Genetic Variation, Gene Transfer and the Evolution of Virulence Flashcards Preview

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Flashcards in Genetic Variation, Gene Transfer and the Evolution of Virulence Deck (21):

Tyeps of bacterial regulation of gene expression (2)

1. 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
-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

1. acquisition of transposable elements
2. bacteriophage conversion
3. acquisition of plasmid
4. 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

1. Transformation
2. Transduction
3. Bacterial conjugation


Bacterial transformation process

1. naked DNA in vicinity
2. transformable species becomes competent for DNA uptake @ certain point in growth cycle --> take up naked DNA


Bacterial transduction definition

DNA transfer mediated by bacteriophage


Growth process of a "virulent" bacteriophage

1. virus attaches and injects genome into bacteria
2. viral genome replicated and proteins produced ("latent period"
3. 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

1. error in DNA packaging results in incorporation of host bacterial DNA into new bacteriophage --> injected into a new bacteria to produce genetic recombinant ("transductant")
2. bacterial plasmid is incorporated and transduced
3. 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

1. certain F+ cells may transfer sections of chromosomal DNA = "Hfr mating type"
2. sex factor is linearly inserted into bacterial chromosome (instead of extrachromosomal)
3. transfer is unidirectional w/polarity