Introduction to Virology and Laboratory Methods

virus structure

• simple structure with nucleic acid genome (DNA or RNA)

• protected by protein shell

• may or may not be enveloped

• usually small - 20-200 nm

• metabolically inert - need host to replicate

viruses can have either ____ or ____, not both

RNA, DNA

why are viruses not seen on light microscopy

they are typically very small (20-200 nm)

some genomes of viruses include genes for _____________

replication (i.e. polymerase)

capsid

protein shell that surrounds and protects the genome th

capsomers

multiple identical subunits that make up the capsid

two types of capsids

icosahedral (20 sided) and tubular

nucleocapsid

a capsid surrounding the genome of a virus

envelope

lipoprotein membrane that surrounds viruses made from the membrane of the host cell as it exits the cell (budding)

glycoprotein

proteins found in the envelope of the virion that is usually glycosylated (act as a virulence factor)

positive sense

in reference to single stranded genome where the genetic material is readily available to be translated by host machinery

• 5’→3’

• represents the template strand

negative sense

in reference to single stranded genome where the genetic material is not readily available to be translated by host machinery

• requires polymerase to convert to 5’→3’

• 3’→5’

• represents the complimentary strand

Defective viruses

composed of viral nucleic acid and proteins but cannot replicate without the help of another virus

• Ex: Hepatitis D needs Hepatitis B to occur

Pseudovirus

contains host DNA instead of viral DNA

• Ex: viral vector for vaccines

viroids

consists of single molecule of circular RNA with no protein coat or envelope (partial viruses)

prions

smallest known infectious particle

• causes Creutzfeldt-Jakob disease

virus life cycle

1. virus attaches to host cell

2. virus enters host cell

3. virus replicates genome using host machinery

4. virus multiplies

5. viruses burst or bud out of cell

antivirals

inhibit some stage of viral replication cycle without being toxic to the body’s cells

modes of action of antivirals

• attachment or entry

• replication viral genome

• synthesis of specific viral proteins

• assembly or release of new infectious virions

• inactivate extracellular virus particles

the use of antivirals can result in viruses that are—

drug resistant and are a concern

viral detection and diagnosis

• serology - antibody response (4x increase titer is significant)

• direct viral antigen detection

• viral culture and assessment of cytopathic effects

• viral nucleic acid detection - NAAT (PCR)

the preferred specimen for viruses

aspirations

types of aspiration specimen

swabs and liquid

swab virus specimen

Dacron or rayon - flock swabs

• calcium alginate & wooden application swabs can interfere with some molecular tests

liquid virus specimen

saline, TSB (tryptic soy broth), transport media (albumin gelatin, etc.)

limitations of transport media

do not use when collecting blood, CSF, urine, amniotic, pleural, pericardial

• mostly sterile site

storage of viral specimen

• refrigerate at 2-8 C short term

• freeze at -70 C if storing for longer than 4 days

  • especially if RNA virus

gold standard of isolation of live viruses and lab ID

cell culture (no longer routine method)

diagnosis of viruses is based on—

cytopathic effect (CPE)

cytopathic effect (CPE)

tissue monolayer grown in cell culture - looking for abnormal tissue under the microscope

• cells look different if infected

shell vial culture

considered to be the more rapid culture - designed to show CPE within 1-2 days after inoculation