Unsuccessful search for DNA transfer from transgenic plants to bacteria in the intestine of the tobacco horn worm, Manduca sexta

Horizontal gene transfer from genetically modified plants - Regulatory considerations

Gene technology regulators receive applications seeking permission for the environmental release of genetically modified (GM) plants, many of which possess beneficial traits such as improved production, enhanced nutrition and resistance to drought, pests and diseases. The regulators must assess the risks to human and animal health and to the environment from releasing these GM plants. One such consideration, of many, is the likelihood and potential consequence of the introduced or modified DNA being transferred to other organisms, including people. While such gene transfer is most likely to occur to sexually compatible relatives (vertical gene transfer), horizontal gene transfer (HGT), which is the acquisition of genetic material that has not been inherited from a parent, is also a possibility considered during these assessments. Advances in HGT detection, aided by next generation sequencing, have demonstrated that HGT occurrence may have been previously underestimated. In this review, we provide updated evidence on the likelihood, factors and the barriers for the introduced or modified DNA in GM plants to be horizontally transferred into a variety of recipients. We present the legislation and frameworks the Australian Gene Technology Regulator adheres to with respect to the consideration of risks posed by HGT. Such a perspective may generally be applicable to regulators in other jurisdictions as well as to commercial and research organisations who develop GM plants.

Potential Effects of Horizontal Gene Exchange in the Human Gut

Many essential functions of the human body are dependent on the symbiotic microbiota, which is present at especially high numbers and diversity in the gut. This intricate host-microbe relationship is a result of the long-term coevolution between the two. While the inheritance of mutational changes in the host evolution is almost exclusively vertical, the main mechanism of bacterial evolution is horizontal gene exchange. The gut conditions, with stable temperature, continuous food supply, constant physicochemical conditions, extremely high concentration of microbial cells and phages, and plenty of opportunities for conjugation on the surfaces of food particles and host tissues, represent one of the most favorable ecological niches for horizontal gene exchange. Thus, the gut microbial system genetically is very dynamic and capable of rapid response, at the genetic level, to selection, for example, by antibiotics. There are many other factors to which the microbiota may dynamically respond including lifestyle, therapy, diet, refined food, food additives, consumption of pre- and probiotics, and many others. The impact of the changing selective pressures on gut microbiota, however, is poorly understood. Presumably, the gut microbiome responds to these changes by genetic restructuring of gut populations, driven mainly via horizontal gene exchange. Thus, our main goal is to reveal the role played by horizontal gene exchange in the changing landscape of the gastrointestinal microbiome and potential effect of these changes on human health in general and autoimmune diseases in particular.

Horizontal gene exchange in environmental microbiota

Horizontal gene transfer (HGT) plays an important role in the evolution of life on the Earth. This view is supported by numerous occasions of HGT that are recorded in the genomes of all three domains of living organisms. HGT-mediated rapid evolution is especially noticeable among the Bacteria, which demonstrate formidable adaptability in the face of recent environmental changes imposed by human activities, such as the use of antibiotics, industrial contamination, and intensive agriculture. At the heart of the HGT-driven bacterial evolution and adaptation are highly sophisticated natural genetic engineering tools in the form of a variety of mobile genetic elements (MGEs). The main aim of this review is to give a brief account of the occurrence and diversity of MGEs in natural ecosystems and of the environmental factors that may affect MGE-mediated HGT.

Origin and diversity of metabolically active gut bacteria from laboratory-bred larvae of Manduca sexta (Sphingidae, Lepidoptera, Insecta)

Cultivation-independent analyses based on genetic profiling of partial bacterial 16S rRNA genes by PCR-single-strand conformation polymorphism (PCR-SSCP), reverse transcriptase (RT)-PCR-SSCP of the 16S rRNA itself, and stable isotope probing (SIP), followed by RT-PCR-SSCP, were applied to characterize the diversity of metabolically active bacteria in the larval gut of Manduca sexta bred on tobacco leaves under greenhouse conditions. For SIP, hatching larvae were fed with leaves from tobacco plants grown in a (13)CO(2)-enriched atmosphere. Dominant SSCP bands were sequenced and phylogenetically analyzed. Only one major gut colonizer, an Enterococcus relative, was detected; it occurred in the heavy RNA fraction, demonstrating its metabolic activity, and it originated from eggs, where its metabolic activity was also indicated by rRNA-based SSCP profiles. In contrast, a Citrobacter sedlakii relative was detected on eggs by DNA-SSCP, but rRNA-SSCP and SIP-rRNA-SSCP were negative, suggesting that these bacterial cells were inactive. A Burkholderia relative was dominant and metabolically active on the tobacco leaves but inactive inside the gut, where it was also quantitatively reduced, as suggested by lower band intensities in the DNA-based SSCP profiles. SIP-RNA-SSCP detected another metabolically active gut bacterium (Enterobacter sp.) and more bacteria in the light RNA fraction, indicating low or no metabolic activity of the latter inside the gut. We conclude that the larval gut supported only a low diversity of metabolically active bacteria.

An assessment of the potential of herbivorous insect gut bacteria to develop competence for natural transformation

Whereas the capability of DNA uptake has been well established for numerous species and strains of bacteria grown in vitro, the broader distribution of natural transformability within bacterial communities remains largely unexplored. Here, we investigate the ability of bacterial isolates from the gut of grass grub larvae (Costelytra zealandica (White); Coleoptera: Scarabaeidae) to develop natural genetic competence in vitro. A total of 37 mostly species-divergent strains isolated from the gut of grass grub larvae were selected for spontaneous rifampicin-resistance. Genomic DNA was subsequently isolated from the resistant strains and exposed to sensitive strains grown individually using established filter transformation protocols. DNA isolated from wild-type strains was used as a control. None of the 37 isolates tested exhibited a frequency of conversion to rifampicin-resistance in the presence of DNA at rates that were significantly higher than the rate of spontaneous mutation to rifampicin-resistance in the presence of wild-type DNA (the limit of detection was approximately < 1 culturable transformant per 10(9) exposed bacteria). To further examine if conditions were conducive to bacterial DNA uptake in the grass grubs gut, we employed the competent bacterium Acinetobacter baylyi strain BD413 as a recipient species for in vivo studies. However, no transformants could be detected above the detection limit of 1 transformant per 10(3) cells, possibly due to low population density and limited growth of A. baylyi cells in grass grub guts. PCR analysis indicated that chromosomal Acinetobacter DNA remains detectable by PCR for up to 3 days after direct inoculation into the alimentary tract of grass grub larvae. Nevertheless, neither transforming activity of the DNA recovered from the alimentary tract of grass grubs larvae nor competence of bacterial cells recovered from inoculated larvae could be shown.

Field study results on the probability and risk of a horizontal gene transfer from transgenic herbicide-resistant oilseed rape pollen to gut bacteria of bees

Bees are specifically subjected to intimate contacts with transgenic plants due to their feeding activities on pollen. In this study, the probability and ecological risk of a gene transfer from pollen to gut bacteria of bees was investigated with larvae of Apis mellifera (honeybee), Bombus terrestris (bumblebee), and Osmia bicornis (red mason bee), all collected at a flowering transgenic oilseed rape field. The plants were genetically engineered with the pat-gene, conferring resistance against glufosinate (syn. phosphinothricin), a glutamine-synthetase inhibitor in plants and microorganisms. Ninety-six bacterial strains were isolated and characterized by 16S rRNA gene sequencing, revealing that Firmicutes represented 58% of the isolates, Actinobacteria 31%, and Proteobacteria 11%, respectively. Of all isolates, 40% were resistant to 1 mM glufosinate, and 11% even to 10 mM. Resistant phenotypes were found in all phylogenetic groups. None of the resistant phenotypes carried the recombinant pat-gene in its genome. The threshold of detecting gene transfer in this field study was relatively insensitive due to the high background of natural glufosinate resistance. However, the broad occurrence of glufosinate-resistant bacteria from different phylogenetic groups suggests that rare events of horizontal gene transfer will not add significantly to natural bacterial glufosinate resistance.

Leaf-feeding larvae of Manduca sexta (Insecta, Lepidoptera) drastically reduce copy numbers of aadA antibiotic resistance genes from transplastomic tobacco but maintain intact aadA genes in their feces

The objective of this study was to evaluate the effect of insect larval feeding on the fate and genetic transformability of recombinant DNA from a transplastomic plant. Leaves of tobacco plants with an aadA antibiotic resistance gene inserted into their chloroplast genome were incubated with larvae of the tobacco hornworm Manduca sexta (Lepidoptera). The specifically designed Acinetobacter strain BD413 pBAB(2) was chosen to analyze the functional integrity of the aadA transgene for natural transformation after gut passages. No gene transfer was detected after simultaneous feeding of leaves and the Acinetobacter BD413 pBAB(2) as a recipient, even though 15% of ingested Acinetobacter BD413 cells could be recovered as viable cells from feces 6 h after feeding. Results with real-time PCR indicated that an average of 98.2 to 99.99% of the aadA gene was degraded during the gut passage, but the range in the number of aadA genes in feces of larvae fed with transplastomic leaves was enormous, varying from 5 x 10(6) to 1 x 10(9) copies.g(-1). DNA extracted from feces of larvae fed with transplastomic leaves was still able to transform externally added competent Acinetobacter BD413 pBAB(2) in vitro. Transformation frequencies with concentrated feces DNA were in the same range as those found with leaves (10(-4)-10(-6) transformants per recipient) or purified plasmid DNA (10(-3)-10(-7)). The presence of functionally intact DNA was also qualitatively observed after incubation of 30 mg freshly shed feces directly with competent Acinetobacter BD413 pBAB(2), demonstrating that aadA genes in feces have a potential to undergo further horizontal gene transfer under environmental conditions.