Chapter 6- Acellular pathogens Flashcards
Who is considered the founder of the field of virology?
Dmitri Ivanovski, a Russian botanist. He discovered the source of tobacco mosaic disease (TMD) by using a porcelain filtering device. The device has pores that are small enough to remove small bacteria from an extract from an infected plant. The cause of TMD could not be removed using a filter. It was thought that TMD was caused by a biological poison. Virus is Latin for poison and was used to describe the cause of TMD
How can viruses be seen?
Using an electron microscope. They are too small to be seen using a light microscope, although their size helps them to infect larger cells
Viruses
Distinct biological entities, but their evolutionary origin is unclear. They are acellular, so they not included in the tree of life. Viruses are considered obligate intracellular parasites because they have to infect a cellular host to survive and reproduce
Virions
New virus particles that are formed as viral components like proteins and nucleic acids are produced. These viral components are produced when the viral genome takes over the genome of the host cell. The new virions transport the viral genome to another host cell to carry out another round of infection.
Host range
The types of hosts a virus is able to infect. Most viruses will only be able to infect the cells of one or a few species, but some viruses have a wider host range
Bacteriophages
Viruses that infect bacteria
Vector
An animal that transmits a pathogen from one host to another. Includes mosquitoes, ticks, and flies
3 ways that viruses can be transmitted
Direct contact, indirect contact with fomites, or through a vector
Mechanical transmission
When an arthropod carries a viral pathogen on the outside of its body and transmits it to a new host, through physical contact
Biological transmission
When the arthropod carries the viral pathogen inside its body and transmits it to the new host through a bite
Zoonoses
When viruses are transmitted from an animal host to a human host and cause disease
Reverse zoonoses
Caused by an infection of an animal by a virus that originated in a human
What size are viruses?
Range from 20 nm for small viruses up to 900 nm for typical, large viruses. There are some giant viral species that are closer in size to a bacterial cell
Wendell Stanley
The first scientist to crystallize the structure of the tobacco mosaic virus in 1935. He discovered that it is composed of RNA and proteins. He isolated the influenza B virus in 1943, which helped with the development of the flu vaccine
Capsid
A protein coat that surrounds the virus’ nucleic acid genome. A capsid is not filled with cytosol, it is filled with the genome and enzymes that are needed to make new viruses. Capsids are composed of protein subunits called capsomeres. Different types of capsomere proteins interlock to form the capsid
Components of a virus (2)
- Nucleic acid- DNA or RNA, never both
- A capsid surrounding the nucleic acid
Naked (non-enveloped) viruses
Viruses formed with only a nucleic acid and a capsid, no envelope
Enveloped viruses
The capsid is surrounded by a lipid bilayer
Viral envelope
A small portion of the phospholipid membrane that is obtained as a virion buds from the host cell. The viral envelope can be intracellular or cytoplasmic.
Spikes
Protein structures that extend from the capsid on enveloped viruses and some naked viruses. The spikes have structures at their tips that help the virus to attach to and enter a cell. One example is the H and N spikes found on influenza viruses
How are influenza viruses identified?
By their H and N spikes. Hemagglutinin spikes (H) are found in influenza viruses. Influenza viruses also have enzymes like the neuraminidase (N) influenza virus spikes that help the virus to detach from the cell when new virions are released.
Shapes of viral capsids (3)
Helical, polyhedral, or complex
Helical capsids
The capsid is cylindrical or rod shaped, and the genome is found inside the capsid. TMV is a naked helical virus and Ebola is an enveloped helical virus
Polyhedral capsids
Consist of a nucleic acid
surrounded by a polyhedral (many-sided) capsid in the form of an icosahedron. It forms the shape of poliovirus and rhinovirus
Icosahedral capsid
A three-dimensional, 20-sided structure with 12 vertices. These capsids somewhat resemble a soccer ball
Complex viruses
Viruses that may have features of both polyhedral and helical viruses. A bacteriophage form is one example. Poxviruses that have complex shapes are often brick shaped, with intricate surface characteristics not seen in the other categories of capsid.
Bacteriophage structure
A complex virus- one end is helical, the other is polyhedral (the end with the capsid and the genome). The tailed end (tailed fibers and tail pins) helps to insert the viral genome into a cell by making a hole in the membrane. The sheath contracts to insert the genome.
ICTV’s viral classification system
Classifies viruses into families and genera based on viral genetics, chemistry, morphology, and mechanism of multiplication. Using this system, there are 7 orders, 96 families, and 350 genera of viruses
ICTV’s nomenclature
Viral family names end in -viridae (e.g, Parvoviridae) and genus names end in −virus (e.g., Parvovirus). The names of viral orders, families, and genera are all italicized. When referring to a viral species, we often use a genus and species epithet such as Pandoravirus dulcis or Pandoravirus salinus.
Baltimore classification system
An alternative to ICTV nomenclature. It classifies viruses according to their genomes (DNA or RNA, single versus double stranded, and mode of replication). With this system, there are 7 groups of viruses that have common genetics and biology
IDC classification of viral diseases
The ICD assigns an alphanumeric code of up to six characters to every type of viral infection, as well as all other types of diseases, medical conditions, and causes of death. The code is used to categorize patient conditions for treatment and insurance reimbursement. ICD codes are routinely used by clinicians to order laboratory tests and prescribe treatments specific to the virus suspected of causing the illness. This ICD code is then used by medical laboratories to identify tests that must be performed to confirm the diagnosis.
Virulent phages
Lead to the death of the cell through cell lysis
5 stages of the lytic cycle of virulent phage
- Attachment
- Penetration
- Biosynthesis
- Maturation
- Lysis
Temperate phages
Can become part of a host chromosome and are replicated with the cell genome until they are used to make new viruses (progeny viruses)
Attachment
The first stage of the lytic cycle. The phage interacts with specific receptors on the surface of the host cell (bacteria).