6.3 bacteriophages

phases

Phages only infect bacterial cells, and do not pose a medical threat to humans 

1. More bacterial cells in your body than human cells, introducing new genetic material, influencing evolution 

2. New tool to kill bacteria 

structure

Structure

• Capsid - icosahedral w/collar that attaches 

• Sheath - contractile protein that moves nucleic acids

• Baseplate - interact w/ host cell

• Tail fibers  - bind to receptor on target cell 

Only come into contact by chance. 

replication

Only come into contact by chance. 

Replication 

Lytic Replication pathway lyses and kills the host cell 

1. Attachment - phage binds to bacterial cell 

   1. Tail fibers interact with proteins on the membrane after random encounter 

2. Entry - phage injects its genome into the host; empty capsids remain outside the cell 

   1. Sheath contracts 

3. Replication - protein synthesis makes phage parts and genome is replicated; host cell DNA is broken down by bacteriophage DNAases

4. Assembly - genome packed into capsid and the phage is assemble 

5. Release - bacterial cell lyses and new phages are released 

lysogenic pathway

1. Attach and entry is the same. Phage genome is integrated and replicated 

   1. Becomes a prophage = phage dna integrated into host cell’s genome 

   2. Lysogenic steps 

      1. Integrated - integrated and becomes prophage 

      2. Cell division - prophage also becomes replicated 

      3. Lytic cycle - re-entry into cycle 

   3. If host cell dies, the phage also dies 

2. Assembles and releases the same way 

viral titer via turbidity

Viral Titer via Turbidity 

• turbidity = measure of cloudiness 

• titer= amount of viral particles present 

• Put into test tube, add phage in, alters turbidity bc of size. If phages lyse, clearer solution. 

• Instruments to measure turbidity in a lab.