Bacterial Genetic Variation, Gene Transfer and Evolution of Virulence

Question 1

What does the bacterial genome consist of?

Answer 1

1. Chromosome = DNA molecule(s) that encode genes essential for growth under “normal” conditions.
2. Plasmids
3. Bacterial viruses

Question 2

What are the three mechanisms of genetic variation?

Answer 2

1. Spontaneous mutation
2. Recombination
3. Acquisition of new DNA segments

Question 3

How often do spontaneous mutations occur?

Answer 3

once in approx 10^8 – 10^10 organisms

Question 4

Where can you see recombination?

Answer 4

• Either site-specific or homologous recombination within a particular organism,
• or genetic exchange and recombination between closely related organisms that results in new strains

Question 5

What’s an example in bacteria of recombination?

Answer 5

Recombinational exchange between expressed and non-expressed pilin genes of N. gonorrhoeae can result in new pilin gene at the expression site, and therefore production of a new antigenetically distinct pili with new unique antigenic properties

Question 6

What types of DNA segments can bacteria acquire?

Answer 6

1. Transposable elements (IS elements and complex transposons)
2. Bacteriophage transfer (Lysogenic conversion)
3. Plasmids
4. “Pathogenicity islands”

Question 7

What is a transposon?

Answer 7

Segment of DNA contained within a bacterial or phage chromosome, or within a plasmid that has the property of being enzymatically moved from one DNA location to another

DNA that jumps places
transpose=move places

Question 8

T/F: Plasmids are self-replicating while transposons are not?

Answer 8

True

Question 9

What are pathogenicity islands?

Answer 9

• Large segments of DNA present in some chromosomes that are acquired from an unrelated organism
• PI appear as “insertions” when comparing the genomes of two isolates of the same species
• May contribute to virulence

Question 10

Just recognize that everything we talked about contributes to virulence of bacteria

Answer 10

They do this by conferring some kind of antibiotic resistance or provide additional virulence factors (adhesion, toxins, survival, etc)

Question 11

T/F: Spontaneous mutation occurs during replication or during natural growth are usually advantageous to bacteria?

Answer 11

False

• That’s why its so rare to get antibiotic resistance in one strain
• But when there is a selective advantage, mutation can persist if environment selects for it

Question 12

What are mechanisms (actions) for genetic exchange between bacteria?

Answer 12

Transformation
Transduction
Conjugation

Question 13

Which one is the genetic transfer of naked DNA strands/plasmids?

Answer 13

Transformation

Question 14

T/F: bacteria are able to acquire naked DNA at any stage of growth cycle?

Answer 14

False
Many transformable species become competent for DNA uptake at only certain points in the growth cycle, and competence requires synthesis of specialized proteins to mediate the uptake

Question 15

What key component is required for transfer of segments of DNA in transduction?

Answer 15

Bacteriophage (virus)

*you need an extra duck, or virus to do the transfer. Stupid I know… but when this question comes up on the test, you are going to think of transDUCKtion and then thank me. Duck Viruses…

Question 16

What’s the difference between the lytic infection and lysogenic formation with temperate bacteriophages?

Answer 16

Lytic: virus lysis bacteria and spills out components

Lysogenic: host remains viable and infecting phage DNA is maintained in host genome in non-infectious state (LYSed to GENes”)

Question 17

What is a prophage?

Answer 17

• Bacteriophage genome that is incorporated into bacterial DNA where it becomes passively replicated (lysogenic stage)
• Can lead to lytic stage. But when you hear prophage, think lysogenic.

Question 18

What other material can be transferred by transduction?

Answer 18

Bacterial plasmids, including those which confer antibiotic resistance, may also be transferred by generalized transduction from phages
-prevalent with Gram +

Question 19

What plasmid mediates conjugation?

Answer 19

Plasmid F (for “fertility”)

Question 20

The F plasmid contains genetic information encoding what traits?

Answer 20

1. Autonomous replication of plasma DNA

2. Synthesis of sex pili (F pili)

Question 21

What’s the difference between conjugative/ non-conjugative plasmids? (it’s what you think)

Answer 21

Conjugative Plasmids: self-transmissible, mediate their own transfer between cells.
Non-Conjugative Plasmids: mobilizable; can be passively transferred during conjugation

Question 22

What do you call the cell that is receiving the sex pilus?

Answer 22

F- =recipient cell

F+ = sex pilus

Question 23

What can conjugative transposons do?

Answer 23

• mediate conjugation transfer between cells
• also can transpose into recipient genome
• Often encode antibiotic resistance, especially Tetracycline

Question 24

T/F: bacteria can transfer through conjugation between both gram (+) to gram (-), and also gram (-) to plants and fungi?

Answer 24

True

Question 25

LO: Describe how errors in bacteriophage development can lead to phage-mediated gene transfer.

Answer 25

1. During phage assembly, it can accidentally insert bacteria DNA instead of phage DNA into capsized
2. It can still attach to new host and inject its bacterial DNA
3. Bacterial DNA then recombines in recipient genome to produce a transductant

Question 26

LO: Define lysogenic conversion, and how its different from normal transduction.

Answer 26

• During lysogenic state, temperate phage DNA may be expressed in host cells causing appearance of new phenotypic trait in lysogenic host.
• These genes are found only as a component of the phage genome (where as, generalized transduction is of normal bacterial genome).