Acellular Pathogens — Chapter 6

Chapter 6

Virus

Infectious agents that are too small to be seen under a light microscope and are not considered cells → they need a living host to be able to carry out functions

• Shape is determined by capsomeres or envelope

Transmission

Direct contact, indirect contact, or via a vector

Zoonose

When a virus is transmitted from an animal host to a human host

Reverse Zoonose

When a virus is transmitted from a human host to an animal host

Virion

Complete virus particle

Nucleic Acid

DNA or RNA, single or double stranded; linear or circular

Capsids

Protects and encloses the nucleic acid → composed of capsomeres

Envelope

Bilayer membrane that is acquired when they bud from the cell

• Composed of lipids, proteins, carbohydrates

Spikes

Glycoproteins that mediate attachment to the host

International Committee on Taxonomy of Viruses (ICTV) Viral Nomenclature

Based on viral genetics, chemistry, morphology, and mechanism of multiplication

Baltimore Classification System on Viral Nomenclature

Based on genome type and mRNA generation

-viridae

Viral classification for the ending of Family names

-virus

Viral classification for the ending of Genus names

Viral Species

A group of viruses sharing the same genetic information and ecological niche (host)

Coronaviridae

Enveloped (+) sense RNA with 1 segment

• Spherical and bacilliform, cause upper respiratory infections

• Infectious agent: SARS and SARS-Cov-2

Avian influenza

Bird flu — both transmitted via zoonosis (animals to humans) and reverse zoonosis (humans to animals)

Influenza Antigenic Shift

Occurs when two different strands of the virus infect the same cell and exchange genetic material → results in new virus strain with different combination of antigens

Emerging Viruses

Viruses that were previously endemic and expand their host range to other species — changed mainly due to population traveling and densities

• Ex. Measles, Polio

Host Recognition/Attachment

All viral replication cycles must achieve the following:

• Viruses must contact and adhere to a host cell that can support their type of replication

Genome Entry

All viral replication cycles must achieve the following:

• The viral genome must enter the host cell and gain access to the cell’s machinery for gene expression

Assembly of Progeny Virions

All viral replication cycles must achieve the following:

• Viral components must be expressed and assembled

  • Components usually self-assemble spontaneously

Exit and Transmission

All viral replication cycles must achieve the following:

• Progeny virions must exit the host cell to reach new host cells

• If multicellular, new hosts to infect

Bacteriophage Absorption

Attachment of viruses to host cells

Bacteriophage Penetration

Phage lysozyme opens cell wall; tail sheath contracts to force tail core and DNA into cell

Bacteriophage Biosynthesis

Formation of new nucleic acid molecules, capsid proteins, and other viral components

Bacteriophage Maturation

Assembly of newly synthesized viral components into completer virions

Bacteriophage Lysis

Phage lysozyme breaks cell wall

Intercellular Viral Communication

Refers to how viruses and infected cells communicate with neighboring cells, influencing viral spread, host immune responses, and even altering cellular behavior

• Involves transferring viral components like RNA, DNA, or proteins, or even exploiting host cell mechanisms for communication

Latent Infection

In which environmental stress triggers reactivation of a virus that was dormant within cells

• Ex. Herpes

Animal Host Virus Attachment

Step 1 of Life Cycle

• Attachment of viruses to host cells

Animal Host Virus Penetration

Step 2 of Life Cycle

• Through endocytosis or fusion

Animal Host Virus Biosynthesis

Step 3 of Life Cycle

• Production of nucleic acid or proteins

Animal Host Virus Assembly

Step 4 of Life Cycle

• Nucleic acid and capsid proteins assemble

Animal Host Virus Release

Step 5 of Life Cycle

• Budding (enveloped viruses) or rupture

+ssRNA Synthesis

RNA viruses that contain this can be directly read by the ribosomes to synthesize viral proteins

-ssRNA Synthesis

Viruses containing this must first use it as a template for synthesis of its counterpart before viral proteins can be synthesized

Papovavirus

Typical DNA-containing virus that attacks animals cells

Chronic Infections

Recurrent or persistent symptoms of disease

Viroids

Infectious RNA particle smaller than a virus → not clear how they cause disease

• Circular, single-stranded RNA

• Exist inside cells as particles of RNA without capsids/envelopes

• Do not require helper virus, nor do they produce proteins

  • Not apparent in infected tissues

Virusoids

Similar size and structure of viroids but are dependent on, and encapsulated in, a helper virus

Prions

Proteinaceous infectious misfolded particles

• Cause fatal neurodegenerative diseases

• Can replicate without a genome

• Misfolding may be result of a mutation

Creutzfeldt and Jakob Observed Illness

Characteristics:

1. Resistant to inactivation by heating to 90ºC

2. Not sensitive to radiation treatment

3. Not destroyed by enzymes that digest DNA or RNA

4. Sensitive to protein denaturing agents — phenol, urea

5. Direct pairing to amino acids

Retroviridae

Family of RNA viruses that replicate by converting their RNA into DNA and integrating it into the host cell’s DNA

• Reverse transcription → 7 steps