Viruses: Structure, Classification, and Impact in Microbiology

Introduction to Virology

Definition and Scope

Virology is the branch of microbiology that studies viruses, which are acellular infectious agents. Viruses are responsible for a wide range of diseases in humans, animals, and plants, and have been the cause of recent epidemics such as SARS-CoV-2 and Monkeypox virus.

Characteristics of Viruses

Obligate Intracellular Parasites

• Viruses cannot replicate or carry out metabolic processes independently.

• They require a living host cell to provide resources such as ATP, enzymes, and ribosomes for replication.

Acellular Structure

• Viruses are not composed of cells.

• A single viral particle, or virion, consists of genetic material (DNA or RNA) enclosed in a protein shell called a capsid.

Small Size

• Viruses are ultramicroscopic, typically 20–400 nm in size, much smaller than bacteria or eukaryotic cells.

Diverse Hosts and Specificity

• Viruses infect animals, plants, fungi, bacteria (bacteriophages), and archaea.

• Many viruses are highly specific, infecting only certain cell types within a host.

Evolution and Mutation

• Viruses mutate rapidly, leading to new strains and sometimes drug resistance.

Virus Structure

Genetic Material (Genome)

• Can be DNA or RNA, single- or double-stranded, linear or circular.

• Contains instructions for viral replication.

Capsid

• Protective protein shell made of repeating subunits called capsomeres.

• Determines the virus's shape and symmetry (helical, icosahedral, or complex).

Envelope (Optional)

• Some viruses have an outer lipid membrane (envelope) derived from the host cell.

• Enveloped viruses are generally less stable outside the host.

Glycoprotein Spikes (Optional)

• Spikes on the envelope help the virus attach to host cell receptors.

Virus Architecture and Nomenclature

Virion vs. Virus

• Virion: Fully assembled, infectious virus particle outside a host cell.

• Virus: Can refer to the virion, the virus inside a host cell, or viral genetic material integrated into host DNA.

Function of a Virion

• Responsible for infection and delivery of genetic material into host cells.

• Directs host machinery to produce new virus particles.

Capsid Structure and Function

• Made of capsomeres, assembled symmetrically.

• Shapes: Icosahedral (e.g., Adenovirus), Helical (e.g., Tobacco Mosaic Virus), Complex (e.g., Bacteriophage).

• Protects genetic material and aids in infection.

Viral Envelope

• Derived from host cell membrane during viral exit.

• Contains viral glycoproteins for host cell recognition.

Virus Classification

Virus Nomenclature

• Governed by the International Committee on Taxonomy of Viruses (ICTV).

• Hierarchical system: Order (-virales), Family (-viridae), Genus (-virus), Species.

• Binomial species naming: Genus + species epithet (e.g., Orthoflavivirus zikaense).

Baltimore Classification

The Baltimore classification system groups viruses based on their type of nucleic acid and the mechanism of mRNA synthesis. All viruses must produce mRNA to synthesize proteins and replicate.

• Seven classes based on nucleic acid type and replication strategy:

Group	Genome Type	Example
I	dsDNA	Adenovirus
II	ssDNA	Parvovirus
III	dsRNA	Reovirus
IV	+ssRNA	Poliovirus
V	–ssRNA	Influenza virus
VI	RNA reverse-transcribing	HIV
VII	DNA reverse-transcribing	Hepatitis B virus

Special Types of Viruses

Giruses and Virophages

• Giruses: Giant DNA viruses (e.g., Mimivirus, Mamavirus).

• Virophages: Subviral agents that infect other large viruses (e.g., Sputnik).

Virome and Human Health

Definition and Impact

• The virome is the collection of all viruses present in a particular environment, such as the human body or aquatic ecosystems.

• Human virome influences health and disease states, with viral communities affecting immunity and disease susceptibility.

• Metagenomics is used to study viromes and their ecological roles.

Applications of Viruses in Medicine

Bacteriophage Therapy

• Use of bacteriophages to treat bacterial infections, especially antibiotic-resistant strains.

• Potential for treating bacterial biofilms.

Gene Therapy

• Viruses (e.g., retroviruses, adenoviruses) are used to deliver functional genes to correct genetic disorders.

Review Questions and Key Concepts

• True or False: The genome of a virus can be composed of RNA or DNA, but only in double-stranded forms. False (can be single- or double-stranded).

• Which term describes the viral nucleic acid genome enclosed within a protein coat? A) Nucleocapsid

• Which of the following best describes an icosahedral virus? B) 20-sided symmetrical structure

• Capsomeres are: C) Structural proteins forming the capsid

• The envelope of a virus is primarily derived from: B) The host cell membrane

Summary Table: Virus Structure and Classification

Component	Description	Function
Genome	DNA or RNA, ss or ds, linear/circular	Genetic instructions
Capsid	Protein shell (capsomeres)	Protection, shape, infection
Envelope	Lipid membrane (optional)	Host recognition, entry
Spikes	Glycoproteins (optional)	Attachment to host