Aug 8, 2002

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Horizontal gene transfer

Stability and behaviour of DNA in the gut. Summary of the FSA studies

In late July 2002 the British Food Standards Agency published the preliminary results of several studies which they had commissioned some time previously. The studies focused on the ability of plant DNA to survive in the intestinal tract of humans and mammals and its potential uptake by micro-organisms living in the gut.

The key findings of the individual studies are summarised below.

Study 1: Survival of ingested DNA in the gut and the potential for genetic transformation of resident bacteria

Survival of ingested DNA in the gut and the potential for genetic transformation of resident bacteria

The stability of transgenic DNA was studied in a laboratory simulation of the digestive system (in vitro) and in the rat gut (in vivo).

  • In vivo the DNAbreaks down extremely quickly: the first 50% after only 6 seconds. After 60 seconds the initial concentration of DNA in the rat gut had decreased 100-fold.
  • In laboratory conditions it could be shown that in rare cases certain bacteria which reside in the oral cavity can take up free transgenic DNA.
  • Integration into the bacterial genome was detected only when the DNA sequence was designed so that the bacterial genome shared identical DNA sequences with a specific fragment of the plant-based test DNA (homologous sequences). However, several of these homologous sequences are needed to achieve this. No bacterial transformation is observed when only one is present in a free DNA fragment.

Study 2: Evaluating the risks associated with using GMOs in human foods (two reports)

This report contains three research components.

(1) The first project deals with the potential for the transformation of antibiotic resistance to micro-organisms. It focused mainly on developing and testing a model system which can be used to determine whether horizontal gene transfer has occurred. Testing for the presence of these kinds of marker genes has previously been difficult, because resistances to certain antibiotics commonly occur naturally in micro-organisms. A pre-existing resistance is difficult to distinguish from one which has been newly acquired by horizontal gene transfer from a GM plant (GMO).

(2) The second project is concerned with testing a simulation model of the human digestive system. The persistence and stability of transgenic DNA from maize and soya were investigated. The breakdown behaviour of different DNA in the individual simulated parts of the digestive system (stomach, gut) appears to vary. However, after about three hours, around 95% of the initial concentration of DNA had broken down in all cases.

(3) The third part of the project, conducted at Newcastle University, concerns the studies which aroused public interest. These too focused on the stability and behaviour of DNA from genetically modified soya beans, but in the human gut rather than in a model system. It is this study in particular which is the subject of public debate.

DNA detection is no proof of gene transfer.

Various experts question whether a positive PCR test for a specific DNA fragment from the plant alone is enough to prove that gene transfer has occurred. Kornelia Smalla of BBA Braunschweig, who has been working on this topic for several years in the context of biosafety research, believes that there are other plausible explanations. The detected transgenic DNA could also have adhered to the bacteria or have been ingested by them as food.

Rolf Einspanier from the TU München also expressed concerns, particularly about the methods used in the Newcastle study. In his view, gene transfer could be proven only if it could be shown that the resistance gene from the plant had been integrated in the bacterial genome and was also biologically active. The positive PCR test referred to in the research report could have numerous other more likely causes.

Human volunteers with normal digestive systems took part in the study, alongside those with a colostomy, which enabled the DNA to be traced partway through the digestive process.

  • No components of the ingested transgenic soya DNA were found in the faeces of the twelve volunteers with normal digestive systems.
  • Three of the seven volunteers with colostomies had detectable residues of transgenic DNA in the faecal mass of the colostomy bag.

The Newcastle team also investigated whether the gut micro-flora of these volunteers had actually taken up plant DNA. Specific DNA fragments derived from the transgenic plants were detected in certain bacteria using PCR methods. Whether the mere presence of transgenic DNA provides evidence of gene transfer is not clear and is contentious among experts. Individual transformed bacteria were not isolated, nor was it possible to detect the integration of plant DNA in the genome of these bacteria.

Study 3: Assessment of the risks of transferring antibiotic resistance determinants from transgenic plants to micro-organisms

Assessment of the risks of transferring antibiotic resistance determinants from transgenic plants to micro-organisms

The release from transgenic maize of a gene that confers resistance to the antibiotic ampicillin and the likelihood of gene transfer to micro-organisms in sheep and hens were investigated.

  • No release of the resistance gene from the transgenic plant material was observed under natural conditions.
  • In model systems – silage effluent, sheep saliva and sheep rumen fluid - the resistance gene very rapidly lost its activity and was detectable only in minute quantities. By contrast, in sheep saliva it remained available for transformation by micro-organisms for up to eight minutes.
  • In feeding experiments with hens, the resistance gene was undetectable once it had passed through the stomach into the intestinal tract

Study 4: Dissemination of GM DNA and antibiotic resistance genes via rumen micro-organisms**

This project traced transgenic DNA from maize in silage, sheep rumen fluid and sheep saliva.

  • DNA fragments from the fodder plants were found in some studies. The DNA broke down at varying speeds depending on the environment.
  • A bacterial species capable of taking up foreign DNA was identified in the rumen. It was discovered that this take-up process is hindered by rumen fluid, which would make gene transfer difficult under natural conditions.
  • In laboratory trials it was shown that E.coli bacteria present in the oral cavity of sheep can absorb certain DNA.

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