Microbiology Chapter on Viruses

Learning Objectives

• Differentiate a virus from a bacterium
• Describe the chemical and physical structure of enveloped and nonenveloped viruses
• Define viral species
• Provide an example of a family, genus, and common name for a virus
• Describe how bacteriophages are cultured
• Describe how animal viruses are cultured
• List three techniques used to identify viruses
• Describe the lytic cycle of T-even bacteriophages
• Describe the lysogenic cycle of bacteriophage lambda
• Compare and contrast the multiplication cycle of DNA and RNA-containing animal viruses
• Define oncogene and transformed cell
• Discuss the relationship between DNA- and RNA-containing viruses and cancer

Check Your Understanding

• How did the small size of viruses aid in their detection before electron microscopes?
• Diagram a non-enveloped polyhedral virus with spikes
• Differentiate virus species from bacterial species
• Provide the correct family and genus for HPV, along with a completed name for example.
• Explain the plaque method
• Explain why continuous cell lines are more practical for culturing viruses than primary cell lines
• Explain tests that can identify influenza virus
• Explain how bacteriophages obtain nucleotides and amino acids without metabolic enzymes
• Describe lysogeny in Vibrio cholerae
• Describe the principal events of enveloped DNA virus attachment, entry, uncoating, biosynthesis, maturation, and release
• Define provirus
• Explain how an RNA virus can cause cancer without DNA insertion

Viral Structure

• A virion is a complete viral particle composed of nucleic acid surrounded by a coat
• Viruses contain either DNA or RNA (never both), which can be single or double stranded, linear or circular, or divided into multiple molecules
• The proportion of nucleic acid relative to protein in viruses ranges from 1% to 50%
• The protein coat surrounding the nucleic acid is called a capsid, which is composed of subunits called capsomeres
• Some viruses have an envelope consisting of lipids, proteins, and carbohydrates surrounding their capsid
• Some envelopes contain spike like structures (carbohydrate-protein complexes)
• Helical viruses have a hollow cylinder shape
• Polyhedral viruses have a many-sided structure (often icosahedral)
• Enveloped viruses have a pleomorphic spherical shape

Growing Bacteriophages/Animal Viruses

• Bacteriophages are grown using a plaque method involving mixing them with host bacteria and nutrient agar
• Animal viruses are sometimes grown in whole animals, or embryonated eggs, or in cell cultures (primary cell lines/embryonic diploid cell lines)
• Viral growth can sometimes be seen as cytopathic effects in cell cultures

Viral Identification

• Viral identification often involves serological tests
• Viruses/proteins can be identified via PCR and RFLPs

Viral Multiplication

• Viruses lack the enzymes necessary for energy production and protein synthesis; viral multiplication relies on the host cell's machinery
• Lytic cycle: Phage causes lysis and death of the host cell
• Lysogenic cycle: Phage DNA is incorporated into the host cell's DNA as a prophage.
• Animal virus multiplication: Viruses attach, enter, uncoat, replicate, assemble & release
• DNA viruses generally replicate in the nucleus, while RNA viruses typically do so in the cytoplasm
• RNA viruses can include RNA dependent RNA polymerase

Viruses and Cancer

• Oncogenic viruses cause cancer
• Some DNA viruses are oncogenic, like those within the Adenoviridae, Herpesviridae, Poxviridae, Papovaviridae families
• EBV is a herpesvirus that can cause Burkitt's lymphoma and nasopharyngeal carcinoma
• Hepadnavirus are associated with liver cancer
• Retroviruses are also oncogenic

Latent and Persistent Infections

• Latent viral infection: The virus remains in the host cell without producing infection, like cold sores or shingles
• Persistent viral infection: Virus occurs over a long period and is fatal

Prions

• Infectious proteins that cause degeneration of brain tissue (e.g., CJD, mad cow disease)
• Result from altered proteins (either a mutation or interaction w/ altered protein)

Plant Viruses and Viroids

• Plant viruses enter hosts via wounds or parasites (like insects)
• Viroids are infectious RNA molecules that cause plant diseases (e.g., potato spindle tuber disease)

Clinical Applications

• Viruses can be studied by using tissue cultures and animal models.
• Virus identification is often done using serological tests.
• There are numerous viruses, and viral infection can cause a wide range of diseases or symptoms that are sometimes detected (e.g., hepatitis viruses and encephalitis).