Horizontal gene transfer

Do bacteria absorb plant genes?

Bacteria are capable of exchanging genetic material directly between each other - and even across species boundaries. But are they also able to absorb plant DNA? If this were the case, bacteria could also exploit those genes which have been newly inserted into genetically modified plants. And yet the results of all research projects which have looked into this issue are consistent: although this type of horizontal gene transfer is theoretically possible, it is nonetheless an extremely rare event.

Bacteria: Diverse and flexible. In plants, the transfer and mixing of genetic material takes place by propagation, generally between partners of the same species. Bacteria are able to exchange genetic material directly, even across species boundaries. This is made possible by so-called "mobile genetic elements" such as plasmids for example. These are small, ring-shaped DNA elements which are found in bacterial cells alongside the main chromosome and are able to replicate independently. The copy of this type of plasmid can enter a receptor cell through direct contact. Other mobile genetic elements include transposons ("jumping genes"), and bacteriophages , viruses which infect bacteria and inject genetic material into their host.

Using "mobile genetic elements" - in this case a plasmid, a small DNA unit which is independent of the main chromosome - bacteria can exchange genetic material very easily on a large scale.

A safety research project looked at whether a gene that confers herbicide resistance can be transferred from rape pollen to micro-organisms in the bee intestine by horizontal gene transfer. Researchers were unable to demonstrate this type of gene transfer. They did find herbicide-resistant bacteria, but such occurrences of resistance are widespread in nature.

E.coli-bacteria are present in the intestine of humans and animals. Many of these bacteria are already resistant to antibiotics.

The classic method used to identify resistance genes involves cultivating bacteria on a nutritive medium. If they are able to survive on the nutritive medium, which contains antibiotics, they are resistant. But only very small numbers of bacteria can be cultivated. Today it is possible to remove DNA directly from soil samples and examine it for the presence of resistance genes.

In the illustration above, discs containing different antibiotics are placed in the cell solution of a specific bacterium. So-called "zones of inhibition" develop around the antibiotic discs to which the bacterium is susceptible.

Some bacteria can even absorb free DNA from their environment. But for this sort of DNA to be integrated into the bacterial genome, equivalent sections known as "homologous sequences" must be present on the bacterial chromosome. This type of gene transfer is called transformation and so far it has been observed in only very small numbers of the bacteria under investigation.

Transfer of plant DNA to bacteria. If bacteria can absorb free DNA, then in theory at least, they can also absorb plant DNA. Therefore studies have been conducted in those areas where foreign genes from transgenic plants could conceivably spread in the presence of bacteria, such as the rhizosphere of plants and the intestine of animals. All the studies indicate that there are no insurmountable barriers to horizontal gene transfer between bacteria. However, transfer of plant DNA to bacteria could not be demonstrated under natural conditions.

What must happen to enable a gene to actually be transferred from the plant to a bacterium?

The foreign gene must be released intact from a dead plant cell.
It must survive in the bacteria's natural environment. The DNA often breaks down quickly and is only present in fragments.
This gene must then encounter a "competent" bacterial cell which is capable of absorbing DNA and penetrate it.
Finally it must be integrated into the bacterial genome in order to become effective, i.e. so that it can be expressed and converted to a protein. But this is extremely unlikely, since foreign DNA generally undergoes degradation in the bacteria, i.e. it is broken down further.

Provoking transfer in the laboratory. So far horizontal gene transfer has been demonstrated under "optimum laboratory conditions" in two safety research projects. In the laboratory it is possible to adjust bacterial concentrations and climatic conditions, use isolated plant DNA and specifically target bacteria capable of absorption.

Furthermore, in both these projects a bacterial strain was prepared in such a way that a horizontal gene transfer was actually provoked: An incomplete gene for kanamycin resistance was introduced into the bacteria. The bacteria needed a specific gene segment from the transgenic plants to repair this gene. These contained the complete kanamycin resistance gene as a marker gene . Because the same DNA sequences were present in both the plant and the bacterium, the bacteria were able to restore the incomplete resistance gene relatively easily by absorbing transgenic plant DNA. However, the transfer rate was extremely low. In one of the projects with transgenic sugar beet there was one transfer to 5.36 billion bacteria.

Do antibiotic-resistance genes pose a risk?

Debate about the risk of transgenes spreading via horizontal gene transfer focuses in particular on antibiotic‑resistance gene s. These genes are used as marker genes to identify plants in which genetic transformation has successfully taken place. Successfully transformed plants are able to grow on a nutritive medium containing antibiotics.

Pathogenic bacteria which are resistant to one or more antibiotics are becoming increasingly problematic. The fear is that antibiotic-resistance genes from transgenic plants could find their way back into bacteria. For instance, in the intestine they could be transferred from ingested transgenic plant material to intestinal bacteria, rendering ineffective antibiotics used in the event of illness.

Based on our current scientific knowledge, there is a range of plausible arguments which put this fear into perspective:

Transfer of plant DNA to micro-organisms is theoretically conceivable, but extremely unlikely.
Antibiotic-resistance genes come from bacteria and are already widespread. In the human intestine for instance on average 27% of all E.coli bacteria are resistant to ampicillin. It is much more likely that such resistances are exchanged by direct transfer between bacteria rather than by the circuitous route via transgenic plants.
A possible horizontal gene transfer from transgenic plants to bacteria makes only a negligible contribution to the spread of resistance genes, if any. The actual problem lies elsewhere, in the extensive use of antibiotics in human and animal medicine and also in animal husbandry. Antibiotics have been routinely added to animal feed to promote growth, particularly in pigs. As a result the selection pressure on bacteria is very high in pig slurry, i.e. antibiotic resistances spread because they give the bacteria a survival advantage.
We now know much more about how these transfer processes work than we did a few years ago. Antibiotic-resistance genes are usually located on mobile genetic elements. Special methods can be used to identify and characterise these elements, even those which do not themselves carry the resistance gene, but transport genes between bacteria. If "carriers", or even plasmids with an extremely broad range of hosts, which confer resistances to several antibiotics are found in an environmental sample, it is indicative of a high selection pressure. Researchers from the Federal Biological Research Centre for Agriculture and Forestry (BBA) in Braunschweig working on an EU project have been able to demonstrate the first time that there is a high level of "gene-mobilising activity" in soil fertilised with liquid manure, but not in unfertilised soil.

Do bacteria absorb plant genes?

Do antibiotic-resistance genes pose a risk?

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