Bacteriophages Flashcards
What are bacteriophages?
A bacteriophage are viruses that only infect and replicate inside of bacterial cells
Only infect bacteria not human cells so there is no risk to humans
What does the timeline for the discovery of bacteriophages look like?
1915: Frederick twist discovered viruses infecting bacteria
He purposed 3 things:
- stage in life cycle of bacteria
- an enzyme produced by bacteria themselves
- a virus that grew on and destroyed bacteria
He couldn’t demonstrate which theory was true
1917:
Felix dherelle independently discovered viruses of bacteria and named them
Allowed the study of bacteria replication
What are the characteristics of bacteriophages?
Have different shapes
Genetic composition of DNA or RNA
Genomes may encode as few as 4 genes and as many as 100s of genes
Give 3 applications of bacteriophages
1) models for animal cell viruses
2) gene transfer in bacteria
3) medical applications - treatment (anti microbial resistance) alternative to antibiotic
How are bacteriophages classified?
19 families according to morphology and nucleic acid
RNA GENOMES (2 families)
Envelope (5 families)
Single stranded DNA genome (2 families)
Circular DNA genome (8 families)
Linear DNA genomes (9 families)
Explain the structure of a bacteriophage
Capsid head on top
Nucleic acid (DNA) inside of head
Collar and sheath attached to head make up the tail
At the bottom of the tail is the baseplate
Attached to the base plate are spiked and tail fibres (lil leg looking things)
Shapes differ
What are the 3 types of bacteriophage structures?
Myoviridae
But capsid thick tail tube with sheath medium length has baseplate
Siphoviridae
Long tail tube small capsid no base plate
Podoviridae
No tail tube small capsid
No base plate
What are the 2 types of bacteriophages in terms of life cycles?
Lytic phages
- lytic cycle
Temperate phages
- lytic cycle
- lysogenic cycle
Explain the lytic cycle
1) absorption
Protein in tail of page recognised and bind to specific receptors on bacterial cell wall
2) penetration
Phage injects it’s DNA into bacteria or RNA (genome)
3) replication
The page DNA is copied and expressed to make proteins like capsid proteins
4) maturation
Capsids assembled from proteins to make new phages
5)release
A bacteriophage- coded enzym breaks down the peptidoglycan in the bacterial cell wall causing osmotic lysis to release newly formed bacteriophages
6) reinfection
From 50 to 200 phages may be produced per infected bacterium
Explain the lysogenic life cycle of a bacteriophage
1)absorption
Phage attaches to bacteria cell through specialised receptors
2)penetration
genome injected into bacteria cell
3)prophage formation
Viral DNA integrates into bacterial cell DNA
4)maintenance
Bacteria continues to divide without knowing its infected
5) spontaneous induction
6)replication
7) maturation
8) release
What’s the main side effect of the lysogenic cycle?
Lysogenic conversion
- non virulent bacteria can transform into highly virulent pathogens through lysogenic conversion
Give 4 examples of lysogenic conversion
1) corynebacterium diphtheriae produces the diphtheria toxin only when it is infected by the phage beta
2) vibrio cholerae is a non toxic strain that can become toxic producing cholera toxin, when it is infected with the phage CTXpsi
3) shigella dysenteriae produces dysentry toxins, stX1 and stX2, when infected with lambdoid prophages
4) certain strains of clostridium botulinum, which causes botulism, express botulinum toxin from phage induced genes
Give 5 fields in which bacteriophage’s are used
Phage therapy
Food industry
Diagnostic
Counteracting bio weapons and toxins
Antimicrobial drug discovery
Give 2 examples where phages have been used in therapy
Eli is institute of bacteriophages, microbiology and virology - phages studied and found that phage therapy has an 80% success rate against entericoccus infections
In Poland doctors had a 90% success rate against cases of staphylococcus aureus, pseudomonas aeruginosa, klebsiella pneumoniae and escherichia coli
What is phage therapy?
Phage therapy is the therapeutic use of lytic bacteriophages to treat pathogenic bacterial infections
What are the principles of phage therapy?
1) phage must be lytic
2) a single dose of phage should treat an infection
3)phage is non toxic and highly specific for targeted bacterial populations
4)antibiotic-resistant bacteria Ramon sensitive to phage-mediated lysis
Why can lysogenic bacteriophages not be used for phage therapy?
Because there is a chance of lysogenic bacteriophages turning into dangerous pathogens through lysogenic conversion
Compare bacteriophages vs antibiotics
Phages
- Lytic phages have good antibacterial activity
- Usually affect only the targeted bacterial species
- disease causing bacterium must be identified only kills desired bacterium
- less good bacteria death
- the rate of developing resistance to phages is approx 10 fold lower than antibiotics
- selecting new phages against phage resistant bacteria is a relatively rapid process that can frequently be accomplished in days or weeks
- rare side effects for therapeutic phages
- phages target specific bacteria and have no effect in eukaryotic cells
- possibility of liberation if endotoxins from bacteria lysed in vivo by phages
Antibiotics
- target both pathogenic microorganisms and normal micro flora ( non harmful bacteria)
- resistance to antibiotics is not limited to targeted bacteria
- developing new antibiotic, for example against antibiotic resistant bacteria is a time consuming process and may take several years
- multiple side effects including intestinal disorders, allergies and secondary infections
What are some factors concerned with the effectiveness of phage treatment?
Phage concentration
- what is the correct concentration of the dosage (MOI ratio of phage given to parents at which it’s effective)
- is a single dosage effective (unknown) and what time to give different doses
Phage administration
- how can dose be delivered to patient so it can reach the infection (oral, topical)
Environmental conditions
- what environment can a phage survive in can it survive in a human body ( more info needed)
Neutralisation
- don’t know how to neutralise the phages ( to stop unnecessary replication)
Resistance to phage
- bacteria may become resistant
How can phages be used in diagnostics and why is it used?
Uses cocktail of bacteriophages ti detect S. aureus in positive blood cultures and determine methicillin (narrow spectrum antibiotic from penicillin class) resistance or susceptibility
Quicker to diagnose
5 hour turn around compared to 2 to 3 days for standard testing methods
