Acellular Pathogens (Viruses)

virus

protein (capsid) + nucleic acid + some have an envelope

covering of virus

• Protection of nucleic acid and

  attachment to host cells

• capsid & envelope (not found in all viruses)

central core of virus

nucleic acid molecule(s) (RNA or DNA) & matrix proteins enzymes (not found in all viruses)  

capsid

protein coats that enclose and protect the nucleic acid of the virus

capsomers

identical protein subunits that make up capsid

helical and icosahedral

possible shapes for capsid

obligate intracellular pathogen

what a virus is

• must live in a host cell

  • needs machinery (ribosome) to replicate

envelope 

virus’ external covering 

naked virus

lacks envelope

spikes

• exposed proteins on the outside of the envelope

• essential for attachment of the virus to the host cell 

• ex: influenza, coronavirus 

poxvirus

• no capsid, covered by dense layer of lipoproteins

• ex: small pox, cow pox

True or False: Chicken pox is a herpes virus.

True

bacteriophage

Infect bacteria & have a polyhedral nucleocapsid along with a helical tail and attachment fibers

ssRNA (+) → dsDNA → ssRNA (+) → protein

Class 6 virus 

ex: HIV

host specificity

virus can only infect certain cells

host range

Spectrum of cells a virus can infect

animal viruses lifecycle

1. adsorption/penetration

   1. membrane fusion 

   2. endocytosis 

2. replication 

3. maturation/latency 

4. release

   1. budding

   2. lysis

adsorption

the virus attaches to its host cell by specific binding of its spikes to cell receptors

penetration

the virus is engulfed into a vesicle and its envelope is uncoated, freeing the viral RNA into the cell cytoplasm

replication and protein production (synthesis)

under the control of viral genes, the cell synthesizes the basic components of new viruses: RNA molecules, capsomers, spikes

assembly

viral spike proteins are inserted into the cell membrane for the viral envelope; nucleocapsid is formed from RNA and capsomers

release

enveloped viruses bud off membrane, carrying away an envelope with the spikes. This complete virus or virion is ready to infect another cell 

budding

exocytosis; nucleocapsid binds to membrane which pinches off and sheds the viruses gradually; cell is not immediately destroyed

lysis

nonenveloped and complex viruses released when cell dies and ruptures

persistent infections

cell harbors the virus and is not immediately lysed

chronic latent state

• can last weeks or host’s lifetime; several can periodically reactivate 

• ex: Measles virus (hidden in brain cells for years), Herpes simplex virus (cold sores & genital herpes), Herpes zoster virus (chickenpox and shingles)

HPV

cervical, anal, and other canceres

EBV

lymphoma, stomach cancer

HBV and HCV

liver cancer

KSHV

Kaposi sarcoma, lymphoma

HTLV-1

Leukemia

HIV-1

• Leukemia

• Kaposi sarcoma, lymphoma

lytic cycle

a viral reproduction process where:

1. a virus infects a host cell,

2. virus genome hijacks its machinery to produce new phage particles

3. virus assembles and then bursts to release phage particles

lysogenic cycle

a viral replication process where:

1. a virus's genetic material integrates into the bacterial chromosome, forming a prophage

2. prophage is replicated with the host chromosome

3. induction can occur resulting in activation of lysogenic prophage

lysozyme

• in bacteriophage tail

• can digest the peptidoglycan layer 

tissue culture and plaque assay

methods for growing viruses 

viroids

• infectious nucleic acid (RNA)

• ex: potato spindle tuber disease

satellite viruses

• require another virus to replicate

• ex: HDV (Hepatitis Delta Virus) 

virion

virus particle outside host cell

No, because in step 2, they cannot grow in pure cultures

Do viruses obey Koch’s postulates?