Bacillus thuringiensis

Bacillus thuringiensis is a Gram-positive, soil dwelling bacterium of the genus Bacillus. Additionally, B. thuringiensis also occurs naturally in the gut of caterpillars of various types of moths and butterflies, as well as on the surface of plants.^[1]

B. thuringiensis was discovered 1901 in Japan by Ishiwata and 1911 in Germany by Ernst Berliner, who discovered a disease called Schlaffsucht in flour moth caterpillars. B. thuringiensis is closely related to B. cereus, a soil bacterium, and B. anthracis, the cause of anthrax: the three organisms differ mainly in their plasmids. Like other members of the genus, all three are aerobes capable of producing endospores.^[1]

Upon sporulation, B. thuringiensis forms crystals of proteinaceous insecticidal δ-endotoxins (Cry toxins: Bacillus thuringiensis Toxin Nomenclature) which are encoded by cry genes. Cry toxins have specific activities against species of the orders Lepidoptera (Moths and Butterflies), Diptera (Flies and Mosquitoes) and Coleoptera (Beetles). Thus, B. thuringiensis serves as an important reservoir of Cry toxins and cry genes for production of biological insecticides and insect-resistant genetically modified crops.

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Use in pest control

Spores and crystalline insecticidal proteins produced by B. thuringiensis are used as specific insecticides under trade names such as Dipel and Thuricide. Because of their specificity, these pesticides are regarded as environmentally friendly, with little or no effect on humans, wildlife, pollinators, and most other beneficial insects. The Belgian company Plant Genetic Systems was the first company (in 1985) to develop genetically engineered (tobacco) plants with insect tolerance by expressing cry genes from B. thuringiensis.^[2][3]

B. thurigiensis-based insecticides are often applied as liquid sprays on crop plants, where the insecticide must be ingested to be effective. It is thought that the solubilized toxins form pores in the midgut epithelium of susceptible larvae. Recent research has suggested that the midgut bacteria of susceptible larvae are required for B. thuringiensis insecticidal activity.^[4]

Bacillus thuringiensis serovar israelensis, a strain of B. thuringiensis is widely used as a larvicide against mosquito larvae, where it is also considered an environmentally friendly method of mosquito control.

Genetic engineering for pest control

Usage

Bt crops (in corn and cotton) were planted on 281,500 km² in 2006 (165,600 km² of Bt corn and 115900 km² of Bt cotton). This was equivalent to 11.1% and 33.6% respectively of global plantings of corn and cotton in 2006 ^[6].

This technology has delivered major economic and environmental benefits. In the first ten years of use (1996-2005), the farmers who used GM (Bt) insect resistant technology derived a total of nearly $9.9 billion worth of extra farm income, with the much of this benefit going to small, resource poor farmers in developing countries (especially from the use of Bt cotton). Over this ten year period insecticide use on these two crops fell by 35.6 million kg of insecticide active ingredient which is roughly equal to the amount of pesticide applied to arable crops in the EU in one year. Using the Environmental Impact Quotient (EIQ) ^[7] measure of the impact of pesticide use on the environment, the adoption of Bt technology over this ten year period resulted in 24.3% and 4.6% reduction respectively in the environmental impact associated with insecticide use on the cotton and corn area using the technology ^[8].

Advantages

There are several advantages in expressing Bt toxins in transgenic Bt crops: i). The level of toxin expression can be very high thus delivering sufficient dosage to the pest, ii). The toxin expression is contained within the plant system and hence only those insects that feed on the crop perish, iii). The toxin expression can be modulated by using tissue-specific promoters, and iv) replaces the use of synthetic pesticides in the environment. The latter observation has been well documented world-wide ^[9]

Safety

Bt crops appear to be safe for the farmers and for consumers. The toxicity of each Bt type is limited to one or two insect orders, and is nontoxic to vertebrates and many beneficial arthropods. The reason is that Bt works by binding to the appropriate receptor on the surface of midgut epithelial cells. Any organism that lacks the appropriate receptors in its gut cannot be affected by Bt ^[10], ^[11]

A recent study funded by the European arm of Greenpeace, suggested the possibility of a slight but statistically meaningful risk of liver damage in rats.^[12] While small statistically significant changes may have been observed, statistical differences are both probable and predictable in animal studies of this kind, and are known as Type I errors- that is, the probability of finding a false-positive due to chance alone. In this case, the number of positive results was within the statistically predicted range for Type I errors. The observed changes have been found to be of no biological significance by the European Food Safety Authority ^[13].

Limitations to Bt crops

 Constant exposure to a toxin creates evolutionary pressure for pests resistant to that toxin. Already, a Diamondback moth population is known to have acquired resistance to Bt in spray form (i.e., not engineered) when used in organic agriculture.^[14]

One method of reducing resistance is the creation of Non-Bt crop refuges to allow some non-resistant insects to survive and maintain a susceptible population. To reduce the chance that an insect would become resistant to a Bt crop, the commercialization of transgenic cotton and maize in 1996 was accompanied with a management strategy to prevent insects from becoming resistant to Bt crops, and insect resistance management plans are mandatory for Bt crops planted in the USA and other countries. The aim is to encourage a large population of pests so that any genes for resistance are greatly diluted. This technique is based on the assumption that resistance genes will be recessive. This means that with sufficiently high levels of transgene expression, nearly all of the heterozygotes (S/s), the largest segment of the pest population carrying a resistance allele, will be killed before they reach maturity, thus preventing transmission of the resistance gene to their progenies ^[15]. The planting of refuges (i.e., fields of non-transgenic plants) adjacent to fields of transgenic plants increases the likelihood that homozygous resistant (s/s) individuals and any surviving heterozygotes will mate with susceptible (S/S) individuals from the refuge, instead of with other individuals carrying the resistance allele. As a result, the resistance gene frequency in the population would remain low.

Nevertheless, there are limitations that can affect the success of the high-dose/refuge stragegy. For example, expression of the Bt gene can vary. For instance, if the temperature is not ideal this stress can lower the toxin production and make the plant more susceptible. More importantly, reduced late-season expression of toxin has been documented, possibly resulting from DNA methylation of the promoter.^[16] Regardless, the durability of Bt crops has exceeded expectations. Possible reasons include key factors independent of management strategy, such as low initial resistance gene frequencies, fitness costs associated with resistance, and the abundance of non-Bt host plants that have supplemented the refuges planted as part of the resistance management strategy. So, while the high-dose/refuge strategy has been successful at prolonging the durability of Bt crops, this success has also had much to do with key factors independent of management strategy, including low initial resistance allele frequencies, fitness costs associated with resistance, and the abundance of non-Bt host plants that have supplemented the refuges planted as part of the resistance management strategy^[17].

Possible problems

The most celebrated problem ever associated with Bt crops was the claim that pollen from Bt maize could kill the monarch butterfly ^[18]. This report was puzzling because the pollen from most maize hybrids contains much lower levels of Bt than the rest of the plant ^[19] and led to multiple follow-up studies. In the end, it appears that the initial study was flawed; based on the way the pollen was collected, they collected and fed non-toxic pollen that was mixed with anther walls that did contain Bt toxin ^[20]. The weight of the evidence is that Bt crops do not pose a risk to the monarch butterfly ^[21].

There was also a report in the prestigious journal, Nature, that Bt maize was contaminating maize in its center of origin ^[22]. Nature later "concluded that the evidence available is not sufficient to justify the publication of the original paper." ^[23] A subsequent large-scale study failed to find any evidence of contamination in the Oaxaca ^[24].

There is also a hypothetical risk that for example, transgenic maize will crossbreed with wild grass variants, and that the Bt-gene will end up in a natural environment, retaining its toxicity. An event like this would have ecological implications, as well as increasing the risk of Bt resistance arising in the general herbivore population. However there is no evidence of crossbreeding between maize and wild grasses.

As of 2007, a new phenomenon called Colony Collapse Disorder (CCD) is affecting bee hives all over North America. Initial speculation on possible causes ranged from cell phone and pesticide use^[25] to the use of Bt resistant transgenic crops.^[26]. A research group called Mid-Atlantic Apiculture Research and Extension Consortium published a report on 2007-03-27 that found no evidence that pollen from Bt crops is adversely affecting bees. CCD has since been attributed to a new virus, unrelated to Bt crops ^[27].

References

See also

• Western corn rootworm