infection, the result is not so readily apparent.
The viral nucleic acid is carried and replicated in the host bacterial cells from
one generation to another without any cell lysis. However, lysogenic phages
may spontaneously become virulent "at some subsequent generation and lyse
the host cells.
In addition, there are some filamentous phages which simply "leak" out of cells
without killing them.
MORPHOLOGY AND STRUCTURE:
1. The electron microscope had made it possible to determine the structural
characteristics of bacterial viruses.
2. All phages have a nucleic acid core covered by a protein coat, or capsid.
3. The capsid is made up of morphological subunits (as seen under the
electron microscope) called capsomeres.
4. The capsomeres consist of a number of protein subunits or molecules
called protomers.
Bacterial viruses may be grouped into six morphological types .
A. This most complex type has hexagonal head, a rigid fail with a
contractile
sheath, and tail fibers.
B. Similar to :A, this type has hexagonal, head. However, it lacks a
contractile
sheath, its tail Is flexible, and it may or may not have tail fibers.
C. This type is characterized by a hexagonal head and a tail shorter than
the
head. The tail has no contractile sheath and may or may not have tail
fibers.
D. This type has a head made up of large capsomeres, but has no tail.
E. This type has a head made up of small capsomeres, but has no tail.
F. This type is filamentous.
a. Types A,B, and C show a morphology unique to bacteriophages — The
morphological types in groups D and E are found in plant and animal
(including insect) viruses as well.
b. The filamentous form of group F is found in some plant viruses.
c. Pleomorphic viruses recently discovered to have a lipid-containing
envelope, have no detectable capsid, and possess double - stranded
DNA (ds-DNA). The representative phage is MV-L2.
d. Phage Structure: Most phages occur in one of two structural forms,
having either cubic or helical symmetry. In overall appearance, cubic-
phages are regular solids or, more specifically, polyhedral, (singular,
polyhedron); helical phages are rod-shaped.
2. Polyhedral phages are icosahedral in shape. (The-icosahedron is a regular
polyhedron with 20 triangular facets and 12 vertices.) This means that, the
capsid has 20 facets, each of which is an equilateral triangle; these facets
come together to the form the 12 corners. In the simplest capsid, there is
capsomere at each of the 12 vertices; this capsomere, which is surrounded by
five other capsomeres, is termed a penton.
For example, the phage X174 exhibits the simplest capsid.
In larger and more complex capsid, the triangular facets and subdivided into
a progressively larger number of equilateral triangles.
Thus a capsid may be composed of hundreds of capsomeres but it is still
based on the simple icosahedron model.
The elongated heads of some tailed phages are derivatives of the
icosahedron. For example the head of the T2 and T4 phages is an icosahedron
elongated by one or two extra bands of hexons.
Rod-shaped viruses have their capsomeres arranged helically and not in
stacked rings. An example is the bacteriophage M l3. . . . . .
Some bacteriophages, such as the T elven coliphages (T2.T4, and T6), have
very complex structures, including a head and a tail. They are said to have binal
symmetry be caused each virion has both an icosahedral head and a hollow helical
tail.
PHAGE REPLICATION :
The Bacteriophage can exist in three phases:
(i) as a free particle virion
(ii) in a lysogenic state as a prophage , and
(iii) in the vegetative state (in lytic cycle )
"As a virion, it is inert and cannot reproduce. In the lysogenic state, the
DNA of the phase is integrated within the bacterial DNA and exists Jrif a tion-
infectious form ( prophage ) and replicates in synchrony with the bacterial'
DNA. In the lytic cycle, the phase particle infects the susceptible host,
multiplies and causes the lysis of bacterial cell with concomittent release of
progeny viral particles. Ateo
when the integrated phase is induced to become the vegetative phage the
lytic
cycle follows. Phase that cause lysis are called virulent phages as opposed to
these which can exist in a lysogenic state which are called as temperate
"phases".
Bacteria which carry temperate phages are called lysogenic bacteria and
such
bacteria are immune to super infection by the same phase .
Reproduction (Life cycle):
LIFE CYCLE OF BACTERIOPHAGE
Bacteriophage exhibits two different types of life cycle-
1. Lytic or Virulent cycle
2. Temparate or Avirulent or Lysogenic cycle:
Lytic or Virulent cycle:
In virulent cycle there is intracellular multiplication of phage followed by
the lysis and release of progeny virion. This is called lytic cycle
Temparate or Avirulent or Lysogenic cycle:
In Lysogenic cycle the phage DNA become integrated with the bacterial
genome, replicating without any cell lysis.
LIFE CYCLE: Multiplication of Bacteriophages
The replication of virulent phage was initially using T even numbered
(T2,T4,T6) phage of E.coli.
The multiplication cycle of phage occur in five steps-
1. Attachment or Adsorption
2. Penetration
3. Biosynthesis of phage component
4. Maturation
5. Release of progeny phage particle
Attachment or Adsorption:
The first step in infection of host bacterial cell by phage is adsorption.
Phage particle come into contact with bacterial cell by random collision.
A phage attaches to the surface of the bacterium by the tail.
Adsorption depends on the presence of chemical group called as receptor
on the surface of bacterial cell.
The receptor of bacterial cell is a lipopolysaccharide.
Host specificity of phage its affinity at the adsorption.
Infection of bacterium by naked phage genetic material is known as
transfection.
Penetration:
Attachment is followed by injection of genetic material (nucleic acid /DNA)
in to the bacterial cell .
The phage DNA is injected into the bacterial cell through the hollow core.
Penetration may be enhanced by the presence of phage tail lysozyme with
break small portion of the cell wall for the entry of phage DNA.
After penetration of DNA the empty head and tail of phage remain outside
the bacterial cell is called shell.
If many phages are attached to the bacterial cell multiple holes are
produced on the bacterial cell with the consequent leakages of cell
component.
Bacterial lysis occurs without viral multiplication.
Phage such as T1 and T5 that do not have contractile sheath also inject
their nucleic acid through the cell envelop by adhesion site between the
inner and outer membrane.
Biosynthesis of phage component:
After the infection and penetration of DNA transcription of part of viral
genome produce early mRNA molecules which is translated into a set of
early protein.
These cause the switch off host cell macromolecule synthesis, degrade
the host DNA/ chromosome and start the synthesis of viral components.
Viral DNA replicate and also produce the late mRNA molecule transcribe
from gene which specify the protein of phage coat.
The late messages are translated into subunit of capsid. Rest of the
structure gets condensed to form phage head, tail and tail fibre.
Maturation:
The phage DNA, head and tail protein are synthesized separately in the
bacterial cell.
DNA condense into compact polyhydron and packaged into head and
finally the tail structure are added.
The process of assembly of the phage from its component is called
maturation.
Release of progeny phage particle
By the sudden explosion or breakaging the bacterial cell wall.
o Lysozyme synthesized with in the cell caused the bacterial cell wall to
breakdown and newly produced Bacteriophage are release from the host
cell
VIROIDS:
Viroids constitute a novel class of micro-organisms and are the smallest known agents of infectious diseases So far, viroid are definitely known to exist only in higher plant.
The first viroid was discovered in attempt to purify and characterize the causative agent of potato spindle tuber, a disease that, for many years, had been assumed to be of viral etiology.
In 1967, Diener and Raymer reported that the transmissible agent of this disease was a free RNA and that no viral micro protein particles (virions) were detectable in infected tissue.
Diener, by using sedimentation and gel electrophoresis, had shown conclusively that the infectious RNA was far smaller than the smaller genomes of viruses. No evidence of the involvement of helper viruses in the replication of RNA could be obtained. Despite in small size, the RNA appeared to be replicated autonomously insusceptible cells. Because of the basic difference between the potato spindle tuber disease agent and the conventional viruses, the term viroid wasintroduced by T. O. Diener
Viroids are low molecular weight RNA and represent minimal genetic and
biological system. Viroids, unlike viruses, lack the protective coat protein and are
composed entirely of single stranded covalently close circuler forms of 1ow mw
RNA.
They are not encapsulated like the viral nucleic acids and are present in
certain species of higher plants infected with specific disease.
They are: not detectable in healthy individuals of the same species but when
introduce in to such individuals, they replicate autonomously despite their small
size and produce the characteristic syndrome. All known viroids infect their hosts
in a persistent manner.
In symptom, viroid disease do not differ, significantly from virus disease,
although stunting of plants is a predominant symptom of most of the viriod
diseases. However, stunting is a symptom of many conventional plant viruses also.
Other important symptoms of viriods include- stunting, veinal discolouration, leaf