Chapter 13 - Acellular Pathogens

Why are viruses not technically organisms

because they are acellular pathogens

Structure of viruses

• protein coat

• envelope

• nucleic acid

• obligatory intracellular parasites - they require a host!

Host range of virus

• spectrum of host cells that a virus can multiply in

• very specific and often very limited

• only able to infect specific types of cells in one host species

How are host ranges determined

• by specific attachment sites or receptors on the host cell 

• availability of certain host cell factors 

size of viruses

• nanometer (10^-9)

• range from 20nm - 1000 nm 

What kind of microscopy is needed to visualize viruses

electron microscopy 

protein coat

• surrounds nucleic acid

• capsid

• subunits = capsomeres

Nucleic Acid 

• DNA or RNA - never both 

• Single-stranded or double of both 

• Linear or circular 

Envelope

• only present sometimes 

• made of lipids, proteins, and carbohydrates 

• glycoproteins = spikes 

Helical

• Hollow cylinder

• Tobacco mosaic virus 

Polyhedral 

• Many sides 

• Icosahedral = 20 sides

  • HPV

Pleomorphic

• nonspecific shape 

• enveloped 

• roughly spherical

Complex

• complex structure

• Bacteriophages 

Viruses must be grown on

living cells

Bacteriophages are grown on

bacterial plate

animal viruses are grown

in embryonated eggs or cell culture/ tissue culture

viral multiplication

Highly reliant on host cell to replicate

Bacteriophage multiplication processes

lytic and lysogenic multiplication cycles

Lytic Cycle

• Attachment 

  • bacteriophage attaches to cell

• Entry

  • bacteriophage injects DNA into the cell

• Biosynthesis

  • Phage DNA makes more of itself 

• Assembly 

  • bacteriophages assemble 

• Lysis

  • cell bursts open, cell dies, phages escape and infect neighboring cells 

Lysogenic Cycle

• Attachment

  • bacteriophage attaches to cell 

• Entry 

  • phage inserts DNA into chromosome of host cell 

  • creates a PROPHAGE 

• Cell multiplies many times

  • prophage is also multiplying 

• External trigger causes prophage to exit the chromosomes

• Once prophage exits the host DNA is enters the Lytic cycle 

  • replicate phage DNA many times 

  • produce phage proteins 

  • assemble new viral particles 

  • lyse 

Multiplication of Animal Viruses

• 1. Attachment

• 2. Entry of bacteriophage 

  • can be by receptor mediated endocytosis or fusion (capsid is brought into cell w/o vesicle) 

• 3. Uncoating 

  • DNA is uncoated in nucleus 

  • RNA is uncoated in cytoplasm 

• 3.2 For DNA in nucleus, DNA is transcribed into RNA (vRNA)

• 4. Translation 

• 5. Synthesis of viral proteins 

• 6. Assembly

• 7. Release

  • can be lytic or non-lytic 

    • lytic: cell is lysed and therefore killed 

    • non-lytic: bacteriophage is expelled via exocytosis, enveloped in membrane of host cell w/ spike proteins 

DNA viruses

must go to nucleus first THEN to cytoplasm after transcription

RNA viruses 

• go directly to cytoplasm (specifically ribosomes) 

  • sense strange 

  • anti-sense strand 

sense strand 

positive-strand

RNA viruses = mRNA → translated directly 

Antisense strand

negative-strand

RNA viruses

complement to mRNA

must first me complimented by viral RNA polymerase → Translation

Retroviruses 

RNA viruses that have a reverse transcriptase enzyme 

Converts RNA to DNA

DNA is then inserted into host genome (creates Provirus) 

• ex: HIV 

Prions 

• Infectious proteins 

• specifically target brain tissue 

• cause spongiform encephalopathy 

• Not an invading protein - result of an altered protein that is already present in the body 

  • PrP^c → PrP^sc

Vibrio Cholerae

• Cholera 

• Cholera toxin 

  • only produces toxin when it is lysogenic, phages make cell more toxic

Lysogenic switch

lysogenic cycle causes change of characteristics of cells