Applied Microbiology 5th lecture: Chapter 6

Viruses

General Structure of Viruses -> Size range – most <0.2 μm; requires electron microscope -> Virion – fully formed virus able to establish an infection

Form and Function of Viruses

General Structure of Viruses Capsids All viruses have capsids - protein coats that enclose and protect their nucleic acid. Each capsid is constructed from identical subunits called capsomers made of protein. The capsid together with the nucleic acid are nucleoscapsid. Some viruses have an external covering called envelope; those lacking an envelope are naked

General Structure of Viruses Capsids - Envelope

General Structure of Viruses Two structural types: -> helical - continuous helix of capsomers forming a cylindrical nucleocapsid -> icosahedral - 20-sided with 12 corners - vary in the number of capsomers - Each capsomer may be made of 1 or several proteins. - Some are enveloped.

General Structure of Viruses helical Influenza virus

General Structure of Viruses Icosahedral Herpes simplex virus Adenovirus

General Structure of Viruses Viral envelope -> mostly animal viruses -> acquired when the virus leaves the host cell -> exposed proteins on the outside of the envelope, called spikes, essential for attachment of the virus to the host cell

Function of Capsid/Envelope Viruses Function of Capsid/Envelope -> Protects the nucleic acid when the virion is outside the host cell -> Helps to bind the virion to a cell surface and assists the penetration of the viral DNA or RNA into a suitable host cell

Viruses Viral Morphology

General Structure of Viruses Complex viruses: atypical viruses -> Poxviruses lack a typical capsid and are covered by a dense layer of lipoproteins. -> Some bacteriophages have a polyhedral nucleocapsid along with a helical tail and attachment fibers.

General Structure of Viruses Nucleic Acids Viral genome – either DNA or RNA but never both -> Carries genes necessary to invade host cell and redirect cell’s activity to make new viruses -> Number of genes is low - varies for each type of virus – few to hundreds

General Structure of Viruses DNA viruses -> usually double stranded (ds) but may be single stranded (ss) -> circular or linear RNA viruses -> usually single stranded, may be double stranded, may be segmented into separate RNA pieces -> ssRNA genomes ready for immediate translation are positive-sense RNA. -> ssRNA genomes that must be converted into proper form are negative-sense RNA.

General Structure of Viruses Pre-formed enzymes may be present. -> polymerases – DNA or RNA -> replicases – copy DNA -> reverse transcriptase –synthesis of DNA from RNA (AIDS virus)

How Viruses are classified -> Main criteria presently used are structure, chemical composition, and genetic makeup. -> No taxa above Family (no kingdom, phylum, etc.) -> Currently recognized: 3 orders, 63 families, and 263 genera of viruses -> Family name ends in -viridae, i.e.Herpesviridae -> Genus name ends in -virus, Simplexvirus -> Herpes simplex virus I (HSV-I)

How Viruses are classified

How Viruses are classified

Modes of Viral Multiplication Viruses Modes of Viral Multiplication General phases in animal virus multiplication cycle: -> Adsorption - binding of virus to specific molecule on host cell -> Penetration - genome enters host cell -> Uncoating – the viral nucleic acid is released from the capsid -> Synthesis – viral components are produced -> Assembly – new viral particles are constructed -> Release – assembled viruses are released by budding (exocytosis) or cell lysis

Modes of Viral Multiplication Viruses Modes of Viral Multiplication

Adsorption and Host Range Viruses Adsorption and Host Range -> Virus coincidentally collides with a susceptible host cell and adsorbs specifically to receptor sites on the cell membrane -> Spectrum of cells a virus can infect – host range -> hepatitis B – human liver cells -> poliovirus – primate intestinal and nerve cells -> rabies – various cells of many mammals

Viruses Adsorption

Viruses Penetration Flexible cell membrane is penetrated by the whole virus or its nucleic acid by: -> endocytosis – entire virus is engulfed and enclosed in a vacuole or vesicle -> fusion – envelope merges directly with membrane resulting in nucleocapsid’s entry into cytoplasm

Viruses Penetration Endocytosis Fusion

Replication and Protein Production Viruses Replication and Protein Production -> Varies depending on whether the virus is a DNA or RNA virus -> DNA viruses generally are replicated and assembled in the nucleus. -> RNA viruses generally are replicated and assembled in the cytoplasm. -> Positive-sense RNA contain the message for translation. -> Negative-sense RNA must be converted into positive-sense message.

Viruses Release Assembled viruses leave host cell in one of two ways: -> budding – exocytosis; nucleocapsid binds to membrane which pinches off and sheds the viruses gradually; cell is not immediately destroyed -> lysis – nonenveloped and complex viruses released when cell dies and ruptures Number of viruses released is variable -> 3,000-4,000 released by poxvirus -> >100,000 released by poliovirus

Viruses Release Budding Budding of HIV from T-cells

Viruses Damage to Host Cells Cytopathic effects - virus-induced damage to cells: 1. Changes in size & shape 2. Cytoplasmic inclusion bodies 3. Nuclear inclusion bodies 4. Cells fuse to form multinucleated cells. 5. Cell lysis 6. Alter DNA 7. Transform cells into cancerous cells

Viruses Damage to Host Cells Cytopathic effects Herpes simplex virus -> multinucleated giant cells (skin cells) Cytosolic inclusion bodies

Viruses Damage to Host Cells

Persistent Infections -> Cells can maintain a carrier relationship Viruses Persistent Infections -> Cells can maintain a carrier relationship -> Persistent infections - cell harbors the virus and is not immediately lysed -> Can last weeks or host’s lifetime; several can periodically reactivate – chronic latent state - measles virus – may remain hidden in brain cells for many years - herpes simplex virus – cold sores and genital herpes - herpes zoster virus – chickenpox and shingles

Persistent Infections -> Cells can maintain a carrier relationship Viruses Persistent Infections -> Cells can maintain a carrier relationship -> Some animal viruses enter host cell and permanently alter its genetic material resulting in cancer – transformation of the cell. -> Transformed cells have increased rate of growth, alterations in chromosomes, and capacity to divide for indefinite time periods resulting in tumors. -> Mammalian viruses capable of initiating tumors are called oncoviruses. - Papillomavirus – cervical cancer - Epstein-Barr virus – Burkitt’s lymphoma

Viruses Bacteriophages -> Bacteriophages – bacterial viruses (phages) -> Most widely studied are those that infect Escherichia coli – complex structure, DNA -> Multiplication goes through similar stages as animal viruses. -> Only the nucleic acid enters the cytoplasm - uncoating is not necessary. -> Release is a result of cell lysis induced by viral enzymes and accumulation of viruses - lytic cycle.

Bacteriophages - Replication Viruses Bacteriophages - Replication 1. Adsorption – binding of virus to specific molecule on host cell 2. Penetration –genome enters host cell 3. Replication – viral components produced 4. Assembly - viral components assembled 5. Maturation – completion of viral formation 6. Release – viruses leave cell to infect other cells

Bacteriophages – Life Cycles Viruses Bacteriophages – Life Cycles

Bacteriophages Lysogeny: The Silent Virus Infection Viruses Bacteriophages Lysogeny: The Silent Virus Infection -> Not all phages complete the lytic cycle. -> Some DNA phages, called temperate phages, undergo adsorption and penetration but don’t replicate. -> The viral genome inserts into bacterial genome and becomes an inactive prophage - the cell is not lysed. -> Prophage is retained and copied during normal cell division resulting in the transfer of temperate phage genome to all host cell progeny – lysogeny. -> Induction can occur resulting in activation of lysogenic prophage followed by viral replication and cell lysis.

Bacteriophages Lysogeny: The Silent Virus Infection Viruses Bacteriophages Lysogeny: The Silent Virus Infection -> Lysogeny results in the spread of the virus without killing the host cell. -> Phage genes in the bacterial chromosome can cause the production of toxins or enzymes that cause pathology – lysogenic conversion. - Corynebacterium diphtheriae - Vibrio cholerae - Clostridium botulinum

Viruses Bacteriophages

Viruses Prions Prions - misfolded proteins, contain no nucleic acid -> cause transmissible spongiform encephalopathies - neurodegenerative diseases (fibrillation of proteins) -> common in animals: - scrapie in sheep & goats - bovine spongiform encephalopathies (BSE), mad cow disease - humans – Creutzfeldt-Jakob Syndrome (CJS) !!! Extremely resistant to usual sterilization techniques !!!

Detection and Treatment of Animal Viral Infections Viruses Detection and Treatment of Animal Viral Infections -> More difficult than other agents -> Consider overall clinical picture -> Take appropriate sample - Infect cell culture- look for characteristic cytopathic effects - Screen for parts of the virus Screen for immune response to virus (antibodies) -> Antiviral drugs can cause serious side effects