Viruses

Not Composed of Cells

Characteristics

• Obligate intracellular parasites

• Single type of nucleic acid

• Protein coat

• Envelope

Two Forms

• Extracellular form

• Intracellular form

Host Range

• Can infect many hosts

• Determined by attachment to host cell– cell wall, flagella/fimbriae

• Receptor sites, or plasma membrane in animal cells

Structure

• Nucleic acid-core

• Either DNA or RNA, not both

• Follows central dogma of molecular biology– Genetic info flows from NA to protein

Structure

• Protein coat-capsid

• Envelope in some virions– May or may not have spikes-glycoproteins

Envelope

• Acquire membrane when budding or pass through membranes

• Advantages of lipid membrane

• Lose infectivity when envelope destroyed

Envelope

• Host’s phospholipids & viral proteins

• Disadvantage -damaged easily

• Without envelope- naked viruses– More resistant to chemicals/ disinfectants

Enzymes

• Required early in infection process

• Bacterial virus or bacteriophage

– lysozyme • Lysis of cell and release of virions

Enzymes

• Some have own NA polymerase

• RNA polymerase in some RNA virions

• Reverse transcriptase in retro viruses– RNA dependent DNA polymerase

• Neuramidases-release of virions

Morphology

• Helical

• Polyhedral- shape of icosohedron-20 sides

• Enveloped- usually spherical

Morphology

• Complex viruses– combination of helical and icosohedral– bacterial viruses-head and tail– poxviruses- several coats of protein

Growth of Bacteriophage

• Grow in suspensions of bacteria or in bacteria cultures on plate

• Plaque method for counting

Growth of Animal Viruses

• Living animals

• Embryonated eggs-influenza

• Cell cultures-continuous lines

  

CPE Cytopathic Effect

• Visible effect of viruses on cells

• Stop multiplication of cells

• Lysosomes release enzymes

• Inclusion bodies

• Syncytium-

• Interferons-

CPE

• Mark infected cells for destruction by immune system

• Transformation-abnormal cells

Multiplication of Bacteriophage

• Lytic cycle: produces virions

• T-even phages, virulent phages, on E. coli– ds DNA for over 100 genes-head–   tail sheath-retracts– DNA moves from head into host–

Lytic Cycle

• Attachment stage (adsorption)– attachment site on virus with complementary

receptor on bacteria cell wall

– use fibers at end of tail as attachment sites– may attach to flagella or fimbriae

Penetration

• Injects DNA

• Tail releases enzyme lysozyme

• Tail core driven through cell wall

• Tail reaches cell membrane

• Capsid remains outside: uncoating

Biosynthesis

• In cytoplasm

• Host protein synthesis is stopped

• Uses host nucleotides and enzymes to synthesis copies of phage DNA

Biosynthesis

• Synthesis of phage capsid proteins

• Uses host ribosomes and amino acids for translation

Maturation

• Assemble into mature phages

• Head assembled and packed with DNA

• Phage tails assembled from plates, sheaths,

• Each head attached to tail

• Then fibers are attached

Release

• Lysis of PM, cell breaks open

• Virulent (lytic) phages

One Step Growth Curve

• One step growth curve

• Always present are mutant bacteria with altered receptors

Lysogenic Cycle

• Temperate phages do not always under go lytic cycle:

• Lysogeny-

• Phage NA incorporated into the host NA

• Lambda phage in E. coli– Integrates into bacterial chromosome– Prophage

Lysogenic Cycle

• Prophage replicated along with host DNA

• On rare event- can lead to popping out of phage DNA

Lysogenic Conversion

• Alteration of characteristics of bacteria

• Cells are immune to reinfection by same phage

•  Not immune to infection by different phage

Phage Conversion

• Medical significance of conversion

• C. diphtheriae and C. botulinum

• Without prophage do not cause disease

• Strep with prophage can cause scarlet fever

Specialized Transduction

  Mediated by lysogenic phage ( only temperate virions)–

DNA on either side of prophage can be picked up

  Phage lambda picks up gene for galactose fermentation-gal from host

 Carry this gene to new host which is gal negative

Multiplication of Animal Viruses

• Attachment– receptor sites on animal cells - proteins and

glycoproteins of PM

– sites are distributed all over surface of virus• Spikes or capsid

Penetration

• Trigger endocytosis-folding inward of PM-vesicle

• Enveloped viruses also can fuse with PM– Fusion protein facilitates this– Releases capsid via endocytosis

Uncoating-Removal of Capsid

• Varies with virus

• Separation of NA and protein coat– lysosomal enzymes inside vesicles

• Some enzymes in host cytoplasm

Biosynthesis of DNA viruses

• DNA viruses replicate DNA in nucleus of host

• Synthesize proteins in cytoplasm

• Early transcription- for enzymes & proteins needed for viral DNA replication

• Late transcription-capsid & structural proteins

Maturation

• Assembly of virus– Takes place in nucleus– Proteins transported via ER into nucleus

• Released from host cell– Budding – Lysis

Biosynthesis of RNA Viruses

• Multiply in host cell’s cytoplasm

• Picornavirus-polio, ss RNA,

• RNA is a sense strand or positive since it acts as mRNA

• Early translation-2 proteins– Inhibits host cell synthesis of RNA & protein– Produces RNA-dependent RNA polymerase

Biosynthesis of SS RNA Virus

 Synthesizes another strand of RNA-antisense strand or negative strand– Serves as template for all + strands

• Late transcription and translation -proteins for capsids

Rhabdoviruses

• Rabies, bullet shaped

• Contains a single minus strand and RNA dependent RNA polymerase – makes + strands from minus strand

• + strand serves as mRNA for new viral RNA and for proteins

Maturation and Release

• Assemble capsids spontaneously

• Enveloped viruses– proteins in envelope encoded by viral genes – envelope wraps around capsid -budding

• Lipids and CH2O encoded by host cell

• Noneveloped viruses released via rupture

Retrovirus

• HIV

– Positive strand RNA virus

– Own RNA polymerase

– RNA dependent DNA polymerase

– Reverse transcriptase

– RNA to DNA

Provirus

• Viral DNA incorporated into host DNA –provirus

• Never comes out of host chromosome

• Protected from host’s immune system and antiviral drugs

Retrovirus Replication

• Provirus may remain in latent state replicating with host DNA OR

• Provirus may be expressed and produces new viruses

Consequences of Virus Infection

• Lytic infection: destruction of host cell– Acute infection-influenza

• Persistent infection : slow release of virions – Budding without lyzing cell

Consequences of Virus Infection

• Latent infections: delay between infection and lytic events

• Transformation: change in cell

Herpes Viruses

• Large, enveloped, latent

• Herpes type 1 and 2 – Cold sores, genital and neonatal herpes

– Varicella zoster- chickenpox, and shingles

• EBV-mononucleosis

Herpes Viruses

• CMV– Salivary gland virus-acute febrile illness, birth

defects

• Roseolovirus (6)– Infants with rash and fever

• HHV-7 rashes in infants

• HHV-8 Kaposi’s sarcoma

Transformation

• Normal cells become tumor cells– Benign and malignant

• Oncogene-cancer gene

Oncogenes

• Genes always turned on– Continuous cell division

• Activated to abnormal functioning by chemicals, radiation and viruses

• Loss of control of cell cycle– Result in formation of tumors

Oncogenic viruses

• 10- 20% of cancers known to be virus induced

• Oncogenic viruses incorporate into host DNA– Cells lack contact inhibition

DNA Oncogenic Viruses

• EBV- herpes virus, causes 2 human cancers– Burkitt’s lymphoma ( rare affecting children in Africa)– Nasopharyngeal cancer is worldwide

• 90% of population carry latent stage of EBV in lymphocytes

• Hepatitis B virus has casual role in liver cancer• Papilloma virus- can cause cervical & penile

cancer-vaccine

RNA Viruses

• Human T cell leukemia viruses

Prions

• Proteinaceous infectious particle – Lacks nucleic acid

• Degenerative changes in brain-large vacuoles– Dementia, wasting, & loss of motor control

Prions-CJD

• Modified forms of normal cellular proteins

• Cause disease by converting normal protein into abnormal forms

CJD

• Transmission– Ingestion of contaminated food

– Sporadic cases

– Contaminated surgical instruments• Neural electrodes or forceps etc.

Influenza

• 8 segments of RNA as genome: minus strands

• Protein capsid

• Envelope with projections (virus can change these so it survives each year)

Spikes

• H spikes -hemagglutinin (binds to host receptors)– Recognize cells and attach

• 100 N spikes -neuraminidase

– Release virus from infected cell

Influenza

• Spread by droplets-use regular mask

• HA bind to ciliated respiratory cells

• Envelope fuses with PM and enters cells

S & S

• Release of cytokines

• Incubation period- 1-3 days, spread day before symptoms

• Ill 7-10 days

Recovery

• Spontaneous

• Secondary infections

• Death

Strains of A Viruses

• Strains H1,H2, H3; N1 and N2

• Antigenic shift-responsible for outbreaks– Reassortment

Antigenic Shift (continued)

• Genetic reassortment

• 2 viral strains infect the same animal/human

• Swine can be infected by both human & avian strains-mixing vessel

Antigenic Shift (continued)

• New virions released from swine

• Must have a full complement of the RNA segments to be infective

• May occur every 10 years or more

Antigenic Drift

• Minor annual variations in genetic make-up of HA or NA

• RNA enzymes lack proofreading capability

H1N1 Virus• Originated in swine

• Virulence similar to seasonal influenza viruses

• Humans have little or no immunity

Healthcare Worker PPE

• CAL OSHA requirement for H1N1• Wear N95 mask

–Must be fit-tested for mask–Wear a visor over mask

–Wear gown & gloves

Vaccination• Get vaccinated yearly

• Seasonal vaccine

–2 types of strains of A: H1N1 & H3N2

–1 B strain

• H1N1 available

Antiviral Drugs for H1N1• Sensitive to neuraminidase inhibitors

–Tamiflu–Relenza

• Prevents virus from leaving cell

• Benefit if started within 48 hours of illness onset

Influenza Pandemics in the

20th Century

1918-1919 “Spanish” Type A (H1N1) 675,000 US

1957-1958 “Asian” Type A (H2N2) 70,000 US

1968-1969 “Hong Kong” Type A (H3N2) 34,000 US

2009 “Swine” Type A (H1N1) 8000 US

Years Flu Virus Deaths

Pandemic Influenza

• Pandemic influenza virus– A new influenza A subtype can infect humans – Causes serious illness– Spreads easily from human-to-human

H5N1 is a likely candidate, but is not a pandemic virus yet

H1N1 is a pandemic virus

AIDS

• Final stage of a long infection with HIV– Attacks immune system

– T helper cells and macrophages

2 Types

• HIV 1-99% of all global cases

• HIV 2 discovered in West Africa– Reduced virulence – Causes milder disease

Structure of HIV

• 2 positive strands of RNA

• 2 identical strands of RNA enzyme- reverse transcriptase – Copies RNA into DNA

• Envelope with spikes termed gp120- glycoprotein of 120,000 mw

Pathogenicity

• Spikes allow attachment to CD4 receptors on host cells

• Coreceptors on T helper cells needed also for attachment

Fusion Protein

• Fuses CM with viral envelope– Nucleocapsid enters cell– Uncoated to release enzyme and RNA

Life Cycle

• RNA plus strands used for template only

• Viral DNA incorporates into host DNA   Integrase enzyme (viral) joins viral DNA

with cellular DNA

• May be latent or cause disease

Protease

• Another enzyme in viral core

• Budding virus is not mature yet

• Proteins in core are in one long strand

• Must be cut by protease then virus is infectious

Mutations

• Rapid antigenic mutations

• Mutations at every position in genome many times each day

AIDS

• Progression to AIDS– based on T cell population

• Progression from HIV to AIDS is about 10 years

HIV Transmission• Contact with infected body fluids

• Blood-

• Semen -

• Heterosexual sex fastest growing risk factor

• Drug use and multiple partners

• Mother to baby, breast feeding

Treatment: Highly Active Anti Retrovirus Therapy

•  Nucleoside analogs-AZT etc. inhibit reverse transcriptase

• Proteases- enzyme that cuts proteins into pieces reassembled into coat of new HIV particles

• Integrase inhibitors- enzyme that incorporates viral DNA into DNA of host

• Fusion inhibitors