Bacteriophages Flashcards
What are bacteriophages?
Viruses that infect bacteria
What are the most studied group of bacteriophages?
Double stranded DNA tailed phages
How can DS DNA tailed phages be catergorised?
According to tail morphology
What are the 3 main groups of DS DNA tailed phages?
Myoviruses
Siphoviruses
Podoviruses
What is the morphology of myoviruses?
Myovirus tail can contract and extend
What is the morphology of siphoviruses?
Siphovirus tail is tall and cannot contract
What is the morphology of podoviruses?
Podovirus tail is short and cannot contract
What are the 5 stages of (lytic) phage infection?
Adsorption
Entry
Synthesis
Assembly
Lysis
Summarise the adsorption stage
Virion attaches to bacterial cell surface using tail fibres
Requires complementary receptors between phage and host cell surface
What are some examples of host receptors for phages?
Receptors tend to be integral components of bacteria. Some examples include: Lipopolysaccharides Teichoic acids Flagella
Summarise the entry stage
- 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
Summarise the synthesis stage
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
Summarise the assembly stage
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)
Summarise the lysis stage
- Phage-encoded lysins degrate bacterial cell wall, releasing newly assembled virions 2. These go on to infect new host bacteria
What are the two different phage lifecyles?
Lytic and lyosogenic
What is the difference between lytic and lysogenic cycle?
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
What phages undergo lytic cycle, with example?
Virulent phages (e.g. E. coli phage T4)
What phages undergo lysogenic cycle, with example?
Temperate phages (e.g. S. aureus 11). They can undergo lysogenic and lytic, depending on environmental conditions
What is a prophage?
The phage genome that has intergrated into its bacterial host’s chromosomal DNA
What is transduction?
The transfer of genetic material between microbes via a bacteriphage
What are the 3 types of transduction?
Generalised Specialised Lateral
What types of phages are involved in generalised transduction?
Virulent or temperate
What is generalised transduction?
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
What are 3 ways in which phages can influence their bacterial hosts?
Predation Lysogenic conversion Horizontal gene transfer
What is predation?
Phages destroying bacterial hosts Impacts/regulates bacterial populations; can be exploited by competing bacteria
What is lysogenic conversion?
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
What is a lysogen?
A bacterial cell infected by and in a state of symbiosis with a temperate phage
What is horizontal gene transfer?
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
What types of phages are involved in specialised transduction?
Only temperate phages
What is specialised transduction?
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)
What types of phages are involved in lateral transduction?
Only temperature phages
What is lateral transduction?
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
What can lateral transduction result in?
The scale of single transfer of DNA from lateral transduction can potentially transform a benign bacterium into a pathogen
