Summary
6.1 Viruses
- Viruses are generally ultramicroscopic, typically from 20 nm to 900 nm in length. Some large viruses have
been found. - Virionsareacellular andconsistofanucleicacid,DNAorRNA,butnotboth,surroundedbyaproteincapsid.
There may also be a phospholipid membrane surrounding the capsid. - Viruses are obligate intracellular parasites.
- Viruses are known to infect various types of cells found in plants, animals, fungi, protists, bacteria, and
archaea. Viruses typically have limitedhost rangesand infect specific cell types. - Viruses may havehelical,polyhedral,orcomplexshapes.
- Classification ofvirusesisbasedonmorphology,typeofnucleicacid,hostrange,cellspecificity,andenzymes
carried within the virion. - Like other diseases, viral diseases are classified using ICD codes.
6.2 The Viral Life Cycle
- Many viruses target specific hosts or tissues. Some may have more than one host.
- Many viruses follow several stages to infect host cells. These stages includeattachment, penetration,
uncoating, biosynthesis, maturation,andrelease. - Bacteriophages have alyticorlysogenic cycle. The lytic cycle leads to the death of the host, whereas the
lysogenic cycle leads to integration of phage into the host genome. - Bacteriophages inject DNA into the host cell, whereas animal viruses enter by endocytosis or membrane
fusion. - Animal viruses can undergolatency, similar to lysogeny for a bacteriophage.
- The majority of plant viruses are positive-strand ssRNA and can undergo latency, chronic, or lytic infection,
as observed for animal viruses. - The growth curve of bacteriophage populations is aone-stepmultiplication curveand not a sigmoidal curve,
as compared to the bacterial growth curve. - Bacteriophages transfer genetic information between hosts using either generalized or specialized
transduction.
6.3 Isolation, Culture, and Identification of Viruses
- Viral cultivation requires the presence of some form of host cell (whole organism, embryo, or cell culture).
- Viruses can be isolated from samples by filtration.
- Viral filtrate is a rich source of released virions.
- Bacteriophages are detected by presence of clearplaqueson bacterial lawn.
- Animal and plant viruses are detected bycytopathic effects, molecular techniques (PCR, RT-PCR), enzyme
immunoassays, and serological assays (hemagglutination assay, hemagglutination inhibition assay).
6.4 Viroids, Virusoids, and Prions
- Other acellular agents such asviroids,virusoids, andprionsalso cause diseases. Viroids consist of small,
naked ssRNAs that cause diseases in plants. Virusoids are ssRNAs that require other helper viruses to
establish an infection. Prions are proteinaceous infectious particles that causetransmissible spongiform
encephalopathies. - Prions are extremely resistant to chemicals, heat, and radiation.
- There are no treatments for prion infection.
Review Questions
266 Chapter 6 | Acellular Pathogens
This OpenStax book is available for free at http://cnx.org/content/col12063/1.2