Bacteriophages

Question 1

What are bacteriophages?

Answer 1

Viruses that infect bacteria

Question 2

What are the most studied group of bacteriophages?

Answer 2

Double stranded DNA tailed phages

Question 3

How can DS DNA tailed phages be catergorised?

Answer 3

According to tail morphology

Question 4

What are the 3 main groups of DS DNA tailed phages?

Answer 4

Myoviruses
Siphoviruses
Podoviruses

Question 5

What is the morphology of myoviruses?

Answer 5

Myovirus tail can contract and extend

Question 6

What is the morphology of siphoviruses?

Answer 6

Siphovirus tail is tall and cannot contract

Question 7

What is the morphology of podoviruses?

Answer 7

Podovirus tail is short and cannot contract

Question 8

What are the 5 stages of (lytic) phage infection?

Answer 8

Adsorption
Entry
Synthesis
Assembly
Lysis

Question 9

Summarise the adsorption stage

Answer 9

Virion attaches to bacterial cell surface using tail fibres
Requires complementary receptors between phage and host cell surface

Question 10

What are some examples of host receptors for phages?

Answer 10

Receptors tend to be integral components of bacteria. Some examples include: Lipopolysaccharides Teichoic acids Flagella

Question 11

Summarise the entry stage

Answer 11

1. After adsorption, the tail fibres retract to enable contact between bacteriophage base plate and host cell outer membrane. 2. Small pores are formed in peptidoglycan by endolysins 3. Tail sheath contracts, driving tail tube through envelope and inner membrane, delivering phage DNA into the cytoplasm

Question 12

Summarise the synthesis stage

Answer 12

Phage DNA uses in rolling circle contameter replication 1. Early proteins produced required for virus DNA/RNA replication. Typically catalytic proteins Produced in smaller quantities 2. More and more phage DNA produced, so middle and late proteins are then produced 3. Late proteins are required for viral packaging. Typically structural proteins Produced in larger quantities

Question 13

Summarise the assembly stage

Answer 13

Newly replicated phage DNA must be packaged into capsids in order to form functional viruses 1. Terminase protein forms a complex with the phage DNA 2. Terminase-DNA complex binds to protal proteins of an empty procapsid 3. Phage DNA enters capside through ATP-dependent translocation 4. Once capside is full, terminase complex dissociates, and moves to a different (empty capsid)

Question 14

Summarise the lysis stage

Answer 14

1. Phage-encoded lysins degrate bacterial cell wall, releasing newly assembled virions 2. These go on to infect new host bacteria

Question 15

What are the two different phage lifecyles?

Answer 15

Lytic and lyosogenic

Question 16

What is the difference between lytic and lysogenic cycle?

Answer 16

Outcome:
Lytic = host cell destroyed
Lysogenic = host cell survives and functions as normal. However, environmental stresses can cause reversible switch from lysogenic to lytic
Viral DNA location:
Lytic = free in host cell
Lysogenic = intergrates into host genome; can be passed onto new generations of host

Question 17

What phages undergo lytic cycle, with example?

Answer 17

Virulent phages (e.g. E. coli phage T4)

Question 18

What phages undergo lysogenic cycle, with example?

Answer 18

Temperate phages (e.g. S. aureus 11). They can undergo lysogenic and lytic, depending on environmental conditions

Question 19

What is a prophage?

Answer 19

The phage genome that has intergrated into its bacterial host’s chromosomal DNA

Question 20

What is transduction?

Answer 20

The transfer of genetic material between microbes via a bacteriphage

Question 21

What are the 3 types of transduction?

Answer 21

Generalised Specialised Lateral

Question 22

What types of phages are involved in generalised transduction?

Answer 22

Virulent or temperate

Question 23

What is generalised transduction?

Answer 23

Random sections of bacterial DNA (instead of phage DNA) are packaged into phage virion. Results in phage that is defective for replication in the recipient cell Low efficiency

Question 24

What are 3 ways in which phages can influence their bacterial hosts?

Answer 24

Predation Lysogenic conversion Horizontal gene transfer

Question 25

What is predation?

Answer 25

Phages destroying bacterial hosts Impacts/regulates bacterial populations; can be exploited by competing bacteria

Question 26

What is lysogenic conversion?

Answer 26

Prophage inducing a phenoytypic change in lysogen. Prophages can carry toxins and virulence factors; provides defence system for host cell to prevent infection by other phages

Question 27

What is a lysogen?

Answer 27

A bacterial cell infected by and in a state of symbiosis with a temperate phage

Question 28

What is horizontal gene transfer?

Answer 28

Acquisition of DNA from another cell that is not inherited from a parent cell. Phages can move DNA between bacterial cells, acting as points for DNA recombination, generating genomic diversity

Question 29

What types of phages are involved in specialised transduction?

Answer 29

Only temperate phages

Question 30

What is specialised transduction?

Answer 30

DNA from a region of the bacterial chromosome directly adjacent to prophage is transferred alongside the phage genome. Specialised transducing virion cannot replicate as a phage anymore (as some of its DNA is missing)

Question 31

What types of phages are involved in lateral transduction?

Answer 31

Only temperature phages

Question 32

What is lateral transduction?

Answer 32

Occurs when packaging commences while prophage is still intergrated in bacterial chromosomes. Results in large spans of chromosomal (host) DNA being packaged into successive particles. High efficiency transfer, rate of occurrence is unknown. Phage can replicate further still

Question 33

What can lateral transduction result in?

Answer 33

The scale of single transfer of DNA from lateral transduction can potentially transform a benign bacterium into a pathogen