Genetic basis for resistance: Bacteriophages, transposons, integrons

Question 1

Bacteriophages:

Answer 1

Bacterial viruses which have a narrow and specific host range. They have DNA or RNA with a protein coat. Replication of phage genome happens alongside that of host DNA
Lysogenic phages: in prophage state, expression of some prophage genes may confer additional properties on cell.

Question 2

Bacteriophage infection stages: (4)

Answer 2

1. )The phage attaches to the bacterium and injects its DNA.
2. ) The phage DNA replicates within host cell
3. ) Capsid components are synthesised, new phage particles are assemble and released.
4. ) After cell lysis, new phage particles are released and are able to infect

Question 3

Lysogeny:

Answer 3

1. ) Phage particle attaches to an E.coli cell and injects its DNA.
2. ) DNA circularizes
3. ) DNA then integrates into host chromosomes.
4. ) Cell division occurs.
   * Next 2 stages are called induction stages*
5. ) In the newly created cells, the DNA excises from the host chromosome.
6. ) New phage particles are produced upon cell lysis

Question 4

bacteriophage in the environment:

Answer 4

Urban sewage and river water have been explored for phage-encoded resistance genes. They found that antibiotic resistance genes in the bacteriophage DNA were a fraction of what they were in environmental sammples.
Faecal waste from livestock was also investigated and found that phages persist well in aquatic environments and could be vehicles for mobilisation of environmental resistance genes.

Question 5

Transposons: The jumping genes

Answer 5

Mobile DNA sequences that are able to transfer from one DNA molecule to another. They can’t replicate independently and must be maintained within functional replicon (chromosome or plasmid).
They do not require extensive DNA homology to insert into the recipient DNA molecule.
Bacteria can acquire several different resistance genes by series of transposon insertions in resident plasmids
They belong to a family of transposable elements which includes insertion sequences; these are required for the DNA to jump via transposase enzymes.
Transposons differ from insertion sequences in that they encode a function that changes phenotype.

Question 6

Integrons:

Answer 6

These are genetic elements which contain a site where additional DNA (in the form of gene cassettes can be integrated by site-specific recombination mediated by integrase. Gene cassettes are discrete genetic elements that may exist as free, circular or non-replicating DNA molecules moving from one DNA site to another. They are normally linear sequences that constitute part of a larger DNA sequence. Gene cassettes usually only carry one gene but also have a 59 base element which acts as a specific recombination site. The genes on gene cassettes lack promoters and need expression from a promoter on an integron. Integrons consist of two conserved segments of DNA which exist either side of the inserted antibiotic resistance genes. The-site specific integration may allow for the formation of new combinations of resistance genes (on the plasmid or transposon) .
Plasmids and transposons can acquire multiple resistance determinants via a site-specific integration mechanism. This is mediated by a family of DNA elements called integrons.
integrons encode components of a recombination system that recognises and captures mobile gene ‘cassettes’. These can be inserted in different orders or combinations.

In summary, they are efficient tools in bacterial adaption and play significant role in antimicrobial resistance.
Integrons may spread resistant genes via mobility of integron itself or following insertion of integron into transposon or horizontal transfer or integrons on plasmids