Horizontal gene transfer (HGT) in prokaryotes refers to the movement of genetic material between organisms, bypassing traditional reproduction. This m

Horizontal gene transfer (HGT) in prokaryotes refers to the movement of genetic material between organisms, bypassing traditional reproduction. This mechanism is crucial for genetic diversity and can have significant consequences, especially in the context of antibiotic resistance. Below are the three main types of horizontal gene transfer:

1. Transformation

  • Definition: Uptake of free DNA from the environment by a prokaryotic cell.

  • Mechanism:

    • Prokaryotes, such as Streptococcus pneumoniae, can take up DNA fragments from lysed cells in their surroundings.

    • This DNA can integrate into the recipient’s genome through homologous recombination if there is sufficient similarity.

  • Consequence for humans:

    • Can result in the acquisition of virulence factors or antibiotic resistance genes. For example, S. pneumoniaecan acquire resistance to penicillin through transformation.

2. Transduction

  • Definition: Transfer of genetic material between bacteria via bacteriophages (viruses that infect bacteria).

  • Mechanism:

    • Generalized transduction: During the lytic cycle, bacterial DNA is accidentally packaged into a viral capsid. When the phage infects another cell, it transfers this DNA.

    • Specialized transduction: During the lysogenic cycle, prophage excision from the bacterial chromosome can carry adjacent bacterial genes. These are transferred when the phage infects a new host.

  • Consequence for humans:

    • Transduction can disseminate genes for toxins, such as the diphtheria toxin encoded by Corynebacterium diphtheriae, or antibiotic resistance.

3. Conjugation

  • Definition: Direct transfer of DNA from one bacterial cell to another through a physical connection (pilus).

  • Mechanism:

    • Requires a donor cell containing a conjugative plasmid, such as the F plasmid (fertility factor) in E. coli.

    • A conjugative pilus forms between the donor and recipient cells, facilitating the transfer of plasmid DNA.

    • The transferred plasmid may carry antibiotic resistance genes, virulence factors, or metabolic genes.

  • Consequence for humans:

    • Conjugation is a major driver of the spread of multi-drug resistance (e.g., plasmids carrying resistance genes for carbapenems, a last-resort antibiotic).

Serious Consequences for Humans

  1. Antibiotic Resistance:

    • HGT allows rapid dissemination of resistance genes, leading to the rise of multidrug-resistant pathogens (e.g., MRSA, Escherichia coli producing extended-spectrum beta-lactamases).

  2. Increased Pathogenicity:

    • Acquisition of virulence factors can make previously harmless bacteria pathogenic. For instance, Vibrio cholerae gains toxin production ability through lysogenic conversion via transduction.

  3. Public Health Threats:

    • Horizontal gene transfer complicates infection control and treatment, resulting in higher morbidity, mortality, and healthcare costs.

 

Horizontal gene transfer - genetic transfer in bacteria

3 methods: Transformation, Transduction, Conjugation

Transformation

- Bacterial transformation: Exchange of genetic information in different strains of

bacteria, in the form of a fragment of the naked DNA by donor to recipient cell. Leads

to recombination in the recipient's chromosome. (incorporating of DNA)

 

- Natural transformation: Binding, take up and recombination of exogenous

DNA.(originating from outside of the cell.) Induced by high cell density and/or

nutritional limitation, associated with the stationary phase of cell growth.(Self

interpretation that it is the response/attempt of the cell inorder to further adapt to the

environment)

Transduction

- Introduction of foreign DNA by virus or virus vector. (Virus that infect bacteria are

called bacteriophages - referred to as phages)

- Happens either through a lytic or lysogenic cycle.

- Lytic cycle - bacteria manipulated by phages to reproduce more phages, eventually

killing the bacteria and releasing the phages.

- Lysogenic cycle - phage DNA is incorporated inside the bacteria’s chromosome,

laying dormant until induced (e.g. by UV light). Phage is excised from the

chromosome initiating the lytic cycle.

 

Conjugation

- Transfer of genetic material between two bacteria by direct contact

- Donor carries Fertility factor - allow donor to produce a thin, tubelike structure called

pilus to contact the recipient.

- Pilus then pulls the bacterias together and genetic material is transferred (typically as

plasmid or small circular piece of DNA.

- Often provides the recipient with adva