ACRE Advice

July 2000

Horizontal Gene Transfer: Genetically Modified Crops and Soil Bacteria

ACRE was asked to consider two recently published research papers (1)(2) that address the issue of horizontal gene transfer between genetically modified crops and soil bacteria and, to advise whether these studies raise any new evidence not previously taken into account when ACRE has reviewed applications for consent to release genetically modified plants.

ACRE's advice:

ACRE thought that both papers were scientifically sound and agreed with the conclusions presented by the authors. The 1998 paper(1) reports that a bacterium (Acinetobacter sp.strain BD413(pFG4deltanptII) can, under optimized laboratory conditions, take up and integrate GM sugar beet DNA into its own genome. A key question addressed in the following paper published in 1999(2) was - is this is representative of what happens in bacterial communities that live in the soil under natural conditions.

It is important to view both papers in the appropriate context. Both focus on the likelihood that horizontal gene transfer occurs. This is only one component of the risk assessment process that ACRE undertakes in reviewing applications - the other major element is the consequence of horizontal gene transfer. It is important to note that risks, including that of horizontal gene transfer, are assessed on a case by case basis.

The 1999 paper reports that long-term persistence of transgenic DNA could be shown under field conditions (up to 2 years) and also in microcosms with introduced transgenic plant DNA. However, and despite this, no construct specific sequences were detected in bacteria isolated from these soils. This study therefore provided no evidence for horizontal gene transfer in the environment, and it is important to consider why this might be so.

ACRE's view is that it reflects the fact that a whole host of conditions have to be satisfied to achieve horizontal gene transfer in the soil(3). The result of this is that the overall likelihood of this process occurring in nature is extremely low. The first paper1 demonstrates just how rare the process is: the study used a) optimised laboratory conditions in a test tube rather than soil, b) a bacterial strain that is unusually competent for taking up exogenous DNA and is constructed to facilitate gene exchange (homologous recombination) and, c) a strong selection pressure which favours multiplication of bacteria that carry the exogenous DNA. Despite these exceptionally favourable conditions, transformation only occurred at a frequency of 1 in 1.5 x 10^-10. The failure to detect such an uptake of exogenous DNA occurring in the soil in these experiments is, therefore, not surprising.

ACRE was also not surprised by the result that transgenic DNA persisted in the soil for up to 2 years. The soil is a repository of anything that falls into it and there will be patches where whatever falls there will be protected and other areas where such material will be degraded. If soil is looked at closely enough with enough samples and with very sensitive techniques, most things will be found including DNA.

Nevertheless, ACRE's view on horizontal gene transfer is that there will always be the opportunity for DNA to be taken up by bacteria in the soil and it is not possible to say that integration will not happen at very low frequency. What is also clear however, is that unless there is very strong selection for the gene that is transferred, it will remain at a low frequency in the population at large. Selection pressure is key.

In both experiments the selection pressure applied was the presence of the antibiotic kanamycin. ACRE, in its consideration of the horizontal transfer considered the consequence of this happening and took into account the fact that antibiotic resistance genes are derived from bacteria and that these genes are widespread in soil bacterial communities. The data collected from these studies illustrate the widespread occurrence of kanamycin resistance in soil bacteria(4). Some of these instances will be due to the presence of genes related to nptII, and some to other resistance mechanisms.

In summary, these papers provide some interesting insights about horizontal gene transfer between soil bacteria and plants. The results are consistent with ACRE's previous advice. These studies do not raise any new evidence not previously taken into account when ACRE has reviewed applications for consent to release genetically modified plants. ACRE keeps a watching brief on horizontal gene transfer and considers this in all applications to release genetically modified organisms into the environment.

1. Frank Gebhard & Kornelia Smalla (1998). Transformation of Acinetobacter sp. Strain BD413 by transgenic sugar beet DNA. Applied and Environmental Microbiology 64 (4) 1550-1554.

2. Frank Gebhard & Kornelia Smalla (1999). Monitoring field releases of genetically modified sugar beets for persistence of transgenic plant DNA and horizontal gene transfer. FEMS Microbiology Ecology 28, 261-272.

3. For horizontal gene transfer to take place, DNA must be present in the soil, the bacteria must develop competence to take this up and finally, these sequences must be integrated into the bacterial genome.

4. The proportion of kanamycin resistance bacteria in the total bacterial population ranged from 0.6% to 5.2% for high level and 3% to 30% for low level kanamycin resistant bacteria.