Lecture 1 - Introduction To Virology.pptx

Malamulo College of Health Sciences
Introduction To Virology
Jones Chipinga
MSc-TDZ, MSc-HS, BSc-BMS,DBMS
2022

Lecture Outline
• Characteristics of Viruses
• Early virus studies
• Learning from viruses
• Theories of viral origin
• The helpful or Collaborative viruses
• Human and aquatic viromes

Lecture Outline
• Applications of viruses in health or medicine
• Viral infections: brief introduction to transmission and
pathogenesis
• Viruses in history
• Recent viral outbreaks

Objectives
•List three advances in techniques/technology that
were needed in order to study viruses in the
laboratory.
•Define virus.
•Summarize a theory about the origin of viruses.
•Explain how viruses are transmitted and cause
disease.
•.

Lecture Objectives
•Discuss an example in which a viral infection is
beneficial to the host.
•Describe at least three applications of viruses in
treating health problems.
•Identify important historical and contemporary
viral epidemics

Learning Outcomes
• Explain three advances in techniques/technology that
were needed in order to study viruses in the
laboratory.
• Explain what a virus is
• Argue for a theory about the origin of viruses.
• Discuss how viruses are transmitted and cause
disease.

Learning Outcomes
•Discuss an example in which a viral infection is
beneficial to the host.
•Pronounce at least three applications of viruses
in treating health problems.
•Recognize important historical and
contemporary viral epidemics

Introduction
•Definition – virology – virus – poisonous – 1970s
•In 1890s scientists began studying filterable
infections agents sickening tobacco plants.
•Intimate relationship with living cells
•Mysterious and insidious nature of viruses
•Popular movies and TV series – World war Z

Dr. Andrew Fassbach - Harvard
• “Mother Nature is a serial killer.… No one’s better …
more creative. Like all serial killers she can’t help but
the urge to want to get caught … and what good are
all those brilliant crimes if no one takes the credit …
sometimes the thing you thought was the most brutal
aspect of the virus, turns out to be the chink in its
armor … and she loves disguising her weaknesses as
strengths.”

Introduction
• Emerging and reemerging viruses
• Virus pestilence timeline – 1518 – 2016
• 2016 – Zika virus – microcephaly and eye abnormalities, Guillain-
Barre syndrome
• 2019 to date (2022) – COVID-19
• 2022 – Monkey pox virus
• 2014 – Ebola, SARS in 2003

TV Series

Characteristics of Viruses
• Small and cannot be seen by naked eye
• Smaller than bacteria – 100 × (0.03 – 0.1 µm), e.g.,
poxviruses are 200 – 400 nm in length, filoviruses can
be up to 1,000 nm
• Can infect bacteria
• Complete dependence on host cell for reproduction

•Receptor-binding protein on the outer
surface, e.g., rhinoviruses bind to
intracellular adhesion molecule-1 (ICAM-1)
•In host cell, ICAM-1 is important in
inflammation and intracellular signaling

•Neither cellular nor microorganisms
•No functional organelles
•Contain only one type of nucleic acid per
particle type
•Transmission is dependent on the
movement of air or fluids

Theories of Origin of Viruses
•Escaped eukaryotic genes evolved to
encode protective protein coats for survival
outside the host cell (transposons and
retrotransposons)
•Degenerate forms of intracellular parasites,
having lost most cellular functions

Theories of Origin of Viruses
•Originated independently along with other
primitive molecules and developed with
self-replicating capabilities.

Why Study Virology
• Bacteriophages
• Livestock and plant species – e.g. Friedrich Loeffler
and Paul Frosch studied foot and mouth disease,
Dimitri Ivanovsky (1892) and Martius Beijerinck (1898)
showed tobacco mosaic virus
• Understanding their role in development of well-
known diseases

Why Study Viruses
• Vaccines e.g., Edward Jenner in 1796, Louis Pasteur in 1885
• In 1900 – yellow fever discovered by Walter Reed (first human virus)
• Karl Landsteiner and Erwin Popper in 1909 - poliomyelitis

The Virosphere
• All multicellular and unicellular organisms can be
infected by viruses
• E.g. every litre of sea water has up to 10 billion viruses
• There are around 5 × 1031 (10 nonillion) individual
viruses on planet earth
• Vast majority are bacteriophages serving to aid the
recycling of organic matter, and even determining
insect behaviour

Bacteriophage Plaque Assays
• For quantifying number of infectious bacteriophages
in a given phage-containing sample
• Bacteriophages are allowed to adsorb to host bacteria
in a test tube
• The mixture is then poured onto a solid agar plate of
medium, and the bacteria and bacteriophages are
allowed to replicate

•The bacteriophages lyse the bacteria that
are present on the surface of agar
•The clearings (plaques) in the bacterial lawn
are areas where bacteria have been killed
by bacteriophages

•When the bacteria are lysed during phage
infection, it is said to be a lytic infection
•A lysogenic infection is one in which
infected host cells are not yet lysed and do
not die during infection.

• This is a bacteriophage plaque
assay.
• Note the lawn of bacteria
growing on the surface of the
medium.
• Circular clearings or plaques
present within the lawn are
areas in which the bacteria were
lysed or killed by
bacteriophages.
• Shors, Teri.

The Nature of Viruses
• Viruses are small particles – experiment by Martinus
Beijerinck and Dimitri Ivanovski made extracts from
diseased plants and passed the extracts through filters
• The filtrates contained and agent that would infect
new plants but no bacteria could be cultured from the
filtrates
• The agent remained infective through several
transfers to new plants, eliminating the possibility of a
toxin.

The Nature of Viruses
• Friedrich Loeffler and Paul Frosch transmitted foot
and moth disease from animal to animal in a highly
diluted inoculum.
• Walter Reed and James Carroll – yellow fever
causative agent is a filtrable agent
• Virion
• Electron microscopy

Properties Common To All Viruses
•Viral genomes are associated with protein that
at its simplest forms the virus particle, but in
some viruses this nucleoprotein is surrounded
by further protein or a lipid bilayer.
•The outermost proteins of the virus particle
allow the virus to recognise the correct host cell
and gain entry.
•Viruses can only reproduce in living cells: they
are obligate parasites.

Properties Common To All Viruses
•Viruses have a nucleic acid genome of
either DNA or RNA.
•Compared with a cell genome, viral
genomes are small, but genomes of different
viruses range in size by over 100-fold (ca
3000 nt to 1,200,000 bp)
•Small genomes make small particles – again
with a 100-fold size range.

Viruses Have Genes
•The virion contains the genome of the virus
•There are four possibilities for a virus genome
•Double-stranded DNA (dsDNA)
•Single-stranded DNA (ssDNA)
•Double-stranded RNA (dsRNA)
•Single-stranded RNA (ssRNA)

Viruses Have Genes
•The genome is enclosed in a capsid
•The genome plus the capsid, plus other
components in many case, constitute a virion
•Virus genomes are much smaller than cell
genomes

Viruses Have Genes
•Viruses use host cell CHONs
•Genomes of large viruses duplicate some
functions of the host cell, but small ones rely
heavily on host cell functions
•RNA virus must have RNA polymerase – cells do
not encode enzymes that can replicate RNA

Viruses Have Genes
•Viruses code efficiently – overlapping genes and
genes encoded with genes e.g. Hepatitis B
•All genomes encode CHONs
•Many viruses are multifunctional e.g., multiple
enzyme activities, e.g.,rhabdovirus L protein
replicates RNA, caps, and polyadenylates mRNA,,
and phosphorylates another virus protein

Viruses Are Parasites
•A new virion is never formed directly from a pre-
existing virion, but by replication inside the host
cell and involves synthesis of components
followed by assembly into virions.

Viruses Are Parasites
•Viruses require:
•Building blocks such as amino acids and
nucleosides
•Protein-synthesizing machinery
•Energy, in the form of ATP
•A virus modifies the intracellular environment of
its host

Some Viruses are Dependent on Other Viruses
•These are known as satellite viruses – unable to
replicate unless the host cell is infected with a
second virus (helper virus)

Comparison of Virus and Cells

Lookup!
1. Burrell J.C. (2017). Fenner and White’s Medical Virology (5th ed.) Elsevier: London
2. Knipe M.D, Howley M.P, et. al Volume 1 and 2 (2013). Fields Virology (6th ed.)
Wolters Kluwer/ Lippincott Williams and Wilkins. U.S.A
3. Carter J & Saunders V (2013). Virology, Principles and Applications (2nd ed) John
Wiley & Sons Ltd: United Kingdom
4. Shors T (2017). Understanding Virology, Principles and Applications (3rd ed) RR
Donnelley, Jones and Bartlett Learning: Massachusetts

Lecture 1 - Introduction To Virology.pptx

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