Comprehensive Study Notes on Viruses, Viroids, and Prions

Viruses, Viroids, and Prions

Definitions and Key Concepts

Understanding the basic definitions and distinctions among viruses, viroids, prions, and bacteriophages is essential for microbiology students.

• Virus: An acellular infectious agent composed of genetic material (DNA or RNA) surrounded by a protein coat (capsid), and in some cases, a lipid envelope. Viruses require a host cell to replicate.

• Viroid: A small, circular, single-stranded RNA molecule that infects plants. Viroids lack a protein coat and do not encode proteins.

• Prion: An infectious protein particle that lacks nucleic acid. Prions cause neurodegenerative diseases by inducing abnormal folding of normal cellular proteins, especially in the brain.

• Bacteriophage: A virus that infects bacteria. Bacteriophages have complex structures, often with a head (capsid), tail, and tail fibers, and play a significant role in bacterial genetics and ecology.

Example: The T4 bacteriophage infects Escherichia coli and is a model organism for studying viral replication.

Structure and Function of Bacteriophages

• Head (Capsid): Contains the viral nucleic acid (DNA or RNA).

• Tail: Used for attachment to the bacterial cell and injection of genetic material.

• Tail Fibers: Recognize and bind to specific receptors on the bacterial surface.

Enveloped vs. Naked Viruses

Viruses are classified based on the presence or absence of a lipid envelope surrounding the capsid.

• Enveloped Viruses: Possess a lipid bilayer derived from the host cell membrane. The envelope contains viral glycoproteins essential for host cell recognition and entry.

• Naked Viruses: Lack an envelope and consist only of the nucleic acid and capsid. They are generally more resistant to environmental stresses.

Example: Herpes simplex virus is enveloped, while adenovirus is a naked virus.

Lytic and Lysogenic Cycles

Bacteriophages and some animal viruses can follow two main replication cycles:

• Lytic Cycle: The virus takes over the host cell machinery to produce new virions, leading to cell lysis and release of progeny viruses.

• Lysogenic Cycle: The viral genome integrates into the host genome (as a prophage) and replicates along with it without causing immediate lysis. Environmental triggers can induce the lytic cycle.

Example: Lambda phage can alternate between lytic and lysogenic cycles in E. coli.

Persistent Viral Infections

• Persistent Infection: The virus remains in the host for extended periods, often with periods of latency and reactivation.

• Examples: Herpesviruses (e.g., HSV, VZV) can establish lifelong latent infections with periodic reactivation.

Properties of Seven Families of DNA Viruses

The seven major families of DNA viruses differ in structure, diseases caused, transmission, and diagnostic methods.

Family	Diseases	Transmission	Diagnosis
Adenoviridae	Respiratory infections, conjunctivitis	Respiratory droplets, fecal-oral	PCR, antigen detection
Herpesviridae	Herpes simplex, varicella-zoster, CMV	Direct contact, respiratory droplets	Serology, PCR
Poxviridae	Smallpox, molluscum contagiosum	Direct contact, fomites	Electron microscopy, PCR
Hepadnaviridae	Hepatitis B	Blood, sexual, perinatal	Serology, PCR
Parvoviridae	Erythema infectiosum (Fifth disease)	Respiratory droplets	Serology, PCR
Polyomaviridae	Progressive multifocal leukoencephalopathy	Respiratory, urine	PCR
Papillomaviridae	Warts, cervical cancer	Direct contact, sexual	PCR, cytology (Pap smear)

Additional info: Table entries inferred from standard virology sources.

Enteroviruses

• Definition: Enteroviruses are a genus of positive-sense single-stranded RNA viruses in the family Picornaviridae.

• Diseases: Poliomyelitis, hand-foot-and-mouth disease, viral meningitis.

• Transmission: Fecal-oral route.

Viruses Causing Hepatitis (A, B, C, D, E)

Virus	Family	Genome	Transmission	Chronicity
Hepatitis A	Picornaviridae	ssRNA (+)	Fecal-oral	No
Hepatitis B	Hepadnaviridae	dsDNA	Blood, sexual, perinatal	Yes
Hepatitis C	Flaviviridae	ssRNA (+)	Blood, sexual	Yes
Hepatitis D	Deltavirus	ssRNA (-)	Blood, sexual	Yes (requires HBV)
Hepatitis E	Hepeviridae	ssRNA (+)	Fecal-oral	No

Additional info: Table entries inferred from standard virology sources.

Diseases Caused by DNA or RNA Virus Families

• DNA Viruses: Examples include smallpox (Poxviridae), herpes simplex (Herpesviridae), warts (Papillomaviridae).

• RNA Viruses: Examples include influenza (Orthomyxoviridae), rubella (Togaviridae), Ebola (Filoviridae), hand-foot-and-mouth disease (Picornaviridae).

Selected Viruses and Associated Diseases

• Rubella: Caused by Rubella virus (Togaviridae); leads to German measles, a mild rash illness but can cause congenital rubella syndrome if contracted during pregnancy.

• Poxviruses: Includes variola virus (smallpox) and molluscum contagiosum; characterized by large, complex, enveloped DNA viruses.

• Herpesviruses: Includes HSV-1, HSV-2, VZV, CMV, EBV; cause oral/genital herpes, chickenpox, mononucleosis, and more.

• Filoviruses: Includes Ebola and Marburg viruses; cause severe hemorrhagic fevers with high mortality rates.

• Influenza: Caused by influenza viruses (Orthomyxoviridae); characterized by fever, cough, and myalgia; undergoes antigenic drift and shift.

• Hand, Foot, and Mouth Disease: Caused primarily by Coxsackievirus A16 (Picornaviridae); presents with fever, oral ulcers, and vesicular rash on hands and feet.

Retroviruses: Oncogenic and Immunosuppressive

• Retroviruses: Enveloped, single-stranded RNA viruses that replicate via a DNA intermediate using reverse transcriptase.

• Oncogenic Retroviruses: Can induce cancer by integrating into host genome and activating oncogenes (e.g., Human T-cell leukemia virus, HTLV).

• Immunosuppressive Retroviruses: Cause immune system dysfunction (e.g., Human Immunodeficiency Virus, HIV, which leads to AIDS).

Example: HIV infects CD4+ T cells, leading to progressive immunodeficiency.