Viruses are one of the smallest infectious agents. They are obligatory intracellular parasites because they have no metabolic activity.
Viruses can infect all organisms in nature:
- Bacteriophages are bacterial viruses.
- Plant viruses: Include complete viruses and viroids.
- Animal viruses: Infect insects or vertebrates including man.
Viruses differ from bacteria in the following:
1. Viruses are very small in size, ranging from 20-300 nm. Therefore:
- They can only be seen under the electron microscope (except poxviruses).
- They can pass through bacterial filters.
- They need ultracentrifugation for sedimentation.
2. Viruses contain only one type of nucleic acid (DNA or RNA), never both.
3. Viruses are obligatory intracellular parasites (can only replicate inside living cells) and do not divide by binary fission.
4. Viruses cannot be cultivated in the laboratory on artificial culture media; however, they can be grown on tissue culture.
5. They are not susceptible to antibacterial antibiotics.
Structure and Composition of Viruses
The typical complete virus particle, called virion, consists of a genome of either DNA or RNA, surrounded by a capsid (protein coat).
The nucleic acid and the protein coat are called nucleocapsid. Some viruses, called enveloped viruses, have an outer lipid- containing envelope whereas others are non-enveloped (naked) (Fig. 1).
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| Fig. 1: The components of the complete virus particle "Virion" |
Viral nucleic acid (genome)
- It is the genetic material of a virus, which may be either RNA or DNA.
- Most DNA viruses are double-stranded (ds) while most RNA viruses are single- stranded (ss). The viral ssRNA may be positive (+ sense) or negative (- sense).
- It is responsible for virulence, i.e. it is the infectious part of the virus.
Viral capsid
Viral capsid is composed of many small protein subunits called capsomers.
It has the following functions:
- It protects the nucleic acid (genome) against harmful environmental factors.
- It mediates attachment to host cell (in non-enveloped viruses).
- It is responsible for the viral symmetry (or morphology) which may be:
1. Icosahedral (many sided) symmetry (Fig. 2a):
Icosahedral or isomeric or cubic viruses resemble a crystal with 20 triangular facets and 12 corners. This includes all DNA viruses, except poxviruses (brick shaped), and some RNA viruses.
2. Helical (coiled tubes) symmetry (Fig. 2b): The viral nucleic acid is closely associated with the protein capsid forming a coil-shaped helical nucleocapsid. This includes many of RNA viruses, e.g. rabies virus.
3. Complex symmetry: Examples include the brick-shaped poxviruses (Fig. 2c) or bacteriophages (Fig. 2d).
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| Fig. 2: Virus symmetry |
Viral envelope
It is a lipoprotein membrane composed of lipids, derived from host cell membrane during release by budding, and protein that is virus-specific.
Frequently, the envelope may have glycoprotein spikes which are the organ of attachment of the enveloped virus to host cell receptors. Therefore, dissolving the envelope inhibits attachment and the virus loses its infectivity.
Enveloped viruses are less stable, i.e. more easily inactivated than naked viruses. They are more sensitive to heat, drying, detergents and lipid solvents. Therefore, enveloped viruses, being unable to survive in the environment, are transmitted essentially by direct contact via blood and body fluids.
N.B.: The surface proteins of the virus, whether they are the capsid proteins (in naked viruses) or the glycoproteins (in enveloped viruses) are:
- responsible for attachment to host cell receptors
- the principal antigens against which the host elicits its immune response to viruses.
Classification of Viruses
There are many methods of classification of viruses, the following are the most important.
A. Classification by symptomatology
This is the old classification, based on diseases that viruses produce i.e. Tropism.
- Neurotropic viruses: These viruses attack the nervous system, causing diseases such as polio, rabies, and encephalitis.
- Enteroviruses: These viruses attack the digestive system, causing diseases such as hepatitis A.
- Arthropod-borne viruses: These viruses are transmitted by insects and ticks, causing diseases such as yellow fever, dengue fever, and Zika fever.
- Respiratory viruses: These viruses attack the respiratory system, causing diseases such as the common cold, influenza, and COVID-19.
- Dermatropic viruses: These viruses attack the skin, causing diseases such as chickenpox, measles, and herpes.
- Others: This category includes viruses that do not fit neatly into the other categories, such as HIV and hepatitis B.
It is important to note that this is an older classification system, and it is no longer widely used, However, it is still useful for understanding some of the different types of viruses that can infect humans and the types of diseases they can cause.
B. Baltimore Classification System for Viruses
It is based on virus genome replication strategy.
The central idea is that all viruses must generate positive strand mRNAs from their genomes, in order to produce proteins and replicate themselves.
The precise mechanisms whereby this is achieved differ for each virus family.
Group I: Double-Stranded DNA Viruses
Replication strategy: The double-stranded DNA genome is transcribed into mRNA, which is then translated into proteins.
The proteins then assemble to form new virus particles.
Examples: Herpesviruses, adenoviruses, poxviruses
Group II: Single-Stranded DNA Viruses
Replication strategy: The single-stranded DNA genome is converted into double-stranded DNA, which is then transcribed into mRNA and translated into proteins.
The proteins then assemble to form new virus particles.
Examples: Parvoviruses, Circoviruses, Geminiviruses
Group III: Double-Stranded RNA Viruses
Replication strategy: The double-stranded RNA genome is transcribed into mRNA, which is then translated into proteins.
The proteins then assemble to form new virus particles. The double-stranded RNA genome can also serve as the template for mRNA synthesis.
Examples: Reoviruses, Orbiviruses
Group IV: Positive-Sense Single-Stranded RNA Viruses
Replication strategy: The positive-sense single-stranded RNA genome serves as the mRNA for protein synthesis.
The proteins then assemble to form new virus particles.
Examples: Picornaviruses, Coronaviruses, Flaviviruses
Group V: Negative-Sense Single-Stranded RNA Viruses
Replication strategy: The negative-sense single-stranded RNA genome is transcribed into positive-sense RNA, which is then translated into proteins.
The proteins then assemble to form new virus particles. The negative-sense RNA genome can also serve as the template for mRNA synthesis.
Examples: Orthomyxoviruses, Rhabdoviruses, Filoviruses
Group VI: Retroviruses
Replication strategy: The single-stranded RNA genome is reverse transcribed into DNA, which is then integrated into the host cell genome.
The integrated DNA is then transcribed into mRNA, which is translated into proteins. The proteins then assemble to form new virus particles.
Examples: HIV, HTLV
Group VII: Double-Stranded DNA Reverse Transcribing Viruses
Replication strategy: The single-stranded RNA genome is reverse transcribed into DNA, which is then integrated into the host cell genome. The integrated DNA is then transcribed into mRNA, which is translated into proteins. The proteins then assemble to form new virus particles.
Examples: Hepadnaviruses
The Baltimore classification system is a useful way to classify viruses based on their replication strategy.
This information can be used to develop new antiviral drugs and vaccines.
C. Hierarchical Virus Classification Scheme
Order
Family
Subfamily
Genus
Species
Classification is based on the following properties:
- Nature of the nucleic acid: RNA or DNA genome
- Virus replication strategy
- Symmetry of the capsid
- Presence or absence of an envelope
- Additional properties: antigenicity, host range, and nucleic acid sequence
Examples of virus families and subfamilies:
Herpesviridae
Alphaherpesvirinae
Simplexvirus
Varicellovirus
Betaherpesvirinae
Cytomegalovirus
Gammaherpesvirinae
Epstein-Barr virus
Picornaviridae
Enterovirus
Poliovirus
Coxsackievirus
Echovirus
Cardiovirus
Enterovirus 71
Coxsackievirus B3
Coronaviridae
Coronavirinae
SARS-CoV-2
SARS-CoV
MERS-CoV
Torovirinae
Bovine coronavirus
This is just a small sample of the many different virus families and subfamilies that exist. For more information, please consult a virology textbook or website.