5.7—Bt-genes don't move from plants to bacteria
Bt-genes will not transfer to gut bacteria.
Analysis of Peer-Reviewed Research:
The Bt protein is a specific biological insect deterrent widely used in organic farming (Zehnder and others 2007) and in GM insect protected crops. We have discussed the safety of this protein in section 3.3, and at Bt protein safety. Crops have been sprayed with Bt protein producing-bacteria for over 50 years and these sprays have a good record of safety. The bacterium that produces the Bt protein, Bacillus thuringiensis is readily detected on fresh fruits and vegetables and humans have been constantly exposed to it. This bacterium does not permanently colonize the gut, and neither do its close relatives. Production of the Bt protein by other bacteria in the gut as postulated in Genetic Roulette would make them very poor competitors with other microorganisms. It is hard to imagine how Smith’s scenario could happen and how it can do harm. Organic farming is more likely to produce adverse outcomes than is biotechnology as far as Bt hazards are concerned, as organic farmers use living Bt producing bacteria on their crops and bacteria might survive briefly in the gut. In genetically engineered crops the Bt gene is been dissected away from bacteria and has lost any plausible method of persisting in the gut.
1. Bacteria that carry the Bt gene already exist. The abbreviation Bt refers to Bacillus thuringiensis which is a microbe closely related to other bacteria which briefly proliferate in the gut but do not permanently colonize the gut. Scenarios that Genetic Roulette envisages for movement of Bt genes from plants to gut bacteria are much less likely than movement of natural Bt genes into the gut bacterial flora. Despite the widespread use of these natural Bt containing bacteria for pesticide control by the organic farming community, and the likely presence of these bacteria in some food items, no establishment of Bt producing bacteria in the gut has ever been reported (Frederiksen and other 2006, Wilcks and others 1998)
2. The existing risk from natural Bt producing bacteria is being ignored by Genetic Roulette. This is another example of biased risk analysis seen in Genetic Roulette where the risks from transgenic genes are overstated, and comparable but more likely risks that already exist in the food supply are completely ignored. The existing Bt producing bacteria are present on food (Frederiksen and others 2006; Kleter and others 2005), and they could easily transfer their Bt genes to other gut bacteria, as in the Bt-producing bacteria the genes are carried on plasmid mini-chromosomes that are easily inherited by other microbes (Wilcks et al. 1998). Such movement of bacterial Bt genes to gut bacteria is far far far more likely than transmission of genes from plants to bacteria. Indeed, even bacteria that are generally recognized as safe and widely used to promote gut health readily accept unfavorable antibiotic resistance genes from other bacteria during their residence in the gut (Mater and others 2008). Why Smith doesn’t realize these facts of life concerning gene movement between gut bacteria is a bit of a mystery. But we never see any problems with bacteria producing Bt in the gut, either because transfer of these genes doesn’t occur, or because it is harmless, or most likely both.
3. Bt proteins would be a burden on gut bacteria. The possible reason why Bacillus thuringiensis does not colonize the gut is that production of extra proteins would be an unnecessary burden on these bacteria and give them a disadvantage in the highly competitive growth environment of the intestinal canal. Similarly any bacteria that inherit an active Bt gene are unlikely to be able to persist in the gut. If a bacteria produces a protein that has not utility or that confers no selective advantage, it confers a fitness cost that becomes a selective disadvantage to the bacteria in the competitive environment of the gut. In any case, these bacteria would not be harmful.
Article by Atte von Wright: [Text appended below].
Transmission of genes for Bt proteins to gut bacteria: Is it likely? by Atte von Wright:
Bt-gene is derived from a soil bacterium Bacillus thuringiensis. It codes for an insecticidal toxin with several variants, each having specificities against different types of insects. For this reason B. thuringiensis-based biopesticides have been used for decades and have been enthusiastically embraced by organic farmers, since they have not been considered as chemical pesticides (Federici, 2005; Zehnder et al. 2007).
Transgenic crops carrying the Bt-gene and expressing the associated insect resistance have been one of the success stories of plant biotechnology resulting in reduced insecticide use and other beneficial environmental effects (Christou and others 2006). Jeff Smith has raised a concern of the possibility of the transfer of the Bt-gene from ingested transgenic plant material to gut bacteria turning them to “living pesticide factories”. As already stated previously when discussing the possibility of the spread of antibiotic resistance markers (see 5.5.) such a transfer is a very unlikely event. In order to put this risk into a better perspective, let us compare it with the situation arising from the biopesticide use of non- genetically modified B. thuringiensis bacterium.
B. thuringiensis is actually a very close relative of B. cereus, a common food poisoning bacterium. The main difference is that most B. thuringiensis strains carry plasmids or self-replicating extrachromosomal DNA molecules, coding for the production of the insecticidal substances. These plasmids are self-transmissible from one B. thuringiensis to another (Wilcks and others 1998) and would show even in the gut conditions clearly the highest potential of being transferred from a bacterium to another. It should be remembered that gene transfer from a bacterium to another (even belonging to a different species) is a regular and natural phenomenon while the transfer of DNA from plant cells to bacteria would be highly exceptional.
Bacillus thuringiensis is a frequent find in foods. In a recent study (Frederiksen and others 2006) both natural and bioinsecticide-derived B. thuringiensis bacteria were regularly detected from fresh fruits and vegetables in retail market. Consequently, consumers are constantly exposed to a background level of B. thuringiensis. Since this exposure, with the highest potential of the intestinal spread of the Bt-gene and which predates both the biopesticide use of B. thuringiensis and the arrival of Bt-crops, did not make our intestinal bacteria to churn out Bt-toxin, then the risk posed by transgenic plants transferring a gene to gut bacteria and creating Bt factories in the intestines is minimal.
Christou P, Capell T, Kohli A, Gatehouse JA and Gatehouse AMR (2006) Recent developments and future prospects in insect pest control in transgenic crops. Trends Plant Sci. 11: 302 – 308.
Federici BA (2005) Insecticidal bacteria: An overwhelming success for invertebrate pathology. J Invertebr Pathol 89: 30 -38. “The cloning in 1981 of the first gene encoding a Cry protein led to an explosion of basic and applied research that culminated in new strains of recombinant insecticidal bacteria and, even more importantly, the development, commercialization, and wide-scale deployment of insecticidal transgenic crops based on Cry proteins. This new and environmentally safe technology has revolutionized agricultural pest control, yielding a multibillion dollar industry that is paving the way to new types of plants that will dominate food and fiber production as the 21st century progresses.”
Frederiksen K, Rosenquist H, Jørgensen K and Wilcks A (2006) Occurrence of natural Bacillus thuringiensis contaminants and residues of Bacillus thuringiensis-based insecticides on fresh fruits and vegetables. Appl Environ Microbiol. 72: 3435 – 3440.
Kleter GA, Peijnenburg AA and Aarts HJ (2005). Health considerations regarding horizontal transfer of microbial transgenes present in genetically modified crops. J Biomed Biotechnol.2005(4):326-52.
Mater DD Langella P, Corthier G, and Flores MJ (2008). A probiotic Lactobacillus strain can acquire vancomycin resistance during digestive transit in mice. J Mol Microbiol Biotechnol. 14(1-3):123-7. Bacteria are used to promote health even except genes from other bacteria when they are living in the gut all
Wilcks A, Jayaswal N, Lereclus D and Andrup L (1998) Characterization of plasmid pAW63, a second self transmissible plasmid in Bacillus thuringiensis subsp. kurstaki HD 73. Microbiology 144, 1263-1270.
Zehnder GF, Gurr GM, Kühne S, Wade, MR, Wratten, SD and Wyss, E (2007) Arthropod pest management in organic crops. Annu Rev Entomol. 52: 57-80
1. Transfer of the Bt transgene could cause our intestinal flora to produce the Bt-toxin.
2. With increased exposure to Bt crops and through selective pressure, the number of gut bacteria producing Bt may increase over time.
3. Since Bt-toxin has been associated with the immune responses and damaged cells in animal intestines, long-term exposure may cause significant health problems.
Genetic Roulette speculates that transgenes in food encoding the Bt protein could be transferred to gut bacteria and that these could permanently produce this Bt protein in the gut.