Bacterial Evolution Flashcards
What are the 3 major evolutionary processes in bacteria?
- Point mutations
- Genomic rearrangements
- Horizontal gene transfer
Transition point mutation
A <-> G (purine<->purine)
OR
T <-> C (pyrimidine<->pyrimidine)
Transversion point mutation
Purine <-> Pyrimidine
Missense point mutation
Amino acid change
Silent point mutation
Codon change, but same amino acid
Nonsense point mutation
Introduces (premature) stop codon
Frameshift point mutation
Caused by insertion/deletion of base - shifts reading frame
What does the impact of a point mutation depend on?
Impact varies depending on:
- Codon redundancy (silent vs missense)
- Important of protein (critical enzyme vs accessory factor)
- Codon position (early = more disruptive)
What are point mutations similar to in viruses?
Antigenic drift
What are SNPs, and what can they be used to do?
Single nucleotide polymorphisms
Can be used to:
- Build phylogenetic trees
- Track transmission chains
What are four types of genomic rearrangements?
- Deletions
- Inversions
- Duplications
- Transposons
What are deletions in genomic rearrangement (with eg)?
Large portions of DNA that are deleted or inactivated (gene loss)
E.g. Mycobacterium leprae has 50% of its genome deleted or inactivated, reflecting its narrow host range
What is inversion in genomic rearrangement?
Flipping of DNA segments, altering gene orientation and regulatory context.
Used in phase variation; switching expression of surfacer antigens for immune evasion (e.g. antigenic variation in Salmonella)
What is duplication in genomic rearrangement?
Duplication of genes provides raw material for evolution of new functions
E.g. B-lactamase duplication (enzyme that breaks down B-lactam ABs) can provide broader resistance profile to ABs
What are transposons in genomic rearrangement?
Mobile genetic elements that move within a genome.
Often used in the transfer of AB resistance genes (e.g. Tn5)
What is the most rapid and impactful evolutionary force in bacteria?
Horizontal gene transfer
What does horizontal gene transfer enable?
- Acquisition of entire genes or operons
- Transfer between strains or species
- Spread of pathogenicity
Why is HGT important?
- Can convert commensal bacteria into pathogens
- Can spread resistance and virulence
- Allows cross-species emergence (zoonotic jumps)
What are the 3 mechanisms of HGT?
- Transformation
- Transduction
- Conjugation
What is HGT transformation?
Uptake of external DNA by a bacterial cell, which then incorporates it into its own genome.
Rare in nature, but high efficiency in select species
What is an example of a bacteria that uses HGT transformation?
S. pneumoniae is naturally competent, and is know to use transformation in capsule switching (uptake of capsule genes).
Makes vaccine development difficult
What is transduction, and what are the 3 types of transduction (HGT)?
Transduction involves the transfer of a DNA fragment from one bacterium to another by a bacteriophage.
The three types are:
- Generalised
- Specialised
- Lateral transduction
What is generalised transduction?
Lytic phages accidentally packaging and transferring bacterial DNA
Random genes transferred at low frequency
What is specialised transduction?
Temperature phages integrate into bacterial genome.
DNA near insertion site is co-packaged during excision.
High frequency transfer of specific genes
