Chapter 13 microbio

Viruses, Viroids, and Prions

General Characteristics of Viruses

Viruses are obligatory intracellular parasites that require living host cells to multiply. Unlike bacteria, viruses lack ribosomes and an ATP-generating mechanism, and they contain either DNA or RNA, but never both. Their replication depends entirely on the host cell's machinery.

• Obligatory intracellular parasites: Must invade host cells to reproduce.

• Genetic material: DNA or RNA, single- or double-stranded, linear or circular.

• Protein coat: Called a capsid, made of subunits called capsomeres.

• No ribosomes or ATP-generating mechanism: Cannot synthesize proteins or generate energy independently.

Example: Chlamydia and Rickettsia are bacteria that also require host cells for replication, but they differ from viruses in other fundamental ways.

Host Range

The host range of a virus refers to the spectrum of host cells it can infect. Most viruses are highly specific, infecting only certain cell types within a host, determined by specific attachment sites and cellular factors.

• Bacteriophages: Viruses that infect bacteria, ranging from 20 nm to 1000 nm in length.

• Examples: HIV infects CD4+ cells; Hepatitis C infects hepatocytes via LDL receptors.

Virus Sizes

Viruses are much smaller than most bacteria and cells, often below the resolution limit of light microscopes (about 200 nm).

Viral Structure

Virion Structure

A virion is a complete, fully developed viral particle. It consists of nucleic acid (DNA or RNA), a protein coat (capsid), and in some cases, an envelope and spikes.

• Nucleic acid: DNA or RNA, single- or double-stranded.

• Capsid: Protein coat made of capsomeres.

• Envelope: Lipid, protein, and carbohydrate coating present in some viruses.

• Spikes: Glycoprotein projections from the envelope, important for attachment.

Nonenveloped Polyhedral Virus

Polyhedral viruses have many-sided capsids, typically icosahedral. Nonenveloped viruses lack a lipid membrane and are more heat-resistant.

Enveloped Helical Virus

Helical viruses have a hollow, cylindrical capsid. Enveloped viruses possess a lipid membrane, making them sensitive to heat and environmental factors.

Complex Viruses

Complex viruses have additional structures attached to their capsids, combining features of both helical and polyhedral forms. Examples include bacteriophages and poxviruses.

Viroids

Viroids are the smallest infectious pathogens, composed solely of a short strand of circular, single-stranded RNA without a protein coat. They infect plants and cause various diseases.

Taxonomy of Viruses

Classification and Nomenclature

Viruses are classified by their genetic material, structure, and host range. Taxonomic names follow specific conventions:

• Order: Ends in –ales (e.g., Herpesvirales)

• Family: Ends in –viridae (e.g., Herpesviridae)

• Genus: Ends in –virus (e.g., Simplexvirus)

• Species: Group sharing genetic information and ecological niche; common names used.

• Subspecies: Designated by numbers (e.g., Herpes simplex virus 1, 2, 3)

Isolation, Cultivation, and Identification of Viruses

Growing Bacteriophages

Bacteriophages are grown in bacteria, forming plaques—clearings on a lawn of bacteria on agar. Each plaque corresponds to a single virus and can be quantified as plaque-forming units (PFU).

Growing Animal Viruses

Animal viruses can be cultured in living animals, embryonated eggs, or cell cultures.

• Embryonated eggs: Virus injected into various egg compartments; growth indicated by changes or embryo death.

• Cell cultures: Tissues treated with enzymes to separate cells; infected cells show cytopathic effects (CPE).

Viral Identification

Viruses are identified by cytopathic effects, serological tests (e.g., Western blotting), and nucleic acid techniques (e.g., RFLPs, PCR).

Viral Multiplication

One-Step Growth Curve

Viral replication follows a one-step growth curve, with an eclipse period followed by a rapid increase in virion numbers as they are released from the host cell.

Multiplication of Bacteriophages

Lytic Cycle

The lytic cycle results in lysis and death of the host cell. Steps include:

1. Attachment: Phage attaches to host cell.

2. Penetration: Phage injects DNA into host.

3. Biosynthesis: Production of phage DNA and proteins.

4. Maturation: Assembly of phage particles.

5. Release: Host cell lyses, releasing new virions.

Lysogenic Cycle

Phage DNA incorporates into host DNA as a prophage, remaining latent. When the host replicates, it also replicates the prophage DNA, which can later initiate a lytic cycle.

Multiplication of Animal Viruses

Animal virus replication involves:

1. Attachment: Virus binds to cell membrane.

2. Entry: By receptor-mediated endocytosis or fusion.

3. Uncoating: Viral or host enzymes remove capsid.

4. Biosynthesis: Production of nucleic acid and proteins.

5. Maturation: Assembly of viral components.

6. Release: By budding (enveloped viruses) or rupture.

Biosynthesis of DNA and RNA Viruses

Biosynthesis of DNA Viruses

DNA viruses replicate their DNA in the host nucleus using viral enzymes and synthesize capsid proteins in the cytoplasm using host enzymes.

• Adenoviridae: Double-stranded DNA, nonenveloped; causes respiratory infections.

• Poxviridae: Double-stranded DNA, enveloped; causes skin lesions.

• Herpesviridae: Double-stranded DNA, enveloped; includes viruses causing cold sores, chickenpox, mononucleosis, and Kaposi's sarcoma.

• Papovaviridae: Double-stranded DNA, nonenveloped; includes HPV, which can cause cancer.

• Hepadnaviridae: Double-stranded DNA, enveloped; includes Hepatitis B virus, uses reverse transcriptase.

Biosynthesis of RNA Viruses

RNA viruses multiply in the host cell's cytoplasm using RNA-dependent RNA polymerase.

• ssRNA (+) strand: Viral RNA serves as mRNA for protein synthesis.

• ssRNA (–) strand: Viral RNA is transcribed to a + strand to serve as mRNA.

• dsRNA: Double-stranded RNA viruses.

Biosynthesis of RNA Viruses That Use DNA

Retroviruses use reverse transcriptase to produce DNA from their RNA genome, which integrates into the host chromosome as a provirus.

• Retroviridae: Includes Lentivirus (HIV) and Oncoviruses.

Viruses and Cancer

Oncogenes and Transformed Cells

Some viruses cause cancer by integrating into host DNA and activating oncogenes, which drive uncontrolled cell growth. A transformed cell harbors tumor-specific antigens.

• Oncogenes: Genes that cause cells to grow and divide too quickly due to mutation (e.g., RAS, MYC, HER2, BCL-2, MUC16).

• Oncogenic viruses: Induce tumors by integrating into host DNA.

Latent and Persistent Viral Infections

Latent Viral Infections

Latent viruses remain in host cells without causing symptoms, but may reactivate. Examples include cold sores and shingles.

Persistent Viral Infections

Persistent infections progress slowly and are often fatal, such as subacute sclerosing panencephalitis (measles virus).

Prions

Characteristics and Pathogenesis

Prions are proteinaceous infectious particles, inherited and transmissible by ingestion, transplant, or surgical instruments. They cause spongiform encephalopathies, such as "mad cow disease" and Creutzfeldt-Jakob disease.

• PrPC: Normal cellular prion protein.

• PrPSc: Scrapie protein; accumulates in brain cells, forming plaques and causing cell death.

Example: PrPSc converts normal PrPC to the pathogenic form, leading to neurodegeneration.

Summary Table: Viruses vs. Bacteria

	Typical Bacteria	Rickettsias/Chlamydias	Viruses
Intracellular Parasite	No	Yes	Yes
Plasma Membrane	Yes	Yes	No
Binary Fission	Yes	Yes	No
Pass through Bacteriological Filters	No	Yes	Yes
Possess Both DNA and RNA	Yes	Yes	No
ATP-Generating Metabolism	Yes	No	No
Ribosomes	Yes	Yes	No
Sensitive to Antibiotics	Yes	Yes	No
Sensitive to Interferon	No	No	Yes

Additional info: This summary covers Chapter 13: Viruses, Viroids, and Prions, directly relevant to college-level Microbiology.