L15 Dissemination of Antibiotic Resistance Genomes II

Question 1

When antibiotics were first introduced, why did people believe resistance would not develop?

Answer 1

Because the frequency of mutation to resistance in bacteria was too low

Question 2

We now know that cells do acquire resistance - how?

Answer 2

By acquiring large pieces of DNA from other bacteria or from bacterial viruses

Question 3

What are the three modes of genetic exchange?

Answer 3

1. Transformation
2. Conjugation
3. Transduction

Question 4

True or false - genetic exchange is two-way.

Answer 4

False - it is one way

Question 5

What is the exogenate?

Answer 5

Donor DNA

Question 6

What is the endogenate?

Answer 6

Recipient DNA

Question 7

Describe the process of transformation.

Answer 7

Transformation involves the release of naked DNA into the environment by the lysis of the donor cell. Donor enzymes chop the released DNA into smaller pieces. The recipient cells secrete a protein competence factor that induces cells to synthesize proteins needed for competence. The double-stranded DNA is bound to proteins on the cell surface of the recipient and taken into the cell; as it passes through the membrane, one strand is degraded.

Question 8

True or false - in many species of bacteria, competence is encoded by chromosomal genes.

Answer 8

True

Question 9

What are three examples of pathogens that use transformation?

Answer 9

Gram +: S. pneumoniae

Gram -: H. influenzae and N. gonorrhea

Question 10

___ uses transformation to alter the appearance of clinically important surface antigens to permit evasion of the host immune system.

Answer 10

N. gonhorrhea

Question 11

___ can destroy naked DNA.

Answer 11

Nuclease detergents

Question 12

What are the three fates of the single-stranded exogenote?

Answer 12

1. Degradation (lost to nucleases)
2. Circularization (will be inherited as a plasmid or lost by dilution)
3. Recombination (stably retained)

Question 13

___ is sometimes called mating, as it involves cell-cell contact.

Answer 13

Conjugation

Question 14

Describe the process of conjugation.

Answer 14

In conjugation, the donor cell contains a conjugal plasmid. The recipient does not carry this plasmid. Genes carried by the R plasmid encode a sex pilus, which facilitates capture of the recipient cell to form a mating pair. A conjugation bridge forms between these two cells through which DNA passes. Transfer of DNA occurs through transfer replication. Replication initiates at the origin of transfer located on the R plasmid and proceeds by the rolling circle mechanism. One of the strands of DNA breaks. Then it is pulled away from its complement through the bridge. The strand left in the donor is replicated to reform the R plasmid. The transferred strand recircularizes and replicates into the R plasmid. The cells detach.

Question 15

What is the clinical relevance of conjugation?

Answer 15

Many important antibiotic resistance determinants are carried by conjugal R plasmids.

Question 16

What is unique about conjugation?

Answer 16

It mediates transfer both within and between species, rapidly disseminating multiple resistance.

Question 17

What is transduction?

Answer 17

Gene transfer that is mediated by bacteriophages

Question 18

What does it mean to say that phages are obligate parasites?

Answer 18

They reproduce only within bacterial cells

Question 19

What are phages (virions) composed of?

Answer 19

1. Protein capsid

2. DNA or RNA genome

Question 20

True or false - bacterial phages can carry genes that are not required for phage propagation.

Answer 20

True

Question 21

To cause the toxic effects of diphtheria and cholera, what must happen?

Answer 21

The host bacterium must be infected by a phage (beta-phage and CTX phage, respectively); these phages carry the genes encoding the necessary toxins.

Question 22

Describe the process of transduction.

Answer 22

A phage infects a sensitive cell by first adsorbing (binding) to a specific receptor. It then injects its genome into the interior of the cell.

Question 23

What are the two types of phages?

Answer 23

1. Lytic

2. Temperate (Lysogenic)

Question 24

What happens in lytic infection?

Answer 24

The phage replicates itself in the host, lyses the host cell, and releases the progeny phage.

Question 25

What happens in lysogenic infection?

Answer 25

The phage becomes latent and does not replicate. The phage genome is considered a prophage - it circularizes or integrates into the host chromosome.

Question 26

What is a lysogen?

Answer 26

A cell that carries a prophage