Medical Microbio - lec 4 - Characterizing pathogens Viruses, Viroids, and Prions

Viruses

-minuscule

-acullular

-infectious agents have either DNA or RNA

-Cause infections of humans, animals, plants, and bacteria

-Uses the host cell's metabolic pathways for replication

Viruses - what do they not do

-Do not carry out any metabolic pathway

-Do not reproduce on their own

Viruses - intracellular and extracellular states

Extracellular - exist as virions

-composed of capsid nucleocapsid, envelop, and nucleic acid

Intracellular - exists as viral nucleic acids

Virions

-Viral particles outside the host cell

-Complete virus particles

-include a nucleic acid, a capsid, and in some cases an envelope

Viral genome

-DNA or RNA but never both

-can be single or double stranded (dsDNA, ssDNA, dsRNA, ssRNA)

-can be linear and segmented or single and circular

-much smaller than genomes of cells

-Theyre classified based on nature of the genome (DNA viruses, RNA viruses)

Hosts of viruses

-All types of organisms are susceptible to some virus

-Most viruses infect only particular hosts cells

-Some can only infect particular kind of cell in a host

-Others infect many kinds of cells or many different hosts

Hosts of viruses - why do most viruses only infect some hosts cells

-due to affinity of viral surface proteins for complementary proteins on host cell surface

Viral capsid

-houses viral nucleic acid (genome)

-protects nucleic acid

-made up of proteinaceous subunits (called capsomeres)

-capsomeres - can be made of single or multiple types of proteins.

-can also determine the shape of the virion

Viral capsid - non-enveloped viruses

-The capsid facilitates attachment and entry into the host's cell

Viral capsid - 3 basic viral shapes

-Helical

-Polyhedral

-Complex

Viral envelope

-Acquired from host cell during viral replication or release

-Envelope if portion membrane system of host

-Enveloped viruses are more fragile than naked viruses

Viral envelope - composition

-composed of phospholipid bilayer and proteins

-Some proteins are virally coded glycoproteins (spikes)

-The proteins and glycoproteins (of envelope) are what recognize hosts -- binding and entry into the host cell

Classification of viruses

-Type of nucleic acid

-Presence of an envelope

-Shape and size

Classification of viruses +ssRNA and -ssRNA

Positive sense (+ssRNA) - similar to mRNA and direct protein synthesis

Negative sense (-ssRNA) - complementary to mRNA, and cannot direct protein synthesis

Example of classification based on nucleic acid type

Viral nucleic acid

DNA or RNA

If DNA

Double stranded or single stranded

If RNA

Double stranded or single stranded

If single stranded

+sense or -sense

Viral replication

-viruses dependent on host's organelles and enzymes to produce new virions

-usually results in the death and lysis of host cell

-5 stages

Viral replication - 5 stages of viral replication cycle

-Attachment

-Entry

-Synthesis

-Assembly

-Release

Viral replication - Attachment of animal viruses

-chemical attraction between viral protein and cell receptor

-Animal viruses do no have tails or tail fibers

-Have glycoprotein spikes or other attachment molecules to make attachment happen

Viral replication - Entry and uncoating of animal viruses

Enters by at least 3 diff mechanisms:

-Direct penetration

-Membrane fusion

-Endocytosis

Viral replication - Synthesis of viral nucleic acid and proteins

-Each type of animal virus requires diff strategy depending on its nucleic acid

Viral replication - Synthesis - DNA vs RNA viruses

DNA viruses - often enter the nucleus

RNA viruses - often replicate in the cytoplasm

Viral replication - Synthesis - What to consider

How mRNA is synthesized

--- DNA -> mRNA, or RNA -> mRNA

And

What serves as a template for nucleic acid replication

--- DNA or RNA (ss or ds)

Viral replication - Synthesis - dsDNA viruses

-Similar to replication of cellular DNA

-replicated in the nucleus

-Each strand of DNA can act as a template for its complementary DNA strand.

-mRNA is synthesized by the host RNA polymerase

-Viral proteins are made in the cytoplasm

Viral replication - Synthesis - dsDNA viruses - some exceptions

Hepatitis B viruses replicate DNA from an RNA intermediary

Viral replication - Synthesis - ssDNA viruses

-Parvoviruses - have ssDNA genomes

-DNA strand folds back on itself to form dsDNA (that is then replicated by cellular DNA polymerase)

-Newly replicated strands released as ssDNA

Viral replication - Synthesis - synthesis of RNA viruses is diff than DNA virus replication

Positive sense (+) viral RNA can act as mRNA

Negative sense (-) viral RNA cannot be directly translated

Viral replication - Synthesis - 4 types of RNA viruses

- +ssRNA

- Retroviruses (+ssRNA)

- -ssRNA

- dsRNA

Viral replication - Synthesis - viral enzymes

-replication of RNA genome is performed by viral enzymes

-RNA-dependent RNA polymerase

-RNA-dependent DNA polymerase (aka reverse transcriptase)

Viral replication - Synthesis - retroviruses (+ssRNA virus)

-does not use its ssRNA as template to synthesize new viral genome or to direct translation of viral proteins

-Uses DNA intermediary (transcribed by viral reverse transcriptase) as template to produce viral genomes

Viral replication - Assembly and release of animal viruses

-Most DNA viruses assemble in nucleus

-Most RNA viruses develop solely in cytoplasm

-# of viruses produced depends on type of virus and size &initial health of host cells

Viral replication - Assembly and release - of naked viruses and enveloped viruses

Naked viruses (not enveloped) - are released by exocytosis or lysis

Enveloped viruses - released by budding out of the infected cell without killing it

-Enveloped viruses cause persistent infections

Viral replication - latency of animal viruses

-when viruses remain dormant in host cells (called latent viruses or proviruses)

-May be prolonged for years with no viral activity

-Incorporation of provirus into host DNA is permanent

Viruses in Cancer - normal cells and appearance of cancers

-Cell division is usually under strict genetic control in normal cells

-Neoplasia or cancers arise due to uncontrolled cell division (due to activation of oncogenes or tumour supressor genes)

Viruses in cancer

-Some viruses carry copies of oncogenes as part of genome

-Other can promote oncogenes already there in the host or interfere with tumor repression.

-20-25% of human cancers are associated with virus infections

Viruses in cancer - exs of human cancers associated with virus infection

Burkitt's lymphoma - Epstein-Barr Virus (EBV)

Hodgkin's disease - Epstein-Barr Virus (EBV)

Kaposi's sarcoma - Kaposi sarcoma-associated herpesvirus

Cervical cancer - Human papilloma virus (HPV)

Culturing viruses in the laboratory

-viruses cannot grow in standard microbiological media bc viruses require a host cell to replicate and produce new virions

Type of media for culturing animal viruses

Mature organism - ethical concerns and expensive to maintain lab animals

Embryonated eggs - inexpensive, sterile, nutrient rich medium

-fertilized chicken eggs are usually used (bc embryonic tissue provide ideal site for growing viruses)

-Used in preparation of some vaccines

Cell cultures - inexpensive and easy to maintain continuous culture

Other parasitic particles: viroids

-extremely small

-circular pieces of ssRNA (that are infectious and pathogenic in plants)

-Similar to RNA virus but lack capsid

Other parasitic particles: viroids RNA

-viroid RNA does not code for proteins

-adheres to complementary plant RNA

--Plant enzyme degrades the dsRNA

--Results in disease state

Other parasitic particles: Prions

-Prions are proteinaceous infectious agents

-Compare Cellular PrP (c-PrP)

to Prion PrP (p-PrP)

Other parasitic particles: Prions - Cellular PrP (c-PrP)

-made by all mammals

-normal, functional structure has alpha helices

-c-PrP function not well understood (but thought to be involved in normal synaptic development and function)

Other parasitic particles: Prions - Prion PrP (p-PrP)

-disease causing form has beta pleated sheets

-prions do not reproduce like other infections agents

Other parasitic particles: Prions - mechanism of infection

-p-PrP causes c-PrP to refold into p-PrP, increasing p-PrP

-p-PrP groups into multimers

---It then gets deposited as amyloid plaques and cause damage to infected tissue

Prion Disease

-spongiform encephalopathies

-Large vacuoles form in the brain (making it look spongy)

-No treatment for prion disease

Prion disease - transmission

-Transmitted by ingestion, transplantation, or contact of mucous membranes with infected tissue.

Prion disease - human diseases

Human prion diseases - Variant Creutzfeldt-Jakob disease (vCJD)

-kuru (fatal neurodegenerative disorder)

Prion disease - animal prion diseases

Bovine spongiform encephalopathy (BSE)

-aka "mad cow disease"

-scrapie (in sheep and goats)