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COURSE TITTLE: BIODETERIORATION
Course code: MCB 321
DURATION: 2HOURS/ WEEK
Course Outline: MCB 321: Biodeterioration (2 Units: LH 15; PH 45)
Principles of microbial deterioration of materials, Materials subject to microbial deterioration:
Foods, Jet fuels, paper, paints, textiles and leather, metals etc. Factors favouring deterioration
of materials, Major microbial groups involved in deterioration. Impact of processing and new
technologies on biodeterioration. Biodeterioration Control
Textbook: The recommended textbooks for this course are:
Introduction to biodeterioration
Dennis Allsop, Kenneth J Seal and Christine C. Gaylarde Cambridge University Press
0-521-821-35-5
2004
Handbook of Material Biodegradation, Biodeteriorationa nd Biostabilization Michalina
Falkiewicz-Dulik; Katarzyna Janda and George Wypych
CRC PressChemTec Publishing
978-1895198447 2010
peer reviewed journal articles will also be used as reference materials.
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WEEK 1: PRINCIPLES OF BIODETERIORATION
Biodeterioration is simply defined as any undesirable change in the property of the material
caused by the vital activities of organisms in that material e.g. bacteria fungi as well as pests
are constantly causing problems in the conservation of our cultural heritage as well as other
materials (food, lethaer, cosmetics, woods, plastics) because of their biodeteriorating
potential.
TYPES OF BIODETERIORATION
In recent times, many materials are complex and much changed from the original raw
materials from which they were derived. New environments are being explored or exploited
and technology advanced. These new materials and their uses presents biological and
environmental problems due to the enormous range of organisms in the environment that
deteriorate them. However, it becomes useful to classify the different types of
biodeterioration which can occur:
a) Physical and mechanical biodeterioration
In this case the organism quite simply disrupts the material by growth or movement and does
not use it as a source of food. There are few if any serious examples of such damages caused
by microorganisms but an example is the expansion of microbial mass between rock layers
leading to spalling of the rock surface. Other examples caused by microorganisms include
cracking of underground pipes by roots, gnawing on electrical cables, cinderblocks,
plasterboards and wood by rodents and bird strikes on aircraft. This latter point shows that
biodeterioration is not necessarily caused by any conscious process by the organisms.
b) Soiling or Fouling (aesthetic) Biodeterioration
This type of biodeterioration is characterized by either the presence of dead insects or
droppings, excreta or metabolic products on food stuffs or materials which renders it
unsaleable. Microorganisms especially fungi and algae can be found growing on the surface
of materials, utilizing surface dirts, reducing its economic value or acceptability. A classic
example is dark fungal colonies growing on damp soap and skin residues on plastic shower
curtains.
Many fungi also release metabolites, soluble or insoluble pigments which discolour the
surface of materials. Fouling can be more serious and transcend the category of purely
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aesthetic damage in that a physical function can be impaired e.g. the extra drags on ship
caused by accumulation of weeds and invertebrates on the hull can increase fuel consumption
dramatically.
Biochemical Assimilatory Biodeterioration
The organism utilizes the material as food or energy source. Examples are: microbial
enzymes breaking down cellulose. Rats and insects eating stored grain and in layer
consuming stored food.
Biochemical Dissimilatory Biodeterioration
In this case, a material suffers chemical damage but not as a result of direct intake of
nutrients by the organism. Many organisms excrete waste products including pigmented or
acidic compounds which can disfigure or damage materials.
WEEK 2: BIODETERIORATION OF DIFFERENT MATERIALS
a) Biodeterioration of stored food materials
Stored (unorocessed) plant materials (fruit or seeds) are uasually decayed due to post harvest
attack of bacteria and fungi. The microbes can damage the plants partially as well as
completely. A large number of bacteria such as Erwinia spp., Corynebacterium spp. And
fungi such as Phytopthora spp., Curvularia spp., Aspergillus spp., are commonly found to be
associated with food materials. The most affected products are soft fruits and salad
vegetables whereas grains, oil seeds and legumes are durable products.
b) Biodeterioration of Leather
Leather is a product of animal hides. Besides its wool and animal glues, there are other
animal products which are attacked my microorganisms since leather contains keratin, animal
fats and proteins. Therefore, these are rapidly deteriorated by lipolytic and proteolytic
microorganisms which screte lipases and proteases. Leather production from animal hides
involved soaking process which is carried in water where Several Bacillus (B. subtilis, B.
megaterium and B. Pumilis) are present. They secrete several extracellular enzymes that may
attack the leather and remain active long after the death of the produced organism. During
leather deterioration, microbial activities cause loss of tensile strength and colouration. Since
finished leather is quite acidic, fungi such as Rhizopus, Mucor, Cunninghamella and
Aspergillus deteriorate quite fairly whereas bacteria are secondary colonizers. The damage on
leather causde by biodeteriogens include: colour change, odour, loss of tensile strength.
c) Biodeterioration of Stone and Building Materials
Old/ancient monuments, natural rocks, building materials are attacked by various
microorganisms such as Cyanobacteria, Pleurococcus, Oscillatoria etc, Lichens, while fungi
include Botrytis Penicillum and Trichoderma spp. The use concrete, bricks and mortar of
building materials as nutrients. Microbes colonize these materials which may cause excessive
expansion and contraction. Entrapment of water within the colonises and cracks can lead to
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enhanced damage. In addition, hyphal penetration into surface layers of these materials can
result in crack formation which may be promoted by excretion of corrosive metabolites.
Several organic acids solubilize calcium carbonate while oxalic and citric acids solubilize
silicates. Desulfovibrio desulfuricans reduce sulphur compounds and produces H
2
S which is
then oxidized to sulphuric acid by Thiobacillus thioxidans. Various niyrifying bacteria such
as Nitrobacter, Nitrosomonas also solubilize calcium associated with building materials.
Additionally, a large number of Pseudomonas corrode steeled iron structures used in building
construction.
WEEK 3: CONTINUATION OF BIODETERIORATION OF DIFFERENT
MATERIALS
d)Biodeterioration of Paper and other Cellulosic Materials
Plant cells are composed of cellulose, hemicellulose, lignin and pectic substances. Therefore,
paper and cards are derived from plants. Although a large number of factors are responsible
for the microbial spoilage of paper. Deterioration of paper occurs by several cellulolytic fungi
such as Trichoderma chaetomium and Aspergillus and bacteria such as Cellulomonas. These
microorganisms secrete cellulose (endo-β-glucanases, exo-β-glucanases and β-glucosidase)
convert cellulose into glucose. Besides, more microorganisms also secret enzyme xylanases
that deteriorate hemicellulose present in paper. However, many other microbial activities
have major effects on paper material strength.
e) Biodeterioration of Metals
Microorganisms are jnown to corrode metals. They make biofilm by colonization of cells
which release corrosive metabolic productsresulting in the removal of hydrogen by sulphate
reducing bacteria (SRB). Microbial concentration of cells appear from an oxygen gradient
that develops as a microbial colonyin contact with the metal and utilizing the available
oxygen. These colonies have both oxygen limited zone (centre) which are having negative
and positive charges respectively. These leads to metal ion formation by producing insoluble
hydroxides
Iron corrosion occurs mainly due to a bacterium Gallionella (chemolihotrouph) that oxidizes
ferrous ion to ferric acid and form insoluble ferric hydroxide deposits at the site of microbial
attack. Biological activities that stimulate corrosion include:
Anodic reactions by acidic metabolism
Cathodic reactions by microbial production of cathodic reactants such as H
2
S
Breakdown of protective films
Increase in conductivity of the liquid in the environment.
f) Biodeterioration of Plastics
Plastics are polymeric materials e.g. polyethylene, polystyrene, polyvinyl chloride (PVC) and
polyesters. Plastics are resistant to microbial attack but addition of various other materials
makes
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the plastics prone to microbial attack. Streptomyces rubireticuli and Penicillum spp are
reported to deteriorate PVC and polyamides (nylon). Polyesters, polycaprolactone and
polybutylene adipate are degraded by bacteria and fungi.
WEEK 4: CONTINUATION OF BIODETERIORATION OF DIFFERENT
MATERIALS
g) Biodeterioration of Pharmaceuticals and Cosmetics
Cosmetics and pharmaceuticals are manufactured in the form of lotion, cream, drugs and
powder forms. They consist of large quantity of water, animal, plant and mineral oils, natural
gums, thickening agents, carbohydrate, aroma and flavouring agents in addition to protein
hydrolysates, milk, beer, egg, plant extracts etc. These product formulations are good sources
of nutrients for microbes. Although preservations are added but due to the complex nature of
the formulations. becomes less effective. Sometimes, creams and lotions are contaminated
with Pseudomonas, although with low levels of these groups of organisms, the individual is
not harmed but when applied to a damaged skin, situation may become worse. Some of the
deteriorated cosmetics impart foul odour due to the production of organic acids, fatty acids,
amines, ammonia, H
2
S or acids which alters the pH changing the consistency and colour of
the product. Sometimes, gas bubbles are generated. Such products may later become unstable
and form separate oil and water phases.
Assignment
Write exhaustively on Biodeterioration of fuels as a global challenge.
FACTORS FAVOURING DETERIORATION OF MATERIALS
The process on biodeterioration is supported on supported on sveveral factors which is a
complex interplay of the effect of climate or meterological factors, biological process and
chemical processes. The major environmental factors involved in the deterioration of
materials include:
Moisture
Temperature
Solar radiation
Air movement and pressure
Precipitation
Chemical and biochemical attack
Intrusion of macro-microorganism
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WEEK 5: CONTINUATION OF FACTORS FAVOURING DETERIORATION OF
MATERIALS
Moisture:
Moisture and temperature affect the chemical, biological, and mechanical processes
of decay. The formation of a moisture layer on the material surface is dependent upon
precipitation. It may also be generated as a result of the reaction of adsorbed water
with the material surface, deposited particles with the material surface, and deposited
particles with reactive gases.
Relative humidity:
Among climatic factors, humidity plays the most important role in outdoor metal corrosion.
In the absence of atmospheric moisture, there will be very limited non pollutant-induced and
pollutant-induced corrosion. The rate and nature of the corrosion is a function of relative
humidity, sunlight radiation, surface contaminants, the properties of the film of electrolytes
formed on the metal surface, and the duration of the effect on the metallic surface
Temperature:
Temperature affects the processes of deterioration of a material gradually and in a variety of
ways. Changes in temperature induce a thermal gradient between the surface layer and the
inner layer of materials (particularly in materials with lower thermal conductivity), which
may result in the degradation of the Mechanical properties of the material land can lead to the
formation of fine cracks. The formation of cracks is accompanied by a loss of strength and by
an increase in material porosity, which may lower the chemical resistance of the material.
Solar Radiation:
Solar radiation causes temperature changes in materials and may induce volume changes of
material in the pores due to expansion of water which is heated by solar radiation. Solar
radiation plays an important role in photochemical reactions since it supplies the energy for
the excitation or splitting of bonds in the reacting molecules. Adequate intensity of solar
radiation at suitable wavelengths is an essential condition for photochemical reactions that
influence the deterioration of different construction materials.
These natural factors continuously promote weathering and material decay including metal
corrosion. There are four mechanisms involved in the deterioration of different materials and
structures. An understanding of these would aid in evaluating the influence of the above
listed environmental factors. They are:
1. Erosion
2. Volume change of the material
3. Dissolution of material and the associated chemical changes
4. Biological processes.
Erosion:
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Can be described as continuous recession of a surface because of localized impact in an
outdoor environment, suspended abrasive particles usally cause erosion of materials mostly
by fine solid particles moving against the material surface by flowing fluids.
Changes in volume of the material:
This is a function of temperature, solar radiation and humidity. The contraction and
expansion of a material caused by heat is influenced by temperature difference which is in the
open environment and depends largely on time and degree of exposure to sun rays (solar
radiation) and the direction the material surface faces. Volume variation occurs due to uneven
distribution
of moisture content on a material surface can be caused by rainfall, fog or wind, an attack of
the material can be caused by differential moisture content through the layer of a
homogenous material since the side with a lower moisture content will expand less than that
with a higher moisture content.
WEEK 6: CONTINUATION OF FACTORS FAVOURING DETERIORATION OF
MATERIALS
Dissolution of material and the associated chemical changes:
Chemically induced damage involves dissolution, oxidation and hydrolysis. The damage will
occur as a result of the interaction between the material and natural constituents, chemicals
and amount of water present. The interactions will vary depending on the reactivity of the
material, the character of the intercepting surface, the exposure time and the nature of the
contaminants. The chemical changes are enhanced by heat (most chemical reactions proceed
more rapidly as the temperature increases). Therefore chemical changes occur more in one
climate.
The dissolution of building materials especially structures with carbonate is most frequently
caused by the action of acidic solution such as rain which contains carbonic acids or both
carbonic acids and sulphuric acids. The effects of these acids is weathering of a surface.
Oxidation of a material by atmosphere oxygen results in the chemical changes in the
formation or composition of the material (especially surface) e.g. reaction of metal ions with
oxygen to form oxides or hydroxides. Hydrolysis can cause the dissolution of a material or
cause a chemical change in its composition simply by a reaction between a material and
water.
Biological processes:
Biological factors cause deteroiation through biochemical effects and intrusion of organisms.
The former is a crucial or essential factor in biodeterioration of building structures as the
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metabolites (enzymes, excrements or feaces) of micro and macro organisms, plant and
animals living in a material can cause chemical damage of the material. Fungi hyphae,
lichens and plant root systems which spread through structures can induce a mechanical
damage. Also, boring insects may destroy structure cohesion which can encourage water
penetration more quickly and deeply facilitating other deteriorating processes
BIODETERIORATION OF MATERIALS: EFFECTS OF MICRO & MACRO
ORGANISMS
Myriads of micro and macro organism cause chemicals, physical changes as well as
mechanical damage to materials. Relative humidity, temperature levels and other parameters
determine whether these organisms flourish or exist at all. Microorganisms can cause damage
through a range of processes.
1. Enzymes secreted by these organisms can be catalysers of chemical reactions that
stimulate an attack on the material
2. Microorganisms may utilize products of this reaction e.g. (corrosion products) or use
certain product of the materials as source of carbon and energy.
3. Chemical damage which is also important may arise by excretion of acids. These
acids are capable of chelating metal ions
4. Stone and wood may be damaged when they serve as substrate for some higher
animals
5. Bushes and trees are observed on buildings. They can cause mechanical and chemical
deterioration on constructions. During growth, plants and roots generate high pressure
which may damage buildings
6. The humic acid present in roots systems attack carbonate compounds.
The bacterial chemical action constitutes the major risk for the deterioration of stone.
Especially harmful are those bacteria that obtain carbon from carbondioxide or energy from
light or by chemical redox reaction. Some are capable of utilizing inorganic compounds e.g.
sulphur and nitrogen to produce sulphuric and nitric acids. These acids influence the pH of
the environment where they live and cause damage to various stone structures. Example of a
bacterium that uses the oxidation of hydrogen sulphide and sulphur as an energy source and
converts this form of sulphur to sulphuric acid are the species in the genus Thiobacillus.
Sulphate reducing bacteria having the unique ability to convert sulphate to produce sulphites
as metabolites are well known for the damage they cause to metals. Desulfovibro vulgaris
and Desulfotomaculum nigrificans produce H
2
S that induce this corrosion on different
systems. Examples of systems that can experience corrosion include oil and gas pipelines, gas
distribution systems and sewage systems. Sulphate reducing bacteria use sulphate ions from
the surrounding micro environment as an oxygen source for the oxidation of metals. The
catalysers of this process are the enzymes of the bacteria. The product of redox reactions in
these processes is sulphide which is released. Microorganisms will support the corrosion of
metals using corrosion products.
Bacteria may lead to a change in the physical characteristics of the wood e.g. permeability
and absorptibility which can lead to loss of strength. Some bacteria such as those belonging
to the genus cytophaga are highly specialised in the only substrate they can use as a carbon
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and energy source is cellulose. They can completely destroy the structure of cellulose fibres.
These bacteria are highly distributed in soil, continental waters and seawaters.
Assignment: As a microbiologist discuss bio deterioration as a societal menace.
WEEK 8: Major microbial groups involved in deterioration
The genus Clostridium is involved in anaerobic decomposition of cycles. Likewise
Actinomycetes play an important role in the decomposition of organic materials. This group
of
bacteria usually a unicellular mycelium with long branching hyphae that attack substances
such as cellulose, hemicellulose, chitin, certain and decompose lignin. They occur in both
freshwater and salt water and soil.
Algae are another group of microorganisms that grow in the water film, on the stone surface
and deteriorate it. They are also found in buildings. They have also been found on limestone
and sandstone as well as historical objects. Algae also attack stones by exhaled CO
2
which in
the presence of water supports the dissolution of the carbonated components of stone. Again,
mechanical deterioration of stone occurs if algae develops to an extent that the multicellular
colonies generated increases the pressure on the walls of the pores thereby damaging the
stones. Some algae living within the stone may contribute to this aggregation of the stone.
FUNGI:
Most fungi are organisms with mycelium as their vegetative structure. Fungi can destroy the
structural integrity of a material mechanically and chemically. Mechanical damage to stone,
concrete and other building materials is caused by the intrusion of the hyphae into the
structure and by the contraction and expansion of the mycelium with changing humidity.
Mechanically, the hyphae grow into the structure. The fungi can colonise and form a film
over the surface of the stone which blocks the pores. Any moisture that does penetrate
through the stone will dry out more slowly which makes the material stay wetter for a long
period which enables dissolved salt to penetrate more deeply. Wood destroying fungi induce
several types of decay ranging from the formation of mycelia on the wood surface to
destruction of wood and rot. Notably each species of fungus may attack different parts within
the wood. Examples of the class fungi that can cause biodeterioration include
Basidiomycetes Ascomycetes and Deuteromycetes
LICHENS:
Excrete organic acids that attack materials and produce compounds such as salts of salicylic
acid and tartaric acid which also degrade carbonates in an alkaline medium. Lichens have
hyphae that can grow through the pores in stones. After absorbing water, they enlarge
considerably in volume and affect the wall of pores by applying pressure on them. Lichens
are extremely sensitive to gaseous sulphur compounds which account for their rare
occurrence in polluted areas.
MOSSES:
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Often grow on the surface of stones that are covered by humus. They have the ability to
absorb large quantity of water and they also produce organic acids.
Pests attack wood in the soil, water and also in the ambient air. Here are different types of
insect such as ants, termites, beetles and insects that frequently cause great damage to
materials. Insects cause a loss of wood strength and in some extreme cases they result in the
collapse of structures.
WEEK9: BIODETERIORATION CONTROL
Biodeterioration is a worldwide challenge, several control measures have been applied to
prevent biodeterioration. These include:
The use of fungicides
Biological control
Prevention of biodeterioration by control of environmental conditions
Periodic cleaning of dirt, dusts and spores
The use of radiation
All of these issues are related to understanding the risk-benefit relationship of each
treatment, relative to the materials or object in question.
In general, one needs to use extreme care in altering the environment conditions
surrounding an object, restrict the use of any active chemical toxins and text how or
whenever possible, the treatment on similar materials before applying a treatment
reported allegedly to be safe.
For insect control, the use of Anoxil treatment represent the safest method currently
available to control insect on wood, museum objects and food.
Fungal infection of materials can be controlled by drying proper cleaning and storage
at 50 60% relative humidity. In the museum at constant temperature, there is little
chance for most fungal spores to germinate. This seems to be due to the fact that the
spores will not enter dormancy. If spores are frozen and dried, they may enter a
dormant state and thus will be potentially viable years later.
Microbes can persist in dry environment. Active metabolism, however requires
appropriate levels of relative humidity and temperature. A combination of low
humidity and low temperature is the simplest way to control microbial growth but this
treatment may be less effective for control of fungi and it is impractical in outdoor
situations. Regular cleaning may be the most effective treatment for preventing
biofilm formation and subsequent biodeterioration of materials such as historic
buildings, monument and other materials.
BIOCIDES:
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The application of biocides has become a routine, a practice in the conservation of
materials. Biocides are chemical substances designed to inhibit or prevent the growth
of microbes when
applied or surfaces. However environmental issues have severely limited the number of
available infective biocides or subsidal biocides used in the conservation of materials.
Biofilm bacterial respond differently to biocide and are generally more resistant than
unattached cells. Because microbes are capable of rapidly acquiring chemical resistance, no
single chemical can be relied on for long term use. Frequently or most times, several
chemicals need to be combine in order to achieve effective eradication of biofilm population.
Biocides are very difficult tools for preservation because many are too caustic for
environmental use. They are not strong enough to discourage microbial growth or the
microbe ultimately develop resistance.
CONSOLIDANTS
They have been used over time to conserve archaeological stones, rocks or building surfaces
from biological and chemical weathering consolidation is a means of generating structural
strength in disintegrating materials and it is an artificial means of repairing the damage
caused by natural processes.
The efficacy of consolidants on outdoor stones is controversial because they can distort or
upset the natural saturation and evaporation of moisture from within the stone often resulting
in exfoliation and cracking of stone surfaces.
Some consolidants may also discolour as they degrade because of aging, photochemical
processed and oxidation. However, the addition of biocides to consolidants would help to
prevent microbial deterioration, increasing the longevity of the treatments. More preferably,
environmentally acceptable biocides should be used as additives in consolidants.
Some scientists have divided the control of biodeterogens into two methods, which are direct
and indirect methods: the indirect methods involves modification of the climatic parameters
of the surrounding environment which include; humidity, light, temperature and nutrient
sources. These can be relatively possible for indoor sites or indoor materials while it is only
partially applicable for outdoor materials e.g. (by avoiding the direct rainfall, wall surfaces,
roofs and surfaces).
The most widely used direct method consists of:
Biomass removal (mechanical method)
Use of lamps with wavelengths that are non-compliable with the photosynthetic
activities
of biodeterogens (physical method)
The application of compounds with biocide activity or consolidants (chemical
method)
WEEK 10: BIODETERIORATION CONTROL
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DRYING AS A MEANS OF CONTROLLING FOOD BIODETERIORATION
Drying is one of the oldest methods of food preservation against microbiological.
spoilage as well as bio-deterioration. Drying helps to maintain the edible status of
food and also extend their shelf
life (life span). The required level of moisture content to prevent spoilage achieved in a
drying process depends on the microbes prevent. Sometimes pre-drying operations such as
osmotic dehydration, evaporation are employed to reduce water concentration to the desired
level. Drying usually refers to the process of liquid water being evaporated from the surface
of the product or from the pores within the product. Sometimes additional heat is usually
required to accelerate the drying process. The heat can be supplied in many ways:
Solar energy
Microwave or
Hot gas strain
They are two different types of drying processes which are in-air or in-vacuum
drying.
For the vacuum drying, they are useful to remove water vapour when the products are
best treated in the absence of air and where relatively low temperatures are preferred.
AIR DRYING
No matter the mechanism of heat - supply, e.g. microwave, radiation or conduction.
Air is frequently used as the medium to remove water vapour from the moist material.
It is slow process although air flows with a high velocity relative to the food product
being dried are employed to increase mass and heat transfer. Hot air drying is the
most common method used in the industries.
FREEZING
Freezing is an effective preservation technology because of the role of temperature in
bio-system stability and the reduction of moisture levels within foods after freezing
and during frozen storage. Both factors combine to significantly slow down the
chemical, physical and biological reaction that governs deterioration of food. The
process of freezing involves the removal of heat from a food material accompanied by
physical change as liquid water becomes solid ice.
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COURSE TITTLE: INTRODUCTORY VIROLOGY
Course code: MCB 313
DURATION: 2HOURS/ WEEK: 3UNITS
COURSE OUTLINE:
Origin and nature of viruses; General characteristics, structure, properties and classification of
viruses;Chemical and physical properties of plant, animal and bacterial viruses; Viral
replication, spread and cytopathic effects. Principles of viral isolation, cultivation, purification
and assay. Regulation of lytic development and maintenance of Bacteriophages; A review of
current infectious endemic disease in Africa such as Polio, Measles, Rabies, Lassa fever,
HIV/AIDS, Ebola, Monkey pox among others. Application of cell culture technique in virology
and modern diagnostic tools in virology.
RECOMMENDED TEXT BOOK
Wiley, J.M., L.M., Sherwood and C.J., Woolverton. 2008. Prescott, Harley and Klein’s
Microbiology. McGraw Hill. New York. 7th ed. 1088pp.
Luria, S.E. et al.,. 1983. General virology. John Wiley and Sons Inc. New York. Flint, S.J. et
al.,. 2004. Principles of virology. ASM Press.
Tortora, G.J., B.R. Funke and C.L. Case. 1982. Microbiology: An introduction. The
Benjamin Cummings Publishing Company. California. 726pp.
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WEEK 1:ORIGIN AND NATURE OF VIRUSES.
Introduction
Virology is the study of viruses. Viruses are small, acellular entities, inert in the extracellular
environment and depend on the machinery of a living host to reproduce. In this unit, we shall
be looking at the origin of viruses, some terms in virology, the general characteristics of
viruses and the chemical composition of viruses.
Origin of Viruses
Virology is defined as the study of viruses, which are small, acellular entities that usually
possess only a single type of nucleic acid and that must use the metabolic machinery of a
living host in order to reproduce.
Viruses are inert in the extracellular environment and only come alive in contact with a living
host cell. They replicate only in the living cells. The viral nucleic acid contains all
information necessary for programming infected cells to synthesize a number of virus
specific macromolecules required for the production of viral progeny. The viral genome takes
control of the metabolism of the host cell. Viruses are known to infect a wide range of host
cells ranging from mycoplasma, bacteria, algae, invertebrates, all higher plants and animals.
Terms in Virology
Capsid: This is the protein shell or coat that encloses the nucleic acid of the virus. Empty
capsids may be by-products of replicative cycle of viruses with icosahedral symmetry.
Nucleocapsid: This refers to the capsid together with the enclosed nucleic acid.
Virion: This is the entire infectious unit or the complete viral particle. The virion serves to
transfer the nucleic acid from cell to cell.
Viriod: This refers to some naked genetic materials that are air-borne but lacks capsid. They
also cause infection on contact with a living host cell. They mostly cause plant infections.
Envelope: This is a lipid containing membrane that surrounds some viruses. It is acquired
during virus maturation by budding process through the cellular membrane. Virus enclosed
glycoproteins are exposed on the surface of the envelope.
Capsomers: This refers to morphologic unit seen in the electron microscope on the surface of
the icosahedral virus particle. They represent clusters of polypeptides.
Capsomeres: They are the protein building blocks that constitute the capsomers.
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Defective virus: this refers to a viral particle that is functionally deficient in some aspect of
viral replication
Replication: This is the mode of multiplication of the viral particles in the host cell for
continuity and maintenance of the virus in the host.
Viroplasm: A virus factory or a modified region in an infected host cell where virus
replication occurs.
WEEK 2 : General characteristics of viruses, structure and properties
All viruses are acellular organisms and are the smallest known infective agents.
They contain only one type of nucleic acid DNA or RNA as their genome but never the two
at a time.
They posses no organelles and are unable to make their own proteins and essential enzymes.
They obligate on their host for energy and replication.
They are metabolically inactive outside their host.
They do not grow in ordinary laboratory or synthetic media used for bacterial cultivation,
they require a living host such as embryo or tissue for their cultivation.
Viruses can only be studied through the aid of an electron microscope that could magnify up
to × 500,000.
Viruses cannot ne gram stained or stained with common laboratory stains.
Viruses are not sensitive to antibiotics.
Chemical Composition of viruses
Chemical constituents of viruses include the following: Proteins, nucleic acid, lipids and
carbohydrates. They are discussed below.
Viral protein
The major purpose of the viral protein is to aid transfer of viral genome. They protect the
viral genome against the action of nucleases of the host cell and participate in the attachment
of the virus particle to a susceptible cell during infection. The viral capsid is made up of
protein sub-units known as capsomers which are made up of capsomeres. The viral protein
determines the antigenic properties of the virus. The surface glycoproteins of some viruses
exhibit specificity in their activities such as the heamagglutinin on influenza virus that
agglutinates the red blood cells. Some viruses carry some enzymes inside the virion which
are essential for the initiation of replication in the host cell such as RNA polymerase carried
by rhabdoviruses and reverse transcriptase carried by retroviruses that makes a DNA copy of
the viral RNA.
Nucleic acid
Viruses contain only a single type of nucleic acid, DNA or RN that encodes all the genetic
information necessary for the replication of the virus. The viral nucleic acid is also referred to
as the viral genome. The viral genome can be single or double stranded, circular or linear,
segmented or linear. The type of nucleic acid, strandedness and the molecular weight of the
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viruses are used in viral classification into families. The molecular weight of viruses ranges
from 1.5×10
6
to 200×10
6
Daltons.
The sequence and composition of the nucleotides of each viral nucleic acid are distinctive.
The G+C content of the nucleic acid is one of the important properties used in characterizing
a viral genome. DNA viral genome can be analyzed using restriction endonucleases and
using molecularly clones CAN copies of RNA and restriction maps can be derived for
analyzing RNA viral genomes.
Viral Lipids
A number of different viruses contain lipid envelopes as part of their structure. The lipid is
required when the virus nucleocapsid buds through a cellular membrane during maturation.
The phospholipid composition is determined by the specific type of cell membrane in the
budding process. The acquisition of a lipid containing membrane is an integral step in virion
morphogenesis in some virus group. Lipid containing viruses are sensitive to ether and
organic solvent.
Viral Carbohydrate
Virus envelopes contain glycoproteins. In contrast to the lipids they are derived from the host
cell, the envelope glycoproteins are virus-coded however the sugars added to the virus
glycoprotein reflect the host cell in which the virus is grown. It is the surface glycoproteins of
enveloped viruses that attaches the virus to a target cell by interacting with cellular receptors
and are also important viral antigens.
Conclusion
Viruses contain lipids, carbohydrate, nucleic acids and a protein coat, require a living host
cell for replication and attack wide range of organism ranging from prokaryotic bacteria to
higher eukaryotes like angiosperms and chordates.
Summary
Viruses are small acellular unit that require a lining host to become living.
They contain a nucleic acid and a protein coat, they could be enveloped or not.
They attack a wide variety of organisms.
Viruses are infective agents.
ASSIGNMENT
1. Draw the structure of the DNA molecule
2. A labeled diagram showing the differences between naked and enveloped virus.
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