Known by some critics as "Frankenstein foods," genetically modified foods have been eyed as a potential risk to human health because they contain microbial DNA.
In plant technology, antibiotic-resistance marker genes, bits of DNA, are coupled with a new desirable gene and the package is then injected into the plant's cells, the British researchers explained in their report published in The Lancet.
The marker gene is used to help track the successful (or failed) introduction of the new desirable gene, which is designed to boost the plant's defenses against environmental threats.
Plants derived from these cells neither contain nor produce antibiotics, the researchers explained. Because only cells that contain the antibiotic-resistance marker can survive exposure to an antibiotic (in the lab), scientists can be sure the surviving plant also contains the injected advantageous gene.
Critics' concerns are that antibiotic-resistant marker genes from these organisms will be passed from the plant to receptive bacteria and ultimately create mutated resistant microbes.
But Stephen H. Gillespie, Ph.D., a professor of medical microbiology at University College London here, said there is no evidence to indicate these markers pose any significant risk. Any recombinant bacteria, he explained, would have to cross several major hurdles during the transfer process, be incorporated into a host, and then be transmitted. Even if the markers accomplished all that, their risk would be minimal at best, he said.
"The contribution that recombinant bacteria might make-should the enormous barriers to transfer be overcome-is so small that its effect would be completely overwhelmed by the effect of resistance that arises through inappropriate (antibiotic) prescribing in medical practice, transmission of mobile genetic elements between bacteria colonizing patients, and hospital environments," Drs. Gillespie and Philippe B. Gay, also a professor at University College London, wrote.
However, they warned that despite the lack of current evidence, the possibility remains. Scientists should look for alternatives and proceed with caution, they said.
The authors came to this conclusion after conducting a review of their own personal files, searches on PubMed, and references from relevant articles. In their review article, they described the many major obstacles these markers would have to overcome.
For starters, conditions must be ideal for these antibiotic resistant markers to make the transition to a new bacterial host. The DNA must not be totally degraded, and there must be a physical encounter between the DNA fragments and the recipient host organism, Dr. Gillespie said.
The outcomes for these antibiotic resistant markers "in genetically modified plants depends on the gene flow in the resistance reservoirs connecting the fields where these plants grow and the bacteria that could be altered to resistance and be transferred to human beings," the authors wrote.
The environment in which these markers exist is also highly competitive, the authors explained. "Antibiotic-resistance markers are only a small proportion of the overall DNA available for transformation," they explained .
The marker gene must also overcome the physiological disadvantage of acquiring a piece of foreign DNA-the gene with which the marker is coupled--probably in conditions where the new gene does not yet provide a selective advantage, the authors said.
The authors noted that there is evidence that fragments of DNA, long enough to encode an antibiotic resistance mechanism, have been found in the gastrointestinal tracts of mice. Despite this evidence, there are so far no signs recombination is taking place in these environments. Still, Drs. Gillespie and Gay said they wouldn't rule out the possibility.
Should any new antibiotic-resistant organisms emerge, they would still face the human medical environment, including hospitals and antibiotic use, the authors said.
"Any contribution to antibiotic resistance made by genetically modified plants must be overwhelmed by the contribution made by antibiotic prescription in clinical practice," the authors wrote. Particularly in hospitals where "bacteria are exposed to a multiplicity of antibiotic agents and selection pressure is at its highest."
The authors concluded that "plants modified genetically have tremendous potential to improve human nutrition." Although the public health cost of these improvements has been questioned, they added, the evidence shows the risks remain very low.
- Reviewed by Robert Jasmer, MD Assistant Professor of Medicine, University of California, San Francisco