Classification and Detection of Viruses

Virus (definition)

The replicating entity; an obligate intracellular parasite requiring host machinery.

Virion (definition)

The infectious physical particle existing outside the host cell.

Capsid

A protein shell that can assemble without genomes; typically icosahedral.

Nucleocapsid

A protein shell that only assembles with the viral genome; typically helical.

Envelope

A lipid bilayer with glycoproteins enclosing the viral capsid or nucleocapsid.

Tegument

A proteinaceous layer between the envelope and capsid of herpesviruses.

Glycoproteins (Spikes)

Trimeric proteins in the envelope that mediate virus attachment and entry.

Virus-like particle (VLP)

Virions that are deprived of a viral genome.

Pseudotyped virus

A virion containing glycoproteins from a different virus than its genome.

Viral Replication Cycle (7 steps)

1. Attachment, 2. Uncoating, 3. Protein synthesis, 4. Nucleic acid synthesis, 5. Assembly, 6. Packaging, 7. Egress.

Intrinsic Properties

Size, structure, genome type, and replication strategy used for taxonomic classification.

Extrinsic Properties

Host range, disease type, and transmission method (not used for taxonomy).

Baltimore Classification

A system of virus classification based on the flow of genetic information.

Viral Strain/Isolate

A genetic variant below the species level.

Viral Serotype

A variant based on unique surface antigens recognized by specific antibodies.

Viral Biotype/Pathotype

Classification based on disease or pathological outcome (e.g., high vs low pathogenicity).

Icosahedral Symmetry

Structure with 20 equilateral triangle faces and 5-fold, 3-fold, and 2-fold axes.

Helical Nucleocapsid

Subunits form a helix where length is determined by genome length.

Animal Helical Viruses

All animal viruses with helical nucleocapsids are enveloped.

Viral Budding

Process where viruses exit the cell while acquiring an envelope from host membranes.

Cytopathic Effect (CPE)

Any change in cell morphology produced by viral infection (e.g., rounding, detachment).

Syncytia

A type of CPE involving cell-cell fusion (typical for paramyxoviruses).

Inclusion Bodies

Diagnostic nuclear or cytoplasmic aggregates, like Negri bodies in rabies.

Viral Titer

The concentration of infectious virus calculated by titration.

TCID50

The dilution of virus where 50% of inoculated replicates show cytopathic effect.

Plaque Assay

Method to calculate PFUs (plaque-forming units) per ml by counting clear zones.

Electron Microscopy (EM)

Direct visualization of virion shape and morphological subunits using negative staining.

ELISA (Antigen Detection)

Fast, high-throughput assay using antibodies to detect viral proteins; doesn't measure infectivity.

PCR / RT-PCR

Extremely sensitive molecular detection of viral DNA or RNA.

Hemagglutination Assay (HA)

Detects viral antigens by their ability to cross-link red blood cells into a lattice.

Neutralization Assay

Measures the ability of patient antibodies to inhibit viral infectivity in culture.

Hemagglutination Inhibition (HAI)

Quantifies patient antibodies that prevent a virus from cross-linking red blood cells.

Virus Isolation (Advantage)

The only test that assesses if infectious virus is being shed.

Virus Isolation (Disadvantage)

Slow (days to weeks) and requires the virus to be cultivatable.

Matrix (M) Protein

Protein frequently involved in virus assembly or budding from infected cells.