Genetic Variation, Gene Transfer and the Evolution of Virulence Flashcards
Tyeps of bacterial regulation of gene expression (2)
- regulation of transcription
2. regulation of transcription via DNA rearrangement
Bacterial regulation of transcription & virulence
- control gene expression based on environmental conditions
- e.g. up-regulation of virulence genes in low-iron conditions as occurs in human/animal host
Bacterial DNA rearrangement & virulence
- “phase variation”
- e.g. salmonella typhimurium: invertible segment of DNA reversibly switches synthesis between two flagellar antigens
- e.g. N. gonorrhoeae: alternation between different antigenic forms of pili
Mechanisms of bacterial genetic variation (3)
- spontaneous mutation
- recombination
- acquisition of new genes
Bacterial spontaneous mutation and virulence
- randomly occurring gene alterations
- low mutation rates
- examples: 1. Pseudomonas/TB antibiotic resistance 2. Strep. pyogenes increased invasiveness due to single amino acid change @ pyogenic exotoxin B
Bacterial recombination and virulence
- site-specific or homologous recombination –> new strains with new properties
- e.g. N. gonorrhoeae
- possibility of new traits/virulence factors
Types of bacterial acquisition of new DNA
- acquisition of transposable elements
- bacteriophage conversion
- acquisition of plasmid
- acquisition of “pathogenicity islands”
Transposon definition + types
- discrete segment of DNA capable of moving itself from one chromosomal location to another
- “insertion sequence”=encdoe transposase
- “complex transposons”=carry additional genes such as those encoding antibiotic resistance, toxins, virulence factors, etc.
Bacterial plasmids definition
autonomously replicating, usually circular, extrachromosomal DNA; may be transferred between bacteria via conjugation or transduction
“Pathogenicity islands” definition
large segments of DNA in some bacterial strains that encode virulence factors.
Types of bacterial gene transfer
- Transformation
- Transduction
- Bacterial conjugation
Bacterial transformation process
- naked DNA in vicinity
- transformable species becomes competent for DNA uptake @ certain point in growth cycle –> take up naked DNA
3.
Bacterial transduction definition
DNA transfer mediated by bacteriophage
Growth process of a “virulent” bacteriophage
- virus attaches and injects genome into bacteria
- viral genome replicated and proteins produced (“latent period”
- viral proteins assembled and released upon lysis of cell
Characteristics of “temperate” bacteriophages
- don’t invariably kill host cell
- lytic response: phage multiplication and cell lysis OR
- lysogenic response: host cell remains viable and viral DNA maintained w/in a prophage state
Prophage state characteristics
- viral DNA inserted into host cell genome
- viral DNA replicated as part of bacterial chromosome
- repressor protein maintains prophage state, usually until stress conditions –> stimulated to enter lytic state
Mechanisms of transduction
- error in DNA packaging results in incorporation of host bacterial DNA into new bacteriophage –> injected into a new bacteria to produce genetic recombinant (“transductant”)
- bacterial plasmid is incorporated and transduced
- bacteriophage conversion: bacteriophage genome encodes genes expressed during lysogenic state that cause new trait in bacteria
Bacterial conjugation definition
genetic transfer via physical contact between recipient and donor cells; usually mediated by plasmids
Conjugative plasmids definition
self-transmissible; mediate own transfer
F plasmid paradigm
- self-transmissible plasmid discovered in E.coli
- contains info for: 1. autonomous replication 2. sex pili 3. conjugative transfer 4. ability to integrate into bacterial chromosome
- transfer initiated by round of DNA synthesis begining at oriT (origin of transfer)
- transferred via conjugation bridge
Process of chromosome transfer by Hfr strains
- certain F+ cells may transfer sections of chromosomal DNA = “Hfr mating type”
- sex factor is linearly inserted into bacterial chromosome (instead of extrachromosomal)
- transfer is unidirectional w/polarity
