Aug 8, 2002

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Horizontal gene transfer

New impetus to old dispute

Can foreign genes from GM crops be absorbed by micro-organisms? And if so, what would be the consequences? These questions have been disputed since the first transgenic plants were released a number of years ago. Today, after several research projects, it is assumed that this type of horizontal gene transfer, although theoretically possible, is nonetheless an extremely rare event in nature. However, a series of British studies has now caused concern amongst consumers once again: the transfer of plant genes to bacteria is thought to have been proven for the first time … in the human gut.

The Food Standards Agency (FSA) in the UK commissioned a number of studies in view of the continuing debate about antibiotic-resistance genes, which are used as marker genes in genetically modified plants. These studies focused on the ability of DNA to survive in the intestinal tract and the ability of micro-organisms living in the gut to absorb marker genes (horizontal gene transfer).

So far, only preliminary reports have been published, but one of the studies has already hit the headlines: a group at Newcastle University has found indications that intestinal bacteria have actually absorbed plant DNA. Does this prove for the first time that marker and other genes from plants that have been consumed are transferred to micro-organisms in the gut?

The Newcastle team did not actually conduct their studies in a natural digestive system, but in people with a colostomy. The plant DNA was isolated from a mass originating from an intermediate phase of the digestive process in which digestion is not yet complete. No plant DNA was detected in the bowel movements of “normal” subjects.

Only in the laboratory, or in the gut as well?

The FSA, which commissioned the study, did not appear to be surprised by the results. “Under realistic conditions it was demonstrated with human volunteers that no DNA material from GM plants survived the passage through the intestinal tract.” The isolated survival of DNA was observed only in artificial trial conditions which simulated the human digestive system, according to the FSA.

GM opponents feel vindicated by the Newcastle findings. For them, this was proof for the first time that intestinal flora actually absorb DNA from GM plants.

Experts harbour doubts about gene transfer

The Newcastle results raise two questions:

• Can the findings obtained under particular circumstances (from people with a colostomy), be generalised? Do they provide evidence that gene transfer events must be expected in the human intestine?
  Most scientists have reacted with caution. John Heritage from the University of Leeds, author of one of the FSA studies, advocates carrying out more experiments on natural systems instead of working so much with model systems.
• Can the detection of plant DNA in the gut bacteria of “positive” human subjects, as in the Newcastle study, be regarded at this early stage as evidence that horizontal gene transfer has occurred?
  Other experts do not support this interpretation. They maintain that the presence of specific DNA does not infer that the corresponding genes have actually been absorbed into the bacterial DNA, and that there are other plausible explanations. The genes can become biologically active only once they have been integrated, but the Newcastle study provided no evidence of integration.

In the USA there is little understanding of European discussions about marker genes and horizontal gene transfer. The idea that pathogens for infectious diseases are becoming increasingly resistant to antibiotics, and therefore more difficult to control, is regarded as a serious problem. This is largely due to the careless overuse of antibiotics in medicine and animal feed additives. The odd antibiotic resistance marker which might possibly be taken up by micro-organisms is of little consequence.

Against a background of growing problems with infectious diseases, Abigail Salyers, President of the American Society of Microbiology, commented on the ongoing controversy surrounding marker genes: “Imagine yourself sitting quietly in your kitchen when you suddenly hear loud noises on your front porch. You run to the window and see several thugs with a crowbar trying to break through your front door. You rush to your phone to call 911, but all you get is a recorded message. The message says that no police are available to answer your call because they are all attending a meeting to determine how they will respond if space aliens land in your town.”

Background: doubts about the safety of marker genes

The antibiotic resistance genes used in genetic engineering as markers to enable genetically modified plant cells to be identified are particularly controversial. Marker genes remain in the GM plants in many cases. In the event of cultivation, marker genes, just like all other plant genes, will be absorbed with the food. There are concerns that marker genes will transfer to the micro-organisms resident in the intestine of humans or animals. In the worst case scenario, pathogens in the gut could no longer be controlled with the antibiotic concerned.