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MICR 201 Microbiology for Health Related Sciences

Lecture 6: Viruses, viroids, and prionsEdith Porter, M.D.

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Lecture Outline

Viruses▪ General characteristics▪ Viral structure▪ Taxonomy▪ Isolation, cultivation and identification▪ Viruses and disease

Viroids Prions

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Viruses

What is a Virus?

Virus is Latin for poison Initially, viral disease described as disease

that could be transmitted with poisonous fluid that had been passed through a “sterile” filter

Acellular infectious agent Exceptionally complex aggregation of non-

living chemicalsOR

Exceptionally simple living organism

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Size of viruses in comparion with bacteria and eukaryotic cells

Bacteria

Prions are not viruses

General characteristics of viruses Obligatory intracellular agents

Require a cellular host for proliferation Multiply inside living cells by using entirely the

synthesizing machinery of the cell Cause the synthesis of specialized structures that can

transfer the viral nucleic acid to other cells Most viruses infect only specific types of cells

in one host Host range is determined by specific host

attachment sites and cellular factors Contain single type of nucleic acid (DNA or RNA) Contain a protein coat Some are enclosed by an envelope Some viruses have spikes

Bacteria and viruses compared

Basic Viral Structure

Contain a single type of nucleic acid Either DNA or RNA Single or double stranded

Protein coat surrounding nucleic acid Capsid Composed of capsomer subunits Protects Vehicle for transmission

May contain a very limited number of enzymes

Virion: complete virus with nucleic acid and protein coat

Main Appearance Forms of Viruses

Polyhedral (spherical, icosahedral)

Helical (filamentous)

Complex

Additional virus structures

Spikes Glycoproteins

projecting from surface Can clump cells▪ Hemagglutination

Envelopes Originate mainly from

host membranes Lipid, protein,

carbohydrate Some virus encoded

proteins

Viral taxonomy

Based on nucleic acid composition, replication mode, morphology

Family names end in –viridae (Herpesviridae) Genus names end in –virus (Herpesvirus) Viral species: A group of viruses sharing the same

genetic information and ecological niche (host). Common names are used for species (Herpes simplexvirus)

Subspecies are designated by a number (Herpes simplexvirus 2)

Often abbreviated names HSV HIV CMV

Viral hosts

In principle, any cell can be infected by a virus

A particular virus can infect only a specific host cell type (receptor mediated entry) Prokaryotic cells▪ Bacterial viruses (phages)▪ Archaeal viruses

Eukaryotic cells▪ Fungal cells▪ Algae▪ Protozoa▪ Plant cell viruses▪ Insect cell viruses▪ Animal viruses

Fits only to specific structures (receptors) on the

host cell

Mastadenovirus

How to Culture Viruses?

Need a living host cell Bacteriophages

Bacteria Plant virus

Plant cells Animal viruses

Living animal Embryonated eggs▪ Large scale production

Cell culture▪ diagnostics

Viral cytopathic effects in cell culture Virus induced changes

of eukaryotic cell morphology Cell rounding

Cell aggregation

Inclusion bodies

Cell fusion

Transformation▪ Continous growth of

eukaryotic cell

Can be used for diagnostic

Normal

Cell rounding

Virus identification

Cytopathic effects Monkey kidney cells Foreskin cells Fibroblasts

Hemagglutination Serology (look for patient antibodies) PCR (polymerase chain reaction, detects

virus specific nucleic acid sequences) RFLP (restriction fragment polymorphism)

How to Quantify Viruses

Electron microscope Count

Plaque Assay Bacterial lawn Add virus Add agar on top to

immobilize virus Incubate Count plaques▪ lack of bacterial growth

where 1 virus had been

Viral multiplication : one-step growth curve

Eclipse is the period immediately after penetration during which not a single intact virus is present

In vitro, no new host cells are provided and number of virions decrease over time

Viral multiplication in bacteria Typically double

stranded DNA viruses Lytic cycle

Rapid large scale production of viruses

Host cell lysis and death

Lysogenic cycle Host cell survives Viral genome

incorporated into host cell genome

Replication with host cell No active virion

production

V

V

V

V V VVV V

VV

V

V

V

DEAD

V

Attachment Penetration Biosynthesis Maturation and

assembly Host lysis and virion

release

Lytic cycle of virus multiplication (1)

VV V

VV V

VV

VV

V

DEAD

V

Lytic cycle of virus multiplication (2)

Lysogenic cycle of virus multiplication

Attachment Penetration Phage DNA integrates into bacterial host

genome by recombination Virus now: prophage Host cell: lysogenic bacterium Lysogenic conversion: bacterium produces

virus encoded proteins ▪ Prevent superinfection with similar phage▪ Some are toxins (e.g. diphteria toxin by C.

diphteriae) Switch to lytic cycle

can be induced by UV light Specialized transduction: accidentally,

bacterial host DNA is cut out too

V

V

V

VV

Lysogenic and lytic cycle of bacteriophage l in E. coli

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Transduction

Virus serves as vector for bacterial DNA

During virus assembly a segment of bacterial DNA is accidentally packed into virus capsids Specialized transduction: a segment of

bacterial DNA along with the proper viral DNA

Generalized transduction: only bacterial DNA is packaged into the capsid

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Specialized transduction

Animal viruses

DNA or RNA viruses Single or double stranded Negative or positive sense Unique viral biosynthesis pathways

RNA viruses require enzymes not present in eukaryotic cells

Key steps in the multiplication of animal viruses

Attachment Entry Uncoating Biosynthesis

Early genes for replication Late genes for structural

elements Assembly (maturation) Release

Host rupture: non-enveloped viruses

Budding: enveloped viruses

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Entry and exit of animal viruses

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Bacteriophage and animal virus multiplication compared

Retroviruses

RNA viruses Include HIV Carry reverse transcriptase

RNA-dependent DNA polymerase Synthesize DNA from RNA Used in molecular biology (RT-PCR)

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Multiplication of a retrovirus

The course of viral diseases

http://pathcuric1.swmed.edu/PathDemo/gbp3/gbp340.jpg

Acute Infection Unspecific: fever, muscle and joint aches Specific: depend on target host cell

Latent Infections Virus retreats in host cells▪ Herpesviridae in neurons (fever blisters)

Persistent Infections Slow virus disease Gradual increase of symptoms▪ Subacute sclerosing panenecephalitis after measles infection

Cancer Chicken leukemia virus, Epstein Barr virus (lymphoma),

HPV (cervix carcinoma) HBV (liver cancer)

Examples for viral diseases Caused by various virus genera

Conjunctivitis Diarrhea Encephalitis “Flu” ▪ Influenza▪ SARS▪ Avian flu

Hepatitis

Diseases with virus specific symptoms Measles Rubella Herpes AIDS

Viroids

RNA only Short piece of naked

RNA RNA does not code for

protein Similarities between

introns and viroids Often found in plant

diseases

Prions

Protein only Proteinaceous infectious particle Infectivity can be reduced with protease treatment Infects central nervous system

Normal protein variant exists (PrPc) Prion protein (PrPsc) induces conformation change

of normal variant and aggregation Snow ball effect Damage in central nervous system due to loss of

cell function and inflammatory host response Neurological disease

Mad cow disease (with limited human transmission) Scrapie Creutzfeldt-Jakob disease

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Multiplication of prions

Important to remember

Viruses Acellular, requires a host cell to mulitply Protein capsid and 1 type of nucleic acid (RNA or

DNA) Spherical, helical and complex structure Can have envelopes and carry a few enzymes Lytic and lysogenic multiplication in bacteria Uncoating and budding is part of animal virus

multiplication Viroids

RNA only Plant diseases

Prions Protein only Neurological diseases