HORIZONTAL GENE TRANSFER
Horizontal Gene Transfer
Horizontal gene transfer (HGT) is the most common mechanism of action used by bacteria to develop antibiotic resistance (Gyles and Boerlin, 2014). Whereas vertical gene transfer involves the movement of DNA from parent to offspring, horizontal gene transfer involves the movement of genetic material between organisms. Some mechanisms of horizontal gene transfer (e.g. transformation) can be considered a primitive sexual process, because of the similarity of combining DNA which is then passed onto future generations.
The mechanisms for horizontal gene transfer include: transformation, transduction, conjugation and transposition. Using these mechanisms, individual bacterial cells can pass on their antibacterial resistant genes to other bacterial cells rapidly, accelerating their evolution, and subsequent potential large-scale resistance to antibiotics.
Chromosomal genes can be transferred from one bacterium to another through a process called transformation. A donor bacterial cell releases chromosomal DNA into the environment, either when it is still alive or after it has died. A similar bacterium in the vicinity can uptake this resistant gene, and may incorporate it into it’s own chromosomal DNA through a process called homologous transformation. For the recipient donor to successfully recombine exogenous DNA into its chromosome, it needs to be competent, whereby it enters a specific physiological state. This often requires the expression of many genes (Solomon and Grossman, 1996). If enough genes are acquired, it may transform the host DNA. This is a relatively common process used in bacteria (Renner and Bellot, 2012).
Figure 1. Chromosomal genes being incorporated into host DNA through transformation.
DNA can be introduced to a bacterial cell by a virus or viral vector through a process known as transduction. This mechanism involves the presence of a bacteriophage (a virus that infects bacteria) and therefore does not require the donor and recipient bacterial cells to make physical contact (see Figure 2). Molecular biologists use transduction as a tool to introduce foreign DNA into the genome of a host bacterium in a stable way (Stearns and Hoekstra, 2005).
Figure 2. A bacteriophage injecting viral genome into a bacterial cell via transduction.
The genes that code for antibiotic resistance may be found on the host DNA, plasmids, or transposable elements; bacteria can transfer copies of plasmids to each other through a process called conjugation, which involves a bridge-like channel called a pilus that forms between the two bacterial cells (see Figure 3). This process requires stable cell to cell contact between the two bacteria, which lasts for an extended time (Stearns and Hoekstra, 2005).
Figure 3. The transferring of plasmid DNA between two bacterial cells through conjugation.
The process of transposition involves the presence of a transposable element (also known as TE, transposon or jumping gene). These are DNA sequences that are able to move from one locus to another between genomes (see Figure 4). The result can lead to an alteration of the cell’s genetic composition, or it can be innocuous. Due to their successful amplification ability, transposable elements comprise a large portion of the genome, and were originally considered to be “selfish DNA”, offering no benefit to the host. However, they are now considered important elements due to their role in genome function, evolution and epigenetic control (Bucher et al. 2012).
Figure 4. The jumping of a transposable element from one locus to another between genomes.
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