Bacteriophage Flashcards
What Are BacterioPhages?
Viruses that Specifically Infect + Replicate In Bacteria
What is A Plaque Essay?
Technique to Study + Quantify viruses
Plaque:
Clear zone in lawn of bacterial cells growing on agar
Forms when bacteriophages Infect + Lyse
How Is A Plaque Assay Prepared?
Dilute Phage sample mixed with Susceptible Bacterial Culture
Mixture poured onto agar Plate + Allowed to solidify.
Bacteria grow into cloudy “lawn,” but wherever phage infects + kills bacteria, a clear plaque forms
What Structure Can Bacteriophages Have?
✓ Icosahedral
✓ Filamentous
✓ Complex
What is The Life Cycle of Bacteriophages?
Lytic Phages
- Lytic Cycle
Temperate Phages
- Lytic Cycle
- Lysogenic Cycle
Describe The Lytic Cycle.
- Bacteriophage binds to Receptors in
Bacterial Cell Wall - Bacteriophage injects genome into
Cytoplasm of Bacterium - Bacteriophage Genome replicates
Nucleic Acid + Capsid Proteins Produced in
Host’s Metabolic Machinery - Bacteriophage Components Assemble
- Bacteriophage-coded Enzyme Breaks down
Peptidoglycan in Bacterial Cell Wall, causes
Osmotic lysis
Describe The Lysogenic Cycle.
- Bacteriophage Bind to receptors on Bacterial
Cell Wall - Bacteriophage Injects Genome Into
Bacterium Cytoplasm - Bacteriophage Inserts Genome Into
Nucleoid → Becomes prophage - Prophage Replicates as part of Bacterium’s
Nucleoid as Bacterium Replicates - Phage Genome Excised from Bacterial
Nucleoid, becomes Lytic - Bacteriophage Genome Replicates and its
Nucleic Acid + Capsid Proteins are Produced
By Bacterium’s Metabolic Machinery - Bacteriophage components assemble
- Bacteriophage-Coded Enzyme breaks down
Peptidoglycan in Bacterial Cell Wall, Causing
Osmotic Lysis
How Does a Phage Switch Between The Lytic and Lysogenic Pathway?
Terms:
- Lambda Repressor (CI)
- Lysogeny Establishment Regulator (CII)
- Control Of Repressor and Others (CRO)
Method:
Using Phage-Encoded Protein CII
CII Monitors Health Of Cell + Impact Decision
CII Acts as switch, Activates CI Transcription
What is The Function of CII?
- CII acts like a “Switch” for Lysogeny
- CII Activates CI gene → Starts producing CI
Protein - Activates Integrase, helps Phage DNA
Integrate Into Bacterial Chromosome
What happens If The Lysogenic Cycle Dominates?
Lysogenic Cycle Dominates = CII Wins
- After phage Injects its DNA Into bacterium,
CRO + CII Genes are expressed - CII is stable + active:
- High Levels of CI Protein made.
- CI Binds to Phage DNA, Blocking CRO Gene +
Other Lytic Genes - CI Activates its own production, Create Self-
Sustaining Loop To Maintain Dormancy - Phage DNA Integrates Into Host Genome +
Replicates Passively with Bacterium.
What happens If The Lytic Cycle Dominates?
After phage injects its DNA into bacterium, CRO and CII genes are expressed
Lytic Cycle Dominates = CII Fails
CII Degradation (e.g., by bacterial proteases like HflB):
- CI production stops + Existing CI Proteins
Break down - CRO Protein Accumulates + Binds to Phage
DNA, repressing CI gene - Lytic genes (for replication, packaging, and
cell lysis) are activated - Phage replicates its DNA, assembles new
viruses, and Lyses Cell
How Can Non-Virulent Bacteria become highly Virulent?
Via Lysogenic Conversion, Where A Bacterium Acquires New Traits and Characteristics
What is Phage Therapy? What Are The key Concepts?
Therapeutic use of Lytic Bacteriophages To Treat Pathogenic Bacterial Infections
Key Concepts:
- Phage Needs To Be Lytic
- Single Dose Should Treat Infection
- Phage Must be Non-Toxic + highly Specific To
Targeted Populations - Antibiotic Resistant Bacteria Remain Sensitive
To Phage-mediated Lysis
Compare Bacteriophages and Antibiotics
Specificity:
B: Target Narrow Host Range
A: Target Pathogenic microorganisms + Normal
Microflora
Resistance:
B:
- 10-Fold lower rate
- New phage Selection Accomplishable In
Days/Weeks
A:
1. Development May Take Several Years
2. Resistance to Antibiotic not limited to
Targeted Bacteria
Side Effects:
B: Minor Side Effects
A: Multiple Side Effects, Including Intestinal
Disorders, Allergies + Secondary Infections
What Are bacterial Host Defense Mechanisms?
Restriction Endonucleases
Host protecting DNA By Modifying It
Antiviral System CRISPR
CRISPR = Clustered Regularly Interspaced Short
Palindromic Repeats
Short DNA Sequences in Bacterial genome → Left behind By Phage that previously Infected Bacteria
CRISPR-Cas9 → Applied in genome Editing
What Are some Applications of Bacteriophages?
✓ Restriction Endonucleases
✓ DNA Ligases
✓ DNA Polymerases
✓ RNA Polymerases
✓ Restriction Endonucleases
✓ prevention of Foodborne Diseases
✓ Vaccines + Vaccine Carriers
✓ Recombinases
