Bacteriophages Flashcards
What are bacteriophages?
These are viruses that infect bacteria.
Briefly discuss the structure of a bacteriophage as well as the function of each structural component.
(1) Head/Capsid: This is an icosahedral protein shell that encases the phage’s genetic material, which can be either DNA or RNA.
(2) Tail: Many bacteriophages have a tail structure that acts like a syringe to inject the phage’s genetic material into the host bacterium. The tail can be long and contractile or short and non-contractile.
(3) Tail fibers: These are long thin structures that extend from the base of the phage tail. They are responsible for recognizing and binding to specific receptors on the surface of the bacterial cell.
(4) Tail pins: These are spike like structures found at the baseplate and are used to puncture the bacterial cell wall once the tail fibers have secured the phage to the bacterial surface.
[Diagram 1] [Diagram 2] [Diagram 3]
[Image 1] [Image 2]
Not all bacteriophages have these components.
Briefly discuss the criteria used in the classification of bacteriophages.
Bacteriophages are classified according to:
(a) Genonme: DNA (ds or ss), RNA (ds or ss)
(b) Shape: Head + tail, Head only, Tail only [Diagram]
(c) Host: Escherichia virus, Salmonella virus, Klebsiella virus, Pseudomonas virus
(d) Cycle behaviour: Lytic vs Lysogenic
Discuss the bacteriophage lytic and lysogenic cycles.
Lytic cycle
🛸 After the phage has injected its genetic material into the host cell, the genetic material will take over the host cell’s machinery to produce phage components - new viral genome copies and proteins.
🛸 New phage particles are assembled from the synthesized components.
🛸 The host cell bursts (lyses), releasing the newly formed phages to infect other bacterial cells.
Lysogenic cycle
🛸 After the phage has injected its genetic material into the host cell, the phage DNA integrates into the bacterial chromosome, becoming a prophage.
🛸 The prophage is replicated along with the host cell’s DNA during cell division, passing the phage DNA to daughter cells.
🛸 Under certain conditions (e.g. stress), the prophage may excise from the bacterial chromosome and enter the lytic cycle, leading to production of new phages and eventual lysis of the host cell.
What is transduction in the context of bacteriophages?
Transduction refers to the process by which bacteriophages transfer genetic material form one bacterium to another.
Discuss the two types of transduction.
Generalized Transduction
🛸 During the lytic cycle, a bacteriophage accidentally packages fragments of the host bacterium’s DNA into its capsid instead of its own viral DNA.
🛸 When this phage infects another bacterium, it injects the bacterial DNA from the previous host.
🛸 This DNA can then recombine with the new host’s genome, leading to genetic variation.
🛸 This process is random and can transfer any part of the bacterial genome.
Specialized transduction
🛸 This occurs during the lysogenic cycle. When a prophage excises from the bacterial chromosome, it sometimes takes adjacent bacterial genes with it. The phage carrying these specific bacterial genes can then infect a new host, integrating the bacterial genes into the new host’s genome.
🛸 This process is not random; it transfers only specific portions of the bacterial genome that are adjacent to the prophage site.
Outline the applications of Bacteriophages.
(1) Phage therapy
🛸 Bacteriophages are used to treat bacterial infections.
(2) Phage typing (bacteria)
🛸 This is a method used to identify bacterial strains based on their susceptibility to different phages.
🛸 A panel of phages is used to infect bacterial samples. The pattern of susceptibility (lysis) helps identify the bacterial strain.
🛸 Applications: Epidemiological studies, tracking sources of bacterial outbreaks, and understanding bacterial diversity.
(3) Phage display (synthesis and study of proteins)
🛸 This is a technique to study protein interactions and discover new peptides or antibodies.
🛸 Phage display libraries are created by inserting DNA sequences encoding peptides or proteins into phage genomes. These phages display the peptides/proteins on their surface.
🛸 Applications: Drug discovery, vaccine development, and understanding protein-protein interactions.
(4) Gene delivery
🛸 This involves the use of phages to deliver genetic material into bacterial cells.
🛸 Engineered phages carry desired genes and inject them into target bacteria, facilitating genetic modification.
🛸 Genetic engineering, synthetic biology, and research on bacterial gene function.
(5) Vaccine delivery
🛸 This involves the use of phages to deliver vaccine antigens.
🛸 Phages are engineered to display antigens on their surface or carry genes encoding antigens. When administered, they stimulate an immune response.
(6) Biocontrol (plant diseases)
🛸 This is the use of phages to control bacterial pathogens in agriculture.
🛸 Phages are applied to crops to infect and lyse bacterial pathogens, reducing disease incidence.
What are some challenges encountered with phage therapy?
(1) Finding a phage that is effective for the target bacteria
(2) Some phages carry genes for bacterial toxins or antibiotic resistance
(3) Pharmacokinetics and pharmacodynamics are not well understood
(4) May trigger a humoral response
(5) Risk of anaphylactic shock due to bacterial endotoxins from impure products or rapid bacterial lysis
T4 Bacteriophage infects the bacterium ____________.
Escherichia coli
Which one of the following is true of bacteriophages?
(a) all have a head carrying the genome
(b) all have a tail for injecting the genome into the host
(c) have been licensed for treatment of drug-resistant tuberculosis
(d) may integrate into the host bacteria genome
(e) can attack human cells when applied in high doses
(d) may integrate into the host bacteria genome
Which one of the following statements about bacteriophages is true?
(a) They can harm human cells if applied in high doses
(b) They may be used to treat multidrug-resistant skin infections
(c) Contain both DNA or RNA
(d) They can transfer genes between bacteria through cross-fertilization
(e) The lysogenic cycle causes lysis of infected bacteria
(b) They may be used to treat multidrug-resistant skin infections
