Bacillus thuringiensis is a common soil bacterium capable of producing spores that have proteins which are toxic to some insect species. It is widely distributed and in one study was detected in approximately 70% of soil samples taken from around the world (Martin). If an insect consumes Bt spores, protein crystals from the spore dissolve and become activated in the insects gut that lead to the formation of pores through the cell membranes of cells within the insect gut. These pores provide a medium that favors bacterial spore germination. A resulting bacterial infection can lead to the death of the insect. When the food supply is exhausted, the bacteria can again enter the spore state and repeat the cycle.
Bt was first cultured from dead silk worm larvae in Japan in 1901. In 1958, Bacillus thuringiensis first became available commercially as a biopesticide. Various preparations based on Bt exist which contain Bt as well as other additives. There are many different varieties of Bt which have different toxicities to different organisms. Bt is primarily effective against the larvae of insects within the orders Lepidoptera (butterflies and moths), coleoptera (beetles), and diptera (flies and mosquitoes), but some varieties also infect nematodes and flatworms.
Before the precise mechanism of toxicity was known, preparations of Bt bacteria were used as biopesticides to control defoliant pests in forests, to protect crops, and to control mosquitoes and black flies that can transmit disease. Formulations of Bacillus thuringiensis continue to be used today, in particular by organic farmers.
Mechanism of action
The toxic nature of Bacillus thuringiensis comes from cry proteins which it produces. Different cry proteins are present in different Bt varieties. The cry proteins are present in a crystalline form in the bacterial spores. Upon ingestion the crystals are solubilized by the high pH of susceptible insect stomachs (low pH will not dissolve most cry proteins, keeping them in a crystallized form in mammalian stomachs) (Swadener, Bravo). Once solubilized, the cry proteins act by creating pores in the cell membranes of the cells lining the insect gut. The exact mechanism by which the pores are formed is not precisely known and may be different for different cry proteins. In general, it is believed that digestive enzymes cleave the cry protein to an active form. The activated cry proteins then bind to specific proteins in the cell membrane and insert into the cell membrane to create pores (Bravo). These pores allow cellular contents to leak form the cells. If enough cells are damaged, the digestive system of the insect will no longer function and it will perish.
Transgenic use of Bt proteins
To harness the insecticidal nature of Bacillus thuriengiensis, cry genes, responsible for the insecticidal activity, can be transformed into plant species. Upon transformation with an appropriate DNA construct the plant species express the cry genes and produce cry proteins. These cry proteins lead to the death of specific insects that feed on the plants. Transgenic plants incorporating cry genes were first produced in the 1980’s. These so called Bt crops have proven to have increased resistance against certain insect species.
Below are a list of advantages and criticisms of Bt crops. This is not yet an exhaustive or well researched list, but hopefully can serve as a good starting point.
By the nature of being proteins, the cry proteins that have insectide activity in Bt crops are completely biodegradeable.
Present in the environment
Bacillus thuringiensis is a common soil bacterium. One study reported finding varieties of Bt in 70% of soil samples taken from around the world. Cry proteins and genes are already present in the environment, though they are not naturally found in plants.
History of use
Preparations of Bacillus thuriengiensis cultures have been used as an applied biopesticide for fifty years.
Cry proteins are specific to certain orders or families of insects. The specificity comes from a combination of the alkaline insect gut environment and from receptor proteins present in the membranes of cells lining the insect gut.
Reduces need for applying pesticide
The use of Bt crops reduces the need for applications of chemical pesticides. Chemical pesticides can be damaging to the environment because they can run off of fields and have undesirable effects on other organisms. Furthermore, there are environment and economic costs associated with the production, transportation, and application of these pesticides.
Disadvantages / criticisms
Can’t be washed off
While Bt preparations have been used as a biopesticide for many years, theses preparations were applied topically and could be washed off the plant surface. With current Bt technology, cry proteins accumulated inside plant tissue and cannot be washed off.
Is Bt really safe?
There have been reports, particularly on the internet, of health problems associated with the use of Bt. There are numerous studies on the safety of Bt that need to be explored for this article. An important consideration is how the safety of Bt compares to the safety of alternative pesticides it can replace.
As with most GM crops, it is a concern that GM crop species may breed with wild relatives and spread GM genes to other species. It is possible to develop plants that are less likely to outcross or pass on transgenic genes to the next generation, though it is likely hard to completely eliminate this possibility.
In laboratories and in the field cases of insect resistance to Bt have been shown. The use of multiple cry genes or complementing cry genes with cyt genes has been shown to reduce or prevent the emergence of resistance. Alternative strategies exist such as providing refuge fields of non-Bt crops for non resistant insects to live, limiting the use of Bt technology, or rotating the use of Bt crops.
Some Bt crops rely on the use of nonspecific, constitutive promoters to drive the expression of Bt genes. The use of these promoters means that the cry proteins accumulate in all tissues, rather than simply the specific tissue that the pest feeds on. For example, the use of a leaf specific promoter would limit accumulation of cry proteins to leaves and leave the fruit / kernel / pollen Bt free.
Use of markers
The use of selectable markers, often antibiotic resistance genes, facilitates the selection of transformed plants. GM plants have been criticized generally for the use of selectable markers. Technological advances have allowed for the production of marker-less transgenetics, although it is more expensive and takes more time.
Toxicity to beneficial insect species
There are criticisms of Bt crops in that they can cause the deaths of non pest species. Bt has been blamed for the deaths of pollinator species and monarch butterflies. Bt does not seem to be have major negative effects for on non target species, but more reading of the research is needed here. Altering the cry genes used or using tissue specific promoters may help to reduce these non target effects, if indeed they are a problem.
Bravo, A., Gill, S., Soberon, M.. Mode of action of Bacillus thuriensis Cry and Cyt toxins and their potential for insect control. Toxicon 49, 423-435. 2007.
Schuler, T., Poppy, G., Kerry, B., Denholm, I.. Insect-resistant transgenic plants. Trends in Biotechnology. 1998.
Swadener, C.. Bacillus Thuringeiensis. Journal of Pesticide Reform. 1994.
Martin, P., Travers, R.. Worlwide abundance and distribution of Bacillus thuringiensis isolates. Applied and Environment Microbiology. 1989.