L9 - Predators of Bacteria: Phages Flashcards
What is a bacteriophage?
specialised virus that kills bacteria (infects and replicates within bacteria)
- virus that infects archaea are also referred to as bacteriophages
How were bacteriophages discovered?
invisible agent was able to be filtered, isolated and cultures (experiments in phage therapy)
Phages contribute to a ____ percentage of virus genus
large
How are bacteriophages classified? (types of morphology)
according to morphology and nucleic acid type.
What are the different virion structures?
icosahedral, filamentous (usually ssDNA), head & tail (usually dsDNA)
ssRNA vs ssDNA vs dsDNA in bacteriophage
phages that has ssRNA are usually smaller <ssDNA (filamentous) < dsDNA (head & tail)
Why do bacteriophages require host machinery?
- they need their metabolites/energy for replication
- to make proteins and replicate genomes
- inserts its dna, switches on a lot of phage genes and off of some bacterial genes
- phage hijacks protein synthesis to make its own heads and tails ad lyses the bacterial cell
How are bacteriophages a route for genetic exchange?
HGT via transduction
Why are bacteriophages used in labs?
can manipulate genes and carry mutations
Bacteriophages can alter _____ of infected cells when not in lytic phase
properties/phenotypes
- - salmonella has lots of phages bc interacts with a lot of genetic diversity in its environment
- but in lab selective pressures are lost so they lose that ability to be diverse in phages
Why are phages more abundant in people with unhealthy microbiome
unhealthy individuals has changes in micrbiota, they are more abundant bc prophages that are usaully in chromosomes have been excited due to SOS response
What are the 2 different phases of phages and describe each
lytic = virulent - causes lysis of host
temperate (lysogenic) = capable of both lytic and lysogenic (depending on environment)
What is the significance of bacteriophage being an episome?
replicates in 2 different ways
- can replicate independently but also in association with chromosome if integrated
What are prophages?
integrated into chromosome, usually dormant and wont cause huge problems just genetic buildup
What are the entry stages of bacteriophage lifecycle?
- recognises surface receptors and absorbed into surface
- phages make enzyme to degrade cell membrane (peptidoglycan if gram positive, inner membrane if gram negative)
- if head and tail phage: tail retracts and nuclear acid inserted into cell. if filamentous: makes holes and inserts nuclear acid
What happens after entry if lysogenic? (head and tail)
can get incorporated into chromosomes as prophages or circularised into plasmids to survive (lysogenic = lives in bacteria)
What happens in the lytic cycle (head and tail)?
- inject nuclear acid into cytoplasm but doesnt incorporate into the chromosome
- they just want to make viral particles into cytoplasm and not live in the bacteria
- uses cells machinery to make copies
Describe the early, mid and late gene expression in lytic lifecycle (head and tail)
- early stages, they try to shut down normal processes of bacteria cell and then hijacks
- mid gene expression: becomes phage factory (dont want the late gene expression bc it doesnt want to leave so it is highly regulated)
- late gene expression = phages encodes for enzyme that lyses the bacterial cell and viral particles can escape the cell
- burst size reached certain number of phage are assembled), late gene expression is switched on
What happens to filamentous phages after entry
makes temporary holes that is filled after they leave so they can infect more bacteria
Lytic vs lysogenic (how is it chosen and converted)
lysogenic can become lytic when conditioins change
- when bacterial cell is exposed to stress/damage (DNA damage of the host, influx of nutrients, encironment etc.) signals thay the phage can survive without the host
- itll become lytic and leave so it doesnt get damage
- poor condition → limited bacteria so doesnt leave (no point)
- good condition → lots of bacteria so could potentially leave
What is aberrant packaging? How does that lead to prophage induction and HGT?
during lytic cycle, it replicates its own materials, sometimes pieces of the host DNA can get incorporated into phage DNA. it becomes a transductant and can conduct horizontal gene transfer
Explain the following properties of phages:
- specific receptors for infection
- little genome redundancy
- orderly gene expression
- protein splicing
- host defence mechanism
- host-subversion systems
- larger host range bc each bacteriophage is specific for one
- all genes are useful, essential and non essential (supplements host gene)
- tightly controlled differential gene expression (early, mid, late)
- encode inteins and econs (doesn’t usually happen in prokaryotes)
- foreign DNA w diff methylation patterns they will recognize and kill them, host recognises CRISPR as previously encountered phages
- nucleic acid modification, anti host restriction proteins, restriction of host chromosome, modification of host RNAP
- - cytosine is modified in phages and the site is not glucosylated (it should be)
- makes it sown DNA unique so host enzymes cant destroy it
- anti host property example: T7 makes Ocr (overcome classical restriction)- mimics structure of bDNA which overlaps nicely with protein
- inactivates post restriction proteins
- shuts down host RNA polymerase and makes its own RNA polymerase at a higher level to improve their survival
-
What is the medical importance of bacteriophage?
phages that kill bacteria (e.g. streptococcus, corynebacterium, - x1 and Stx2 are toxins from E. coli phages
- removal of phages leads to the removal of virulence factors
What are some applications of phages?
- phage therapy
- phage typing
- biocleansing
- food pathogen reduction
- diagnostics
- surface display of peptides and antibodies
Why are phages good for these applications and what are the drawbacks?
- growing AMR so phage therapy > bacterial resistance
- good bc we can idnetify phages that are specific to bacteria (we dont want a phage that is generic and broad we need it to be specific)
- we add phage there might be a stress and bacteria might change its properties so it is very difficult to treat using phage therapy (phage resistance)
- bc of the specificity we cna also use it to identify the bacteria (phage typing to identify bvacteria)
Applications of phages part 2
- vaccine/drug delivery systems
- tools for molecular biology : T7 RNAP (can make long transcripts in short time), gave us promoters for controllable gene expression, T4 ligase, transposing mutagenesis, CRISPR
