1 Applied Microbiology 5 th lecture: Chapter 6. 2 Viruses.

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Applied MicrobiologyApplied Microbiology

55thth lecture: Chapter lecture: Chapter 66

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VirusesViruses

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VirusesViruses

General Structure of VirusesGeneral Structure of Viruses

-> Size range –-> Size range –

most <0.2 most <0.2 μμm; requires electron microscopem; requires electron microscope

-> Virion-> Virion – fully formed virus able to establish an – fully formed virus able to establish an infectioninfection

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VirusesViruses

Form and Function of VirusesForm and Function of Viruses

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VirusesViruses


CapsidsCapsids- All viruses have All viruses have capsidscapsids - protein coats that enclose - protein coats that enclose and and

protectprotect their nucleic acid. their nucleic acid.

- Each capsid is constructed from Each capsid is constructed from identical subunitsidentical subunits calledcalled

capsomerscapsomers made of protein. made of protein.

- The capsid together with the nucleic acid areThe capsid together with the nucleic acid are

nucleoscapsidnucleoscapsid..

- Some viruses have an Some viruses have an external coveringexternal covering called called envelopeenvelope; ;

those lacking an envelope are those lacking an envelope are nakednaked

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VirusesViruses


Capsids - EnvelopeCapsids - Envelope

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VirusesViruses


Two structural types:Two structural types:

-> -> helicalhelical - continuous helix of capsomers forming - continuous helix of capsomers forming a a

cylindrical nucleocapsidcylindrical nucleocapsid

-> -> icosahedralicosahedral - 20-sided with 12 corners - 20-sided with 12 corners- vary in the number of capsomers- vary in the number of capsomers

- Each capsomer may be made of 1 or several - Each capsomer may be made of 1 or several proteins.proteins.

- Some are enveloped.- Some are enveloped.

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VirusesViruses


helicalhelical

Influenza virus

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VirusesViruses


IcosahedralIcosahedral

AdenovirusHerpes simplex virus

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Viral envelopeViral envelope-> mostly -> mostly animal virusesanimal viruses

-> acquired when the virus leaves the host cell-> acquired when the virus leaves the host cell

-> exposed proteins on the outside of the envelope, called -> exposed proteins on the outside of the envelope, called

spikesspikes, essential for attachment of the virus to the host , essential for attachment of the virus to the host cellcell

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VirusesViruses

Function of Capsid/EnvelopeFunction of Capsid/Envelope

-> -> ProtectsProtects the nucleic acid when the virion the nucleic acid when the virion isis

outside the host celloutside the host cell

-> Helps to -> Helps to bindbind the virion the virion to a cell surfaceto a cell surface andand

assists the assists the penetration of the viral DNApenetration of the viral DNA or or

RNA into a suitable host cellRNA into a suitable host cell

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Viral MorphologyViral Morphology

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Complex viruses: atypical virusesComplex viruses: atypical viruses-> Poxviruses lack a typical capsid and are covered by -> Poxviruses lack a typical capsid and are covered by a a

dense layer of dense layer of lipoproteinslipoproteins..

-> Some -> Some bacteriophagesbacteriophages have a polyhedral have a polyhedral nucleocapsid nucleocapsid

along with a helical along with a helical tailtail and and attachment fibersattachment fibers..

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Nucleic AcidsNucleic Acids

Viral genomeViral genome – either DNA or RNA but never both – either DNA or RNA but never both

-> Carries -> Carries genes necessary to invade host cellgenes necessary to invade host cell and redirect and redirect

cell’s activity to cell’s activity to make new virusesmake new viruses

-> -> Number of genes is low -Number of genes is low - varies for each type of virus – varies for each type of virus – fewfew

to hundredsto hundreds

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VirusesViruses


DNA virusesDNA viruses -> usually -> usually double stranded (ds)double stranded (ds) but may be single but may be single stranded (ss)stranded (ss)-> circular or linear-> circular or linear

RNA virusesRNA viruses -> usually -> usually single strandedsingle stranded, may be , may be double double strandedstranded, , may be segmented into separate RNA piecesmay be segmented into separate RNA pieces-> ssRNA genomes ready for -> ssRNA genomes ready for immediate immediate translationtranslation are are positive-sense RNApositive-sense RNA..-> ssRNA genomes that must be -> ssRNA genomes that must be converted into converted into properproper formform are are negative-sense RNAnegative-sense RNA..

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VirusesViruses


Pre-formed enzymes may be present.Pre-formed enzymes may be present.

-> polymerases – DNA or RNA-> polymerases – DNA or RNA

-> replicases – copy DNA-> replicases – copy DNA

-> reverse transcriptase –synthesis of DNA from RNA -> reverse transcriptase –synthesis of DNA from RNA

(AIDS virus)(AIDS virus)

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VirusesViruses

How Viruses are classifiedHow Viruses are classified

-> Main criteria presently used are -> Main criteria presently used are structurestructure, , chemical chemical compositioncomposition, and , and genetic makeupgenetic makeup..

-> No taxa above -> No taxa above Family Family (no kingdom, phylum, etc.) (no kingdom, phylum, etc.)

-> Currently recognized: 3 orders, 63 families, and 263 -> Currently recognized: 3 orders, 63 families, and 263 genera genera of virusesof viruses

-> Family name ends in -viridae, i.e.Herpesviridae-> Family name ends in -viridae, i.e.Herpesviridae

-> Genus name ends in -virus, Simplexvirus-> Genus name ends in -virus, Simplexvirus

-> Herpes simplex virus I (HSV-I)-> Herpes simplex virus I (HSV-I)

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VirusesViruses

Modes of Viral MultiplicationModes of Viral Multiplication

General phases in General phases in animal virusanimal virus multiplication multiplication cycle:cycle:

-> -> AdsorptionAdsorption - binding of virus to specific molecule on - binding of virus to specific molecule on host host

cellcell-> -> PenetrationPenetration - genome enters host cell - genome enters host cell-> -> UncoatingUncoating – the viral nucleic acid is released from the – the viral nucleic acid is released from the

capsidcapsid-> -> SynthesisSynthesis – viral components are produced – viral components are produced-> -> AssemblyAssembly – new viral particles are constructed– new viral particles are constructed-> -> ReleaseRelease – assembled viruses are released by budding – assembled viruses are released by budding

(exocytosis) or cell lysis(exocytosis) or cell lysis

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VirusesVirusesModes of Viral MultiplicationModes of Viral Multiplication

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VirusesVirusesAdsorption and Host RangeAdsorption and Host Range

-> Virus coincidentally collides with a susceptible host -> Virus coincidentally collides with a susceptible host cellcell

and adsorbs specifically to and adsorbs specifically to receptor sitesreceptor sites on the cell on the cell

membranemembrane

-> Spectrum of cells a virus can infect – -> Spectrum of cells a virus can infect – host rangehost range -> hepatitis B – human liver cells-> hepatitis B – human liver cells

-> poliovirus – primate intestinal and nerve cells-> poliovirus – primate intestinal and nerve cells

-> rabies – various cells of many mammals-> rabies – various cells of many mammals

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VirusesVirusesAdsorptionAdsorption

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PenetrationPenetration

Flexible cell membrane is penetrated by the Flexible cell membrane is penetrated by the whole virus or its nucleic acid by:whole virus or its nucleic acid by:

-> -> endocytosisendocytosis – entire virus is engulfed and enclosed – entire virus is engulfed and enclosed in ain a

vacuole or vesiclevacuole or vesicle

-> -> fusionfusion – envelope – envelope merges directly with membranemerges directly with membrane

resulting in nucleocapsid’s entry into cytoplasmresulting in nucleocapsid’s entry into cytoplasm

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PenetrationPenetration

Endocytosis

Fusion

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Replication and Protein ProductionReplication and Protein Production

-> -> Varies Varies depending on whether the virus is a depending on whether the virus is a DNA or RNA DNA or RNA virusvirus

-> -> DNA virusesDNA viruses generally are replicated and assembled generally are replicated and assembled in in the the nucleus.nucleus.

-> -> RNA virusesRNA viruses generally are replicated and assembled generally are replicated and assembled in in the the cytoplasm.cytoplasm.

-> Positive-sense RNA contain the message for -> Positive-sense RNA contain the message for translation.translation.

-> Negative-sense RNA must be converted into positive--> Negative-sense RNA must be converted into positive-sense sense message.message.

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VirusesViruses

ReleaseRelease

Assembled viruses leave host cell in one of two ways:Assembled viruses leave host cell in one of two ways:

-> -> buddingbudding – exocytosis; nucleocapsid binds to membrane – exocytosis; nucleocapsid binds to membrane whichwhich pinches off and sheds the viruses gradually; cell is not pinches off and sheds the viruses gradually; cell is not immediately destroyedimmediately destroyed

-> -> lysislysis – nonenveloped and complex viruses released when – nonenveloped and complex viruses released when cellcell dies and rupturesdies and ruptures

Number of viruses released is variableNumber of viruses released is variable -> 3,000-4,000 released by poxvirus-> 3,000-4,000 released by poxvirus -> >100,000 released by poliovirus-> >100,000 released by poliovirus

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ReleaseRelease

Budding

Budding of HIV from T-cells

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Damage to Host CellsDamage to Host Cells

Cytopathic effectsCytopathic effects - virus-induced damage to cells: - virus-induced damage to cells:

1. Changes in size & shape1. Changes in size & shape

2. Cytoplasmic inclusion bodies2. Cytoplasmic inclusion bodies

3. Nuclear inclusion bodies3. Nuclear inclusion bodies

4. Cells fuse to form multinucleated cells.4. Cells fuse to form multinucleated cells.

5. Cell lysis5. Cell lysis

6. Alter DNA6. Alter DNA

7. Transform cells into cancerous cells7. Transform cells into cancerous cells

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Damage to Host CellsDamage to Host CellsCytopathic effectsCytopathic effects

Herpes simplex virus -> multinucleated giant cells multinucleated giant cells (skin cells)(skin cells)

Cytosolic inclusion bodies

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Damage to Host CellsDamage to Host Cells

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VirusesVirusesPersistent Infections Persistent Infections

-> -> Cells can maintain a carrier relationshipCells can maintain a carrier relationship

-> -> Persistent infectionsPersistent infections - cell harbors the virus and is not - cell harbors the virus and is not

immediately lysedimmediately lysed

-> -> Can last weeks or host’s lifetime; several can periodically Can last weeks or host’s lifetime; several can periodically

reactivate – reactivate – chronic latent statechronic latent state

- measles virus – may remain hidden in brain cells for many - measles virus – may remain hidden in brain cells for many yearsyears

- herpes simplex virus – cold sores and genital herpes- herpes simplex virus – cold sores and genital herpes

- herpes zoster virus – chickenpox and shingles- herpes zoster virus – chickenpox and shingles

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VirusesVirusesPersistent Infections Persistent Infections

-> -> Cells can maintain a carrier relationshipCells can maintain a carrier relationship

->-> Some animal viruses enter host cell and permanently Some animal viruses enter host cell and permanently alter alter its its

genetic materialgenetic material resulting in cancer – resulting in cancer – transformationtransformation of the of the cell.cell.

->-> Transformed cells have increased rate of growth, alterations Transformed cells have increased rate of growth, alterations inin

chromosomes, and capacity to divide for indefinite time chromosomes, and capacity to divide for indefinite time periods periods

resulting in resulting in tumorstumors..

->-> Mammalian viruses capable of initiating tumors are called Mammalian viruses capable of initiating tumors are called

oncovirusesoncoviruses..

- Papillomavirus – cervical cancer- Papillomavirus – cervical cancer

- Epstein-Barr virus – Burkitt’s lymphoma- Epstein-Barr virus – Burkitt’s lymphoma

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VirusesViruses

Bacteriophages Bacteriophages

->-> Bacteriophages Bacteriophages – – bacterial virusesbacterial viruses (phages) (phages)

-> Most widely studied are those that infect -> Most widely studied are those that infect Escherichia Escherichia colicoli – –

complex structure, complex structure, DNADNA

-> Multiplication goes through similar stages as animal -> Multiplication goes through similar stages as animal viruses. viruses.

-> Only the nucleic acid enters the cytoplasm - uncoating -> Only the nucleic acid enters the cytoplasm - uncoating is notis not

necessary.necessary.

-> Release is a result of -> Release is a result of cell lysiscell lysis induced by viral enzymes induced by viral enzymes and and

accumulation of viruses - accumulation of viruses - lytic cyclelytic cycle..

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Bacteriophages - Replication Bacteriophages - Replication

1. Adsorption1. Adsorption – binding of virus to – binding of virus to specific molecule on host cellspecific molecule on host cell

2. Penetration2. Penetration –genome enters host –genome enters host cellcell

3. Replication3. Replication – viral components – viral components producedproduced

4. Assembly - 4. Assembly - viral components viral components assembledassembled

5. Maturation5. Maturation – completion of viral – completion of viral formationformation

6. Release6. Release – viruses leave cell to infect – viruses leave cell to infect other cellsother cells

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Bacteriophages – Life Cycles Bacteriophages – Life Cycles

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VirusesVirusesBacteriophages Bacteriophages

Lysogeny: The Silent Virus InfectionLysogeny: The Silent Virus Infection

-> Not all phages complete the lytic cycle.-> Not all phages complete the lytic cycle.

-> Some DNA phages, called -> Some DNA phages, called temperate phagestemperate phages, undergo , undergo adsorptionadsorption and penetration but don’t replicate.and penetration but don’t replicate.

-> The viral genome inserts into bacterial genome and -> The viral genome inserts into bacterial genome and becomes anbecomes an inactive inactive prophageprophage - - the cell is not lysed.the cell is not lysed.

-> Prophage is retained and copied during normal cell division-> Prophage is retained and copied during normal cell division resulting in the transfer of resulting in the transfer of temperate phage genometemperate phage genome to all to all host host cell progeny – cell progeny – lysogenylysogeny..

->-> InductionInduction can occur resulting in activation of lysogenic can occur resulting in activation of lysogenic prophageprophage followed by viral replication and cell lysis.followed by viral replication and cell lysis.

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VirusesVirusesBacteriophages Bacteriophages

Lysogeny: The Silent Virus InfectionLysogeny: The Silent Virus Infection

-> Lysogeny results in the spread of the virus without killing -> Lysogeny results in the spread of the virus without killing the the host cell.host cell.

-> Phage genes in the bacterial chromosome can cause the -> Phage genes in the bacterial chromosome can cause the production of production of toxinstoxins or or enzymes that cause pathologyenzymes that cause pathology – – lysogeniclysogenic conversion.conversion.

- Corynebacterium diphtheriae- Corynebacterium diphtheriae - Vibrio cholerae- Vibrio cholerae - Clostridium botulinum- Clostridium botulinum

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Bacteriophages Bacteriophages

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PrionsPrions

PrionsPrions - misfolded proteins, - misfolded proteins,

contain no nucleic acidcontain no nucleic acid

-> cause transmissible spongiform -> cause transmissible spongiform encephalopathies - neurodegenerative encephalopathies - neurodegenerative diseases (fibrillation of proteins)diseases (fibrillation of proteins)

-> common in animals:-> common in animals:- scrapie in sheep & goats- scrapie in sheep & goats

- bovine spongiform encephalopathies (BSE), - bovine spongiform encephalopathies (BSE),

mad cow diseasemad cow disease

- humans – Creutzfeldt-Jakob Syndrome (CJS)- humans – Creutzfeldt-Jakob Syndrome (CJS)

!!! Extremely resistant to usual sterilization !!! Extremely resistant to usual sterilization techniques !!!techniques !!!

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VirusesVirusesDetection and Treatment of Animal Viral Detection and Treatment of Animal Viral

InfectionsInfections

-> More difficult than other agents-> More difficult than other agents

-> Consider overall clinical picture-> Consider overall clinical picture

-> Take appropriate sample -> Take appropriate sample - Infect cell culture- look for characteristic cytopathic effects- Infect cell culture- look for characteristic cytopathic effects

- Screen for parts of the virus- Screen for parts of the virus- Screen for immune response to virus (antibodies)Screen for immune response to virus (antibodies)

-> Antiviral drugs can cause serious side effects-> Antiviral drugs can cause serious side effects

1 Applied Microbiology 5 th lecture: Chapter 6. 2 Viruses.

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