General Properties of Viruses

General Characteristics of Viruses

• Unique infectious agents, smallest in size (20-300 nm).
• Non-cellular, with a nucleic acid core of either DNA or RNA.
• Encased in a protective protein coat called capsid for attachment and entry.
• Obligate intracellular parasites; capable of growth only within living cells.
• Lack ribosomes and metabolic enzymes; depend on host cellular resources for reproduction.
• Not affected by antibiotics due to absence of cellular structures; susceptible to antimicrobial agents like alcohol and chlorine.
• Infect a range of organisms including unicellular prokaryotes and eukaryotes.

Major Viral Groups

• Plant viruses.
• Insect viruses.
• Bacteriophages (viruses infecting bacteria).
• Animal viruses (viruses infecting vertebrates).

Virus Structure

• Virion consists of at least DNA or RNA and capsid.
• Capsid made of protein subunits called capsomeres; unique arrangement aids in viral identification.
• Capsid functions include protection, cell attachment, structural symmetry, and antigenicity.

Viral Proteins and Enzymes

• Common enzymes: polymerase, integrase, reverse transcriptase.
• Envelope lipids are derived from the host cell, forming a lipid membrane around the virus.
• Viral envelope contains glycoproteins like neuraminidase and hemagglutinin, aiding in attachment and release of new viral particles.

Viral Envelope

• Formed during replication; contains glycoproteins and lipids.
• Protects nucleic acid, facilitates attachment, enhances structural integrity, and provides antigenic variability.

Morphology of Virions

• Smallest among microorganisms, typically measured in nanometers.
• Size varies significantly between virus types (e.g., picornaviruses vs. poxviruses).
• Morphological classification: naked icosahedral, enveloped icosahedral, naked helical, enveloped helical, and complex viruses.

Viral Stability

• Stability varies with factors like temperature; naked viruses are more heat-stable than enveloped viruses.
• Heat inactivation occurs at 50°-60°C; preservation strategies involve sub-zero storage.
• Salts can stabilize certain viruses, affecting vaccine development and efficacy.
• pH effects: stability generally between pH 5-9; extreme acidity or alkalinity can destroy viruses.

Modes of Physical and Chemical Inactivation

• Ether and detergents disrupt lipid envelopes and capsids effectively.
• Formaldehyde inactivates viral infectivity through nucleic acid reaction.
• Antimicrobial agents such as chlorination and alcohols are necessary to eliminate viruses.

Transmission of Viruses

• Depend on the source and route to target tissue; naked viruses endure drying and extreme conditions.
• Direct transmission through respiratory droplets; indirect through contaminated surfaces.
• Aerosol transmission involves small particles remaining airborne.

SARS-CoV-2 Specifics

• Causative agent of COVID-19; enveloped, positive-sense, single-stranded RNA virus.
• Attach to host cells via spike protein binding to ACE2 receptors; fusion allows entry.
• Primary target is the respiratory tract; other targets include the gastrointestinal tract.

Prevention Strategies for COVID-19

• Vaccination options include mRNA (Pfizer-BioNTech, Moderna), viral vector (AstraZeneca, Johnson & Johnson), and inactivated virus vaccines (Sinopharm, Sinovac).
• Non-pharmaceutical interventions: wearing masks, hand hygiene, physical distancing, and proper ventilation.