5.8—DNA moves between microbes in the mouth and throat

Genes may transfer to bacteria in the mouth or throat from many other microbes.

See Genetic Roulette’s False Claims at Bottom of Page

Analysis of Peer-Reviewed Research:

Genes that confer resistance to two antibiotics — ampicillin and kanamycin — are often present in transgenic plants because they are useful in laboratory manipulation of DNA. Jeffrey Smith is worried about movement of these antibiotic resistance genes to mouth bacteria. This issue has been investigated by many expert medical microbiologists. Unfortunately Genetic Roulette does not provide readers with what these microbiology experts have to say (Bennett and others 2004, Dröge and others 1998, Thomson  2001,  van den Eede and others 2004). From such experts readers would learn that genes conferring ampicillin resistance and kanamycin resistance are widely carried by microbes found in food, the human digestive tract, and the environments in which food crops are grown – particularly in the soil. They would learn that these genes are famous for their ability to be frequently transferred between different species of microbes by means of plasmids. Antibiotic resistance gene movement between different bacteria is indeed expected to occur and to rely on genes that move with plasmids.

Plasmids are circular mini chromosomes that can replicate easily inside bacterial cells. There is very convincing proof that plasmid DNA is transferred frequently between different bacteria in the gut. Smith mentions studies that show that plasmid DNA can be exchanged between different bacteria in the mouth. But plant DNA is very different from plasmid DNA. Plant transgenes are not circular as plasmids are. So the experiments with plasmid DNA that simulate what might happen in the mouth do not extrapolate to transgene DNA carried in plant chromosomes.

In laboratory experiments it is possible to expose mouth bacteria to extremely large amount of bacterial plasmid DNA and get them to successfully take up some plasmid DNA carrying antibiotic resistance genes, but very artificial optimized conditions are needed. These experiment show bacteria might use this route to exchange genes with other bacteria.

Genetic Roulette takes these experiments as evidence that ampicillin and kanamycin genes from plant DNA might be taken up by mouth bacteria when food is eaten. What Jeffrey Smith doesn’t mention is that it is virtually impossible for bacteria to capture these genes from plants unless they already have them. An important reason that these plant genes cannot transfer to bacteria is that plant DNA is not circular and does not generate the circular plasmids which are needed for these bacteria to successfully inherit novel genes. If the genes are not on circular plasmids then gene movement is way way way less frequent. What Jeffrey Smith didn’t understand is that the experiments he quotes about gene uptake by mouth bacteria were carried with bacterial plasmid DNA, not plant DNA. They confirmed what is already well known, that bacteria have well developed options for exchanging plasmid DNA with one another.

Medical microbiologists have concluded that other bacteria which are very common on food and in soil which carry ampicillin and kanamycin resistance genes are a far more serious risk for compromising antibiotic effectiveness than are genetically manipulated foods.

Genetically modified foods have never been shown to donate new antibiotic resistance genes to microbes, despite numerous attempts to check if this can happen. The most likely source of plasmid DNA in the mouth is from other bacteria which frequently carry plasmids often in large numbers. The plasmid DNA transfer experiments described by Smith merely confirm that transfer of antibiotic genes between different species of bacteria may happen very infrequently in the mouth. Gene movement from food to mouth microbes is a theoretical possibility that raises no significant health issues (Bennett 2004 and others 2004). Genetic Roulette doesn’t let its readers know any of this reassuring advice. He speculates a lot, but this is not proof that genes move the way he imagines.

See also

Section 5.2. Genetically engineered plants do not promote movement of genes from plants to bacteria.

From an expert in the field:

“To speculate is not to prove. We have been among the first groups to show that plasmid DNA can be transferred in the gut, the in real situation (Morelli and others 1988.) from one bacterium into another . While this transfer is occurring every day in our gut among bacteria cells, there are no data supporting transfer from crop to bacteria.” L. Morrelli.

We have been among the first groups to show that plasmid DNA can be transferred in the gut, the in real situation (Morelli L, Sarra PG, Bottazzi V.  In vivo transfer of pAMbeta 1 from Lactobacillus reuteri to Enterococcus faecalis) J Appl Bacteriol. 1988 Nov;65(5):371-5.

From one bacterium into another . While this transfer is occurring every day in our gut among bacteria cells, there are no data supporting transfer from crop to bacteria.

1. Laboratory experiments with bacterial DNA do not tell you what  could happen with plant derived transgenic DNA. (From Atte von Wright) Jeffrey Smith is worried about the risk of antibiotic resistance genes being spread from transgenic food into mouth bacteria. This process would depend on the uptake of free DNA by bacteria that are competent, competence meaning a physiological state in which the cell wall is permeable to DNA. While some bacteria are naturally competent under some growth conditions, others have to be manipulated by special treatment to permeabilise the cell wall.

The experiments that worry Smith have been performed in conditions mimicking the situation in the mouth.  Derry Mercer and colleagues (Mercer and others 1999) measured the transfer of plasmid DNA (plasmid being a circular, self-replicating small DNA-molecule) into a naturally transformable mouth bacterium in the presence of human saliva. The transfer could be detected, although the efficiency decreased very rapidly over time scales measured in seconds, due to the degradation of DNA.

Duggan and co-workers (2003) performed similar experiments with bacterial plasmid DNA placed in the mouths of sheep. Again rapid degradation was observed, allowing, however, detectable transformation of Escherichia coli-bacterium made artificially competent (natural competence is not typical to this bacterium).

It should be noted that in neither experiment was actual transgenic plant material used but purified bacterial DNA in a physical form (plasmid) that was optimal for the transfer.  Thus the experiments are not representative of a real situation in the mouth, where only limited amounts of DNA could be expected to be liberated from the food being chewed, and plasmids would not be generated.

2. Ampicillin and kanamycin resistance genes are widespread and move around among bacteria. Regular food often contains large numbers of bacteria that harbor antibiotic resistance genes (e.g. Gevers and others 2003; Ramessar and others 2007) typically carried on plasmids. A major source of these genes is the soil, where there is an astonishingly huge diversity of antibiotic resistant bacteria (D’Costa and others 2007, Demanèche S and others 2008).  A typical soil microbe is resistant to seven or eight different antibiotics and genes for both ampicillin resistance and kanamycin resistance can be readily found in soil microbes (Beneviste, Davies 1973; Demanèche S and others 2008; Dröge and others 1998). There are many specialized mechanisms for gene movement to take place quite frequently between different bacterial species – and movement from from soil microbes to food germs is not surprizing. Such bacterial gene movement certainly occurs in the gut (Morelli and others 1988).

Genes that confer resistance to antibiotics have many times been shown to have migrated between distantly related microbe species, often carried on plasmids. Some plasmids have the capacity to inject themselves into a wide variety of new microbe hosts and spread themselves promiscuously by a process called conjugation (Bennett and others 2004; Dröge M and others 1998; van den Eede G and others 2004). Given these numerous possible ways for antibiotic resistant mouth microbes to arise, medical specialists have concluded that the comparative risk of the spread of antibiotic resistance from transgenic plants is “remote, and that the hazard arising from any such transfer is, at worst, slight.”  (Bennett and others 2004). The available reservoir of genes from huge numbers of bacteria in the environment is the source of antibiotic resistance traits that can be captured by pathogenic germs. Compared to this vast collection of genes, the inaccessible genes of plants are a minute risk.

3. Bacteria are the most likely source of plasmid DNA in the mouth. The main information that worries Jeffrey Smith is experiments that detected uptake of plasmid DNA into mouth bacteria under optimized conditions in the laboratory. The most likely source of plasmid DNA in the mouth– remember that plasmid DNA is a bacterial mini chromosome often carried in large numbers in the bacterial cells — is other bacteria present in the mouth. Many of these bacteria will carry small circular plasmids which are suited to replicate in other bacteria should they get there. They could be released from their original microbe hosts when bacteria are damaged or killed, either prior to entering the mouth, or during chewing of food. It is very unlikely that plasmids could come from genetically engineered food (Bennett and others 2004, Ramessar and others (2007). Thomson 2001) except from the bacteria that it might harbor. Smith has actually missed the elephant in the room. He has misinterpreted information that confirms that ampicillin, kanamycin, and other antibiotic resistance genes carried on bacterial plasmids are relatively easily transmitted at low frequencies to new bacterial hosts.

References

Beever D and Kemp C (2000). Safety issues associated with the DNA in animal feed derived from genetically modified crops. A review of scientific and regulatory procedures. Nutritional Abstract Reviews Series B: Livestock Feeds and Feeding 70:175–182.

Benveniste R, Davies J (1973). Aminoglycoside antibiotic-inactivating enzymes in actinomycetes similar to those present in clinical isolates of antibiotic-resistant bacteria. Proc Natl Acad Sci U S A 70:2276-2280.” This work represents one of the first studies to identify mechanisms of resistance in soil-dwelling antibiotic producers to be identical to those in clinical pathogens – Streptomyces strains that inactivate aminoglycosides by acetylation and phosphorylation”. D’Costa and others 2007.

D’Costa VM, and others (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.

Demanèche S and others (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.”

Dröge M and others (1998). Review: Horizontal gene transfer as a biosafety issue: a natural phenomenon of public concern. J Biotechnol. 64(1):75-90.

Duggan, P.S., Chambers, P.A., Heritage, J. & Forbes, M. (2003) Fate of genetically modified maize DNA in the oral cavity and the rumen of the sheep. Br J Nutr. 89: 159 – 166.

Gevers, D., Masco, L., Baert, L., Huys, G., Debevere, J. & Swings, J. (2003) Prevalence and diversity of tetracycline resistant lactic acid bacteria and their tet genes along the process line of fermented dry sausages. Syst Appl Microbiol. 26: 277-283.

Mercer, D.K., Scott, K.P., Bruce-Johnson, W.A., Glover, L.A. & Flint, H. (1999) Fate of free DNA and transformation of the oral bacterium Streptococcus gordonii DL1 by plasmid DNA in human saliva. Appl Environ Microbiol 65: 6 -10

Morelli L, Sarra PG, Bottazzi V. (1988). In vivo transfer of pAMbeta 1 from Lactobacillus reuteri to Enterococcus faecalis J Appl Bacteriol. 65(5):371-5. Detection of plasmid DNA movement between different bacterial species in the gut of mice.

Ramessar K and others (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. 2007 Jun;16(3):261-80. Epub 2007 Apr 14.”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.”

Thomson J (2001). Horizontal transfer of DNA from GM Crops to bacteria and to mammalian Cells. Journal of Food science 66(2):188-193.

van den Eede G and others (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

(Based on text by Atte von Wright)