Genetic Variation, Gene Transfer and the Evolution of Virulence

Question 1

Tyeps of bacterial regulation of gene expression (2)

Answer 1

1. regulation of transcription

2. regulation of transcription via DNA rearrangement

Question 2

Bacterial regulation of transcription & virulence

Answer 2

• control gene expression based on environmental conditions

- e.g. up-regulation of virulence genes in low-iron conditions as occurs in human/animal host

Question 3

Bacterial DNA rearrangement & virulence

Answer 3

• “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

Question 4

Mechanisms of bacterial genetic variation (3)

Answer 4

• spontaneous mutation
• recombination
• acquisition of new genes

Question 5

Bacterial spontaneous mutation and virulence

Answer 5

• 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

Question 6

Bacterial recombination and virulence

Answer 6

• site-specific or homologous recombination –> new strains with new properties
• e.g. N. gonorrhoeae
• possibility of new traits/virulence factors

Question 7

Types of bacterial acquisition of new DNA

Answer 7

1. acquisition of transposable elements
2. bacteriophage conversion
3. acquisition of plasmid
4. acquisition of “pathogenicity islands”

Question 8

Transposon definition + types

Answer 8

• 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.

Question 9

Bacterial plasmids definition

Answer 9

autonomously replicating, usually circular, extrachromosomal DNA; may be transferred between bacteria via conjugation or transduction

Question 10

“Pathogenicity islands” definition

Answer 10

large segments of DNA in some bacterial strains that encode virulence factors.

Question 11

Types of bacterial gene transfer

Answer 11

1. Transformation
2. Transduction
3. Bacterial conjugation

Question 12

Bacterial transformation process

Answer 12

1. naked DNA in vicinity
2. transformable species becomes competent for DNA uptake @ certain point in growth cycle –> take up naked DNA
   3.

Question 13

Bacterial transduction definition

Answer 13

DNA transfer mediated by bacteriophage

Question 14

Growth process of a “virulent” bacteriophage

Answer 14

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

Question 15

Characteristics of “temperate” bacteriophages

Answer 15

• 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

Question 16

Prophage state characteristics

Answer 16

• 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

Question 17

Mechanisms of transduction

Answer 17

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

Question 18

Bacterial conjugation definition

Answer 18

genetic transfer via physical contact between recipient and donor cells; usually mediated by plasmids

Question 19

Conjugative plasmids definition

Answer 19

self-transmissible; mediate own transfer

Question 20

F plasmid paradigm

Answer 20

• 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

Question 21

Process of chromosome transfer by Hfr strains

Answer 21

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