5.2—GM does not affect gene movement into bacteria
Genetically engineered plants do not promote movement of genes from plants to bacteria
Analysis of Peer-Reviewed Research:
Experiments with transgenic crops and bacteria have shown that it is almost impossible for genes to move from a transgenic plant to bacteria– in fact, such a transfer has never been detected after years of searching by scientists. Many transgenic crops contain bacterial DNA that could theoretically allow for gene transfer from plants to gut or soil bacteria at exceedingly low frequencies. That, however, requires a substantial genetic similarity in the DNA of the receiving bacterium, meaning in practice that it should already have the gene in question. The possibility of a whole plasmid (a self-replicating bacterial mini-chromosome, inserted in some GM-constructions) getting transferred to a gut bacterium is extremely remote. The fear of ingested herbicides forming a selective advantage for hypothetical intestinal bacteria that acquire herbicide resistance from plant transgenic DNA is absurd, because the necessary concentrations of herbicide in our guts would create much more serious acute health concerns than horizontal gene transfer.
1. Enhanced rate of movement of DNA from plants to bacteria from genetically-engineered plants is purely speculative. Enhanced movement has not been detected in practice. As with many of the hypothetical risks speculated about in Genetic Roulette, the idea that genetic engineering has greatly increased the possibility of movement of genes from plants to bacteria is pure speculation. Gene movement from plants to bacteria has been investigated thoroughly and has not been detected. Artificial and deliberately contrived conditions can be created in the laboratory to enable scientists to force it to happen (Nielsen and others 2000). These kinds of conditions, called “marker rescue” are unlikely to exist in nature except in bacteria that already carry the gene being transferred. Several expert scientific panels have considered this issue in the context of movement of antibiotic resistance genes from to bacteria and have concluded that such gene movement is a very improbable, low frequency event, occurring at the most about once in every 100,000,000,000,000,000 bacteria exposed to transgenic plants. Jeffrey Smith does not provide the reader with the conclusions of these experts (Bennett and others 2004, Ramessar and others 2007, van den Eede G and others 2004). He also does not mention that antibiotic resistant genes are already so widespread in nature that the acquisition of an occasional resistance gene from a plant by a bacterium would be inconsequential –the E. coli and other bacteria found in the human gut already carry antibiotic resistance genes (Calva and others 1996, Berche 1998).
2. It is not unprecedented for bacterial genes to be present in plants. One of the mechanisms which Smith postulates as leading to fast rates of gene movement from plants to bacteria is that genetic engineering can provide plant genomes with short segments of bacterial DNA. The presence of these DNA segments provides of regions of DNA similarity between bacteria and plants. Such similarity could theoretically increase the uptake of plant genes by bacteria, because it could help DNA insert into bacterial chromosomes. But Smith overlooks the fact that presence of bacterial DNA within plant genomes already occurs in nature. Therefore there is no absolutely radical precedent with artificially transgenic plants having some DNA sequences that resemble bacterial DNA. Bacteria known as Agrobacterium have specialized mechanisms for insertion of bacterial DNA into plant chromosomes. Genes from these bacteria have already been discovered in tobacco plants (Dröge and others 1998). For many years now, we have known that genes move between greatly dissimilar organisms at low rates (measured over evolutionary time; Keeling and Palmer 2008). Whether genetic engineering is promoting further gene movement is debatable and at present not proven.
3. Health regulations impose limits on the possible levels of herbicides in food so they cannot be effective in selecting for herbicide-tolerant bacteria in the gut. Smith mentions correctly that antibiotics can promote gene movement among gut bacteria. He then illogically argues that herbicides present in food will also increase gene movement among gut bacteria, because they can be classified as antibiotic like substances. The risk of herbicides acting as antibiotic like materials in the gut is negligible compared to the much greater selective effects of antibiotics used in medicine and in animal husbandry. Indeed, one unfortunate aspect of focusing public attention on the negligible risk of antibiotic resistance from genetically modified plants is that it distracts attention away from the serious risk of antibiotic resistant pathogenic bacteria caused by irresponsible use of antibiotics. The major cause of rapid spread of antibiotic resistance is not antibiotic resistance genes—they are already widely scattered in nature—it is the profligate and indiscriminate use of antibiotics by humans.
4. Self-replication of genes from plants in bacteria is unlikely. Genetic Roulette supposes that self-replicating plasmids (plasmids are circular mini-chromosomes that are found in bacteria) will reform if DNA in a transgenic plant DNA is accidently transferred to gut bacteria. It postulates incorrectly that this re-formation of circular plasmids can occur easily when it is in reality highly unlikely. Transgenic DNA in plants does not exist in a circular form, and the extremely low probability of re-formation of DNA circles is a barrier that makes formation of self replicating transgenic DNA in bacteria highly unlikely (Bennett and others 2004). In some instances the genetically engineered DNA in the plant by design does not contain plasmid sequences — called origins of replication — that are required to form a real plasmid, and cannot possibly re-form circular plasmids in bacteria.
5. The risks of transfer of plant transgenic DNA are miniscule compared to the real and present existing traffic of antibiotic resistant gene movement among bacteria. Existing pathways for gene movement pose a greater and much more probable risk than the unlikely events mentioned by Smith. Genetic Roulette correctly mentions that the gut of humans is a natural “hotspot” for gene transfer between species. There is scientific certainty that genes normally move at detectable frequencies between different microbe species in the gut. But Genetic Roulette does not mention that genes constantly move between distantly related bacteria in other environments, and that these other environments provide a huge reservoir of antibiotic resistance genes that can be transferred to gut bacteria. There is vast and diverse array of genetically mobile antibiotic resistance genes in soil bacteria. Genes move between kingdoms in ocean plankton on a vast scale. Multitudes of viruses and other agents that carry genes between different species are known to be active in these different environments. These are proven existing sources of novel genes for bacteria that dwarf the undetectable gene transfers that Genetic Roulette speculates about (Bennett and others; 2004; D’Costa and others 2007; Demanèche and others 2008; Dröge and others 1998; Gladyshev and others 2008; Keeling , Palmer 2008; van den Eede and others 2004).
Berche P (1998) Les plantes transgéniques et la resistance aux antibiotiques. Méd Thérap 4:709–719
Bennett PM and others, Working Party of the British Society for Antimicrobial Chemotherapy(2004). An assessment of the risks associated with the use of antibiotic resistance genes in genetically modified plants: report of the Working Party of the British Society for Antimicrobial Chemotherapy. J Antimicrob Chemother. 2004 Mar;53(3):418-31. Epub 2004 Jan 28.”…the argument that occasional transfer of these particular resistance genes from GM plants to bacteria would pose an unacceptable risk to human or animal health has little substance. We conclude that the risk of transfer of AR genes from GM plants to bacteria is remote, and that the hazard arising from any such gene transfer is, at worst, slight.”
Calva, JJ, Sifuentes-Osornio J, Cer´on C (1996) Antimicrobial resistance in fecal flora: longitudinal community-based surveillance of children from urban Mexico. Antimicrob Agents Chemotherapy 40:1699–1702
Demanèche S, Sanguin H, Poté J, Navarro E, Bernillon D, Mavingui P, Wildi W, Vogel TM, Simonet P (2008). Antibiotic-resistant soil bacteria in transgenic plant fields. Proc Natl Acad Sci U S A. 105(10):3957-62. “Our results indicate that soil bacteria are naturally resistant to a broad spectrum of beta-lactam antibiotics… These high resistance levels for a wide range of antibiotics are partly due to the polymorphism of bla genes, which occur frequently among soil bacteria. The blaTEM116 gene of the transgenic corn Bt176investigated here is among those frequently found, thus reducing any risk of introducing a new bacterial resistance trait from the transgenic material.”
D’Costa VM, Griffiths E and Wright GD (2007). Expanding the soil antibiotic resistome: exploring environmental diversity. Curr Opin Microbiol. 10(5):481-9. There is now growing evidence that bacteria that live in the environment (e.g. the soil) are multi-drug-resistant. Recent research is revealing an unexpected density of resistance genes in the environment.
Dröge M, Pühler A and Selbitschka W (1998). Horizontal gene transfer as a biosafety issue: A natural phenomenon of public concern. Journal of Biotechnology 64:75-90
Gladyshev EA, Meselson M and Arkhipova IR (2008). Massive horizontal gene transfer in bdelloid rotifers. Science 320:1210-1213. Movement of genes across kingdoms in the ocean. You are what you eat.
Keeling PJ, and Palmer JD (2008). Horizontal gene transfer in eukaryotic evolution. Nature Reviews Genetics 9:605-618. The state of play on gene movement between different species showing that gene movement–for example between different plants that are widely unrelated to one another –has happened many times in during evolution of different organisms.
Nielsen KM, van Elsas JD and Smalla K (2000). Transformation of Acinetobacter sp. strain BD413(pFG4ΔnptII) with transgenic plant DNA in soil microcosms and effects of kanamycin on selection of transformants. Applied and Environmental Microbiology 66, 1237–42.
Ramessar K, Peremarti A, Gómez-Galera S, Naqvi S, Moralejo M, Muñoz P, Capell T and Christou P (2007). Biosafety and risk assessment framework for selectable marker genes in transgenic crop plants: a case of the science not supporting the politics. Transgenic Res. 16(3):261-80. “Our conclusion, supported by numerous studies, most of which are commissioned by some of the very parties that have taken a position against the use of antibiotic selectable marker gene systems, is that there is no scientific basis to argue against the use and presence of selectable marker genes as a class in transgenic plants.”
Schlüter K Schlüter K, Fütterer J and Potrykus I (1995). ‘Horizontal’ gene transfer from a transgenic potato line to a bacterial pathogen (Erwinia chrysanthemi) occurs—if at all—at an extremely low frequency. Biotechnology 13:1094–8.
van den Eede G, Aarts H, Buhk HJ, Corthier G, Flint HJ, Hammes W, Jacobsen B, Midtvedt T, van der Vossen J, von Wright A, Wackernagel W and Wilcks A (2004). The relevance of gene transfer to the safety of food and feed derived from genetically modified (GM) plants. Food and Chemical Toxicology 42:1127–1156
Transgene design facilitates transfer into gut bacteria.
1. Genes can naturally transfer between species and even kingdoms, but it is uncommon.
2. GM crops may be especially suited to overcome the natural barriers of this transfer.
3. Short bacterial sequences and higher herbicide residues, may for example, significantly increase the transfer rate.
4. Transgenes may therefore readily travel from GM food into the DNA of gut bacteria.
Jeffrey Smith mentions that genes can naturally transfer between species and even biological kingdoms, and highlights possible problems that can occur because of gene movement from food to gut bacteria, claiming that genetically engineered crops may vastly increase the rate of gene movement from plants to bacteria. He also argues that herbicide in the diet from genetically modified herbicide tolerant plants would promote proliferation of bacteria in the gut that are resistant to herbicide.