Horizontal Gene Transfer
The process of horizontal gene transfer (otherwise known as HGT) can be described as one organism acquiring the DNA of another organism. This can be observed in many types of organisms but is usually performed by prokaryotic organisms – mainly bacteria. This process can be done in a multitude of ways, and results in an “endosymbiotic” organism, containing both sets of DNA.
There are four main methods used by bacteria for HGT: conjugation, transformation, phage-mediated transduction, and introgression.
This method of HGT occurs through direct contact between two bacterial cells. Conjugation starts with the separation of the DNA to be transported over – using a section of bi-functional enzymes, which have trans-esterase activities built into them. Next, the DNA is transported across the covalent bond between the two cells and is unwound by DNA helicase enzymes – ready to now enter the new bacterium and become integrated into its own DNA makeup.
Conjugation has been observed at different levels, through many methods of study. These studies have shown that, a majority of DNA that is transferred via bacterial conjugation is in some way related to antibiotic resistance.
Bacterial transformation is another major method by which bacteria perform HGT – with natural transformation being the main process of incorporating another organism’s DNA into its own genome. This process is observed when a bacterial cell allows a section of its DNA to leave the cell and allows it then to be taken up by another, adjacent bacterium. It can then be integrated into its own DNA, which can then be used for acquiring new functions.
This process, as is with the other methods of HGT, is essential to cell speciation, adaptation, and cellular evolution. It can assist in transferring DNA to another organism to increase virulence and improve the efficiency of metabolic reactions. Most importantly, this process can introduce a major, new defence function: antibiotic resistance.
How antibiotic resistance spreads between bacteria
In general, bacteriophages (or ‘phages’) are a type of viruses, which infect bacterial cells. There is a type of phage called ‘temperate bacteriophages’, which can transfer the DNA of one bacterium into another. This process is called phage-mediated transduction.
Phage-mediated transduction has been observed in bacterial cells, usually transferring genes responsible for virulence, and for antimicrobial resistance. In a study performed in 2011, three types of environmental bacteriophages were isolated, and placed into two species of bacteria: Enterococcus faecalis and Enterococcus gallinarum. These bacteria were then left, along with the phages, to allow transduction to take place.
Once transduction had occurred, the bacteria were subjected to different types of antimicrobials – and it was found that most of the Enterococcus fecalis has become resistant to tetracycline, and that most of the Enterococcus gallinarum bacteria has become resistant to gentamicin. It was therefore concluded that phage-mediated transduction plays a large role in antimicrobial resistance within bacterial colonies.
This method of HGT can take place in many organisms – including plants and bacteria. It is the process of ‘back-crossing hybridization’ between one organism and another. In bacteria, this can mean that one bacterium can hybridize itself with another species of bacterium – thus creating a new, ‘hybrid’ bacteria, which contain parts of both of the original organism’s DNA.
Through this type of DNA hybridization, it has been observed that some new hybrid bacteria have acquired antimicrobial resistance – a characteristic that any of the original species did not possess.