Introduction to Virology

3000BC

History Smallpox was endemic in

China by 1000BC. In response, the practice of variolation was developed. Recognizing that survivors of smallpox outbreaks were protected from subsequent infection, variolation involved inhalation of the dried crusts from smallpox lesions like snuff, or in later modifications, inoculation of the pus from a lesion into a scratch on the forearm of a child.

The concept of virus

Edward Jenner (1798), introduced the term virus in microbiology.

Virus in Greek means poison.Edward Jenner noticed that milk maids who infected with

cowpox develop immunity against smallpox. He inoculated a boy with the vesicle fluid taken from the hand

of infected maid.The boy developed sustained immunity against smallpox.

The concept of virus.

Edward Jenner assumed that the vesicle fluid that has been taken from the hand of the milk maid contained a poison ( virus ), that was responsible for immunity.

9.1 General Properties of Viruses

• Virus: Viruses are the obligate intracellular element that cannot replicate independently of a living (host) cell

• Virology: the study of viruses• Virus particle (virion): extracellular form of a virus

– Exists outside host and facilitates transmission from one host cell to another

– Contains nucleic acid genome surrounded by a protein coat and, in some cases, other layers of material

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General characteristics of viruses

Viruses are smaller than bacteria, they range in size between 20-300 nanometer ( nm ).

Viruses contain only one type of nucleic acid, either DNA or RNA, but never both.

Viruses consist of nucleic acid surrounded by a protein coat. Some viruses have additional lipoprotein envelope.

Viruses lack cellular organelles, such as mitochondria and ribosomes.

General characteristics of viruses

Viruses are obligate cellular parasites. They replicate only inside living cells.

Viruses replicate through replication of their nucleic acid and synthesis of the viral protein.

Viruses do not multiply in chemically defined media.Viruses do not undergo binary fission.Virus particles can only be observed by an electron

microscope

Challenges the way we define life

viruses do not respire, nor do they display irritability应急性 ; they do not move they do not grow they do most certainly reproduce, and may

adapt to new hosts.

Virus structure, classification and replication

Viruses defined

Obligate intracellular parasites

VirionVirion

capsomeres

Nucleocapsid(a nucleocapsid without a genome is a capsid)

envelopedvirus

envelope

A virion

nucleocapsid

13

VIRAL STRUCTURE – SOME TERMINOLOGY

• virus particle = virion

• protein which coats the genome = capsid

• capsid + genome = nucleocapsid

• may have an envelope

Terminology

Virion: The complete virus particle.Capsid: The protein coat that surrounds nucleic acid.Nucleocapsid: The nucleic acid plus the capsid.Capsomeres: The structural protein units that made up the

capsid.Defective virus: the virus cannot replicate by its own, it requires

helper virus.Nanometer : milli-micron.

Genome The genome of a virus can be either DNA or RNA

DNA-double stranded (ds): linear or circular Single stranded (ss) : linear or circular RNA- ss:segmented or non-segmented ss:polarity+(sense) or polarity –(non-sense) ds: linear (only reovirus family)

Basic virus structure

Nature of Viruses

Viral genome is packaged in protein coat

Virion

Capsid

Viral core

envelope

Viral core

Viral core

The viral nucleic acid genome, In the center of the virion, : Control the viral heredity and variation, responsible for the infectivity.

Viral Capsid The protein shell, or coat, that

encloses the nucleic acid genome. Functions: a. Protect the viral

nucleic acid. b. Participate in the viral infection. c. Share the antigenicity

9.2 Nature of the Virion

• Viral Structure– Capsid: the protein shell that surrounds the

genome of a virus particle (Figure 9.2)• Composed of a number of protein molecules

arranged in a precise and highly repetitive pattern around the nucleic acid

– Capsomere: subunit of the capsid• Smallest morphological unit visible with an

electron microscope

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Figure 9.3

Nucleocapsid

Nucleicacid

Capsid(composed ofcapsomeres)

Envelope

Capsid

Nucleic acid

Naked virus Enveloped virus

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Nucleocapsid The core of a virus particle

consisting of the genome plus a complex of proteins.

complex of proteins = Structural proteins +Non- Structural proteins (Enzymes &Nucleic acid binding proteins)

Symmetry of Nucleocapsid

Helical Cubic /Icosahedral Complex

Helical symmetry

26

27

28

Adenovirus

SHAPES MAY DIFFER BUT…

T4 BacteriophageTobacco Mosaic

Virus Influenza Virus

Head

Tail sheath

DNA

Tail fiber

RNACapsid

Surfaceproteins Membrane

envelope

RNA

Capsidproteins

All viruses have 1. Chromosome-like part that carries hereditary information – The Core 2. Protein coat: Protects hereditary information and provides the shape! The Capsid

Size and Shape

Methods Size of Viruses Shapes of Viruses

Introduction to Virology

Recognizing the shape, size, and structure of different viruses is critical to the study of diseaseViruses have an inner core of nucleic acid

surrounded by protein coat known as an envelope

Most viruses range in sizes from 20 – 250 nanometers

Shape of Viruses

Spherical Rod-shaped Brick-shaped Tadpole-shaped Bullet-shaped Filament

Shapes of Viruses:Spherical

Shapes of Viruses :Rod-shaped

Shapes of Viruses :Brick-shaped .

                                                                

           

Tadpole-shaped

Shapes of Viruses :Bullet-shaped

Shapes of Viruses :Filament

39

White, DO and Fenner, FJ. Medical Virology, 4th Ed. 1994

Virus particle = virion

T4 Bacteriophage

Structure of viruses

A – naked, not containing an envelope aroud capsid

B – enveloped, containing an envelope around the capsid

Properties of enveloped virusesProperties of enveloped viruses• Envelope is sensitive to

• – Drying• – Heat• – Detergents• – Acid

• • Consequences• – Must stay wet during transmission• – Transmission in large droplets and secretions• – Cannot survive in the gastrointestinal tract• – Do not need to kill cells in order to spread

Adapted from Murray, P.R. Rosenthal K.S., Pfaller, M.A. (2005) Medical Microbiology, 5th edition, Elsevier Mosby, Philadelphia, PA Box 6-5

Genome

The classification of viruses is based on the type of nucleic acid contained withinRNA viruses---also known as a retrovirusDNA viruses

Classification of virusesClassification of viruses

Viruses are divided into two large groups:Viruses are divided into two large groups:• RNA containing viruses.RNA containing viruses.• DNA containing viruses.DNA containing viruses.

Viral GenomesViral Genomes

Nucleic Acid

DNA

RNA

Double Stranded

Positive

Negative

RNA DNA

Single Stranded

Double Stranded

Single Stranded

Virus Genomes

DNA virusesDNA viruses

adenovirus

Papillomavirus

Herpesvirus

Parvo virus

Hepatitis B virus

Molluscum contagiosum (Pox virus)

RNA virusesRNA viruses

Influenza viruses

Rotavirus

Enterovirus

Paramyxovirus

Rift valley fever

ReplicationViruses replicate within a host cell while

utilizing the host cell’s nucleic acids.

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BASIC STEPS IN VIRAL LIFE CYCLE

• ADSORPTION

• PENETRATION

• UNCOATING AND ECLIPSE

• SYNTHESIS OF VIRAL NUCLEIC ACID AND PROTEIN

• ASSEMBLY (maturation)

• RELEASE

Steps in virus replication

1-Adsorption (attachment ).

Viruses must recognize and bind to specific cellular receptors on the surface of the infected cell via particular glycoproteins.

Steps in virus replication

2--Penetration. A- Enveloped viruses that has the ability to form syncytia

( multi-nucleated giant cell ) enter the cell through fusion of the viral envelope with cell plasma membrane( eg. Paramyo and herpes viruses ).

2- The remaining enveloped viruses enter the cell through endocytosis.

Entry of enveloped viruses, fusion of the viral envelope.

Steps in virus replication

B- Unenveloped viruses enter the cell either by endocytosis ( endosome lyses as with adenoviruses) or by forming a pore in the membrane of the cell. The viral RNA is then released inside the cell (picornaviruses).

Endocytosis

Endocytosis involves invagination of the cell membrane to form vesicles in the cell cytoplasm.

Infected viruses are then engulfed inside these vesicles.Each vesicle fuses with a lysosome to form lysosomal

vesicle.The viral envelope fuses with lysosomal membrane and

the viral nucleocapsid is expelled into the cytoplasm.

Endocytosis.

Steps in virus replication

3- Uncoating. Release of the viral genome from its protective capsid to

enable the viral nucleic acid to replicate.

4- Transcription. Synthesis of m-RNA.

5-Translation. The viral genome is translated using cell ribosomes into structural and non-structural proteins.

Steps in viral replication

6- Replication of the viral nucleic acid.

7-Assembly. New virus genomes and proteins are assembled to form new virus particles.

8-Release. Enveloped viruses are released by budding from the infected cells. Unenveloped viruses are released by rupture of the infected cells.

Release of enveloped viruses by budding

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PENETRATION - ENVELOPED VIRUSES

from Schaechter et al, Mechanisms of Microbial Disease, 3rd ed, 1998

63

smallpox virus cytoplasmic assembly and maturation

F. A. Murphy, School of Veterinary Medicine, University of California, Davis. http://www.vetnet.ucdavis.edu/fam_graphics/download.html

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HIV budding and maturation

Hsiung, GD et al., Diagnostic Virology 1994 p204 (D. Medina)

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HIV – mature form

Briggs JA et al. Structure. (2006) 14:15-20

Why Study Viruses

(From Medical Microbiology, 4th ed., Murray, Rosenthal, Kobayashi & Pfaller, Mosby Inc., 2002, Fig. 65-1.)

Under attack!

Why Study Viruses• Viruses are capable of infecting all forms of life

– Vertebrates, prokaryotes, fungi, algae• Most abundant form of life

– Bacteriophages are extremely abundant– Estimated 1031 tailed bacteriophages

• Excellent molecular biology tools

VirusesViruses

&&• Human HealthHuman Health

June 23, 2009 71SBI3C

Some examples of generalised viral infections:

Measles

Viral haemorrhagic fever

zoonosis

Rabies

Motor neurone

Source of infection:

Shedding virus susceptible

Man > 99%

animals zoonosis

Routes of entry:

sexual

Inhalation

inoculation

Bloodorgan t/plant

ingestion

Congenital / vertical

“vertical transmission”

• Some viruses can cross the placenta

• Infection during pregnancy can damage the foetus

e.g. Rubella, Cytomegalovirus

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Viral diseases are difficult to treat because:

1. No drug is available to kill viruses in the body

2. Some viruses are lysogenic can remain dormant for years (hide inside cells)

ex. a) Herpes Simplex Virus I (HSV I)

(cold sores)

b) HIV

3. some viruses are cause cells

to become cancerous

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(AIDS)

ex. Human Papiloma Virus (HPV)

(genital warts)(cervical cancer)

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 vaccines: only true protection against viral diseases

• people are given a dead or weakened form of the virus• builds up an army of WBC’s & antibodies to kill the living virus immediately before it can reproduceex. polio vaccine, hepatitis B vaccine

Protection Against Viral Diseases

Viral Diseases-Examples InfluenzaRabies HIVHepatitis

PoliovirusPoliovirus

PoliovirusPoliovirus

• Enterovirus.

• Possesses a RNA genome.

• Transmitted by the faecal oral route.

• Cause of gastrointestinal illness and poliomyelitis.

Properties of the virus

Poliovirus InfectionPoliovirus Infection

Gut

VirusInfection

Virus excretionin the faeces

Viraemia

Non-neuronaltissues

Neuronaltissues

Paralysis

Incidence of PoliomyelitisIncidence of Poliomyelitis

40

30

20

10

0

1950 1960 1970 1980

Nu

mb

er o

f ca

ses

(in

th

ousa

nd

s)

A B

Poliovirus vaccinesA: Salk – killed inactivated vaccine.B: Sabin – live attenuated vaccine

Influenza A virusInfluenza A virus

• Myxovirus• Enveloped virus with a

segmented RNA genome

• Infects a wide range of animals other than humans

• Undergoes extensive antigenic variation

• Major cause of respiratory infections

Properties of the virus

Influenza A virus InfectionInfluenza A virus Infection• Spread by respiratory route

• Virus infects cells of the respiratory tract

• Destruction of respiratory epithelium Secondary bacterial infections

• Altered cytokine expression leading to fever

e.g interleukin-1 and interferon

Spread of influenza virusSpread of influenza virus

Respiratory TractRespiratory Tract

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Structure of HIV

April 19, 2023 SBI3C 92SBI3C 92

AIDSHIV (Human Immunodeficiency Virus)

• virus that causes AIDS• RNA core (retrovirus)• can only infect helper T cells (type of white blood cell)• cannot survive outside the body because glycoprotein membrane around its capsid dries out can only be transmitted from 1 bodily fluid to another ex. 1. blood to blood: needles, transfusions, toothbrushes 2. semen & vaginal secretions

3. breast milk

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Transmission of AIDS (Worldwide)

• HIV in Body Fluids

Semen11,000 Vaginal

Fluid7,000

Blood18,000

Amniotic Fluid4,000 Saliva

1

Average number of HIV particles in 1 mL of these body fluids

April 19, 2023 94SBI3C 94

SBI3C 95

Source: UNAIDS, 2006 Report on the Global AIDS Epidemic, 2006

HIV Prevalence Worldwide

Adults Ages 15-49 with HIV

15.01% - 34.0%

5.01% - 15.0%

1.01% - 5.0%

0.51% - 1.0%

0.0% - 0.5%

Not available

Transmission of VirusesTransmission of Viruses• Respiratory transmission

Influenza A virus• Faecal-oral transmission

Enterovirus• Blood-borne transmission

Hepatitis B virus• Sexual Transmission

HIV• Animal or insect vectors

Rabies virus

Treatment and Prevention Treatment and Prevention of Virus Infectionsof Virus Infections

• Antivirals

• Vaccines and immunisation

Antiviral TargetsAntiviral Targets• Attachment/Entry

• Nucleic acid replication

• Virus protein processing

• Virus maturation

Viruses and Human TumoursViruses and Human Tumours• Epstein Barr Virus

Burkitt’s Lymphoma

• Human papillomavirus Benign warts Cervical Carcinoma

• Human T-cell Leukaemia Virus (HTLV-1) Leukaemia

• Hepatitis C virus Liver carcinoma

Virus-induced tumoursVirus-induced tumours

VirusInfection

UninfectedCell

?

[ ]Uncontrolled cell

growth and tumourformation

Virus-induced transformationVirus-induced transformation

Normal cells Transformed cells

Virus-Induced TumoursVirus-Induced Tumours

• Virus infects cell.• Virus nucleic acid, as DNA,

integrates into cellular genome.• Virus causes changes in cellular gene

expression.• Uncontrolled cell multiplication and

tumour formation.

The cultivation of viruses is complex and includes three common methodsChicken egg cultureCell cultureAnimal inoculation

Cultivation of Viruses A. Chick Embryos: Virus growth in an embryonated chick egg may result in the death of the embryo (eg, encephalitis virus), the production of pocks or plaques on the chorioallantoic membrane (eg, herpes, smallpox, vaccinia), the development of hemagglutinins in the embryonic fluids or tissues (eg, influenza), or the development of infective virus (eg, polio virus type 2).

Cultivation of Viruses B. Tissue Cultures:

Primary cultures are made by dispersing cells (usually with trypsin) from host tissues. In general, they are unable to grow for more than a few passages in culture, as secondary cultures.

- Diploid cell strains are secondary cultures which have undergone a change that allows their limited culture (up to 50 passages) but which retain their normal chromosome pattern.

- Continuous cell lines are cultures capable of more prolonged (perhaps indefinite) culture which have been derived from cell strains or from malignant tissues They invariably have altered and irregular numbers of chromosomes.

HeLA,

Hep-2,

Detroit-6,

KB,

Vero,

Fibroblasts of human embryou,

Kidney of rhesus monkey,

WI-38,

RD,

Primary cultures of chiken fibroblasts

Cell Cultures

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CYTOPATHIC EFFECT

• ANY DETECTABLE CHANGES IN THE HOST CELL– MORPHOLOGICAL CHANGES

108

Hockley et al. J Gen Virol 69:2455-2469

uninfected HIV infected

HIV infected (at higher magnifcation)

109

CYTOPATHIC EFFECT

• ANY DETECTABLE CHANGES IN THE HOST CELL– MORPHOLOGICAL CHANGES– DEATH– APOPTOSIS– INDEFINITE GROWTH

110

111

113

tissue culture cells

epithelial epithelioid fibroblasticslides from CDC

114

epithelial cells - adenovirus

uninfected early infection late infectionslides from CDC

115

epithelial cells - respiratory syncytial virus

uninfected respiratory syncytial virusslides from CDC

116

fibroblastic cells - herpes simplex virus

uninfected early infection late infectionslides from CDC

117

fibroblastic cells - poliovirus

uninfected early infection late infectionslides from CDC

Cytopathic Effect (cpe)Cytopathic Effect (cpe)Adenovirus Herpes virus