Bacillus thuringiensis
Higher order taxa
Eubacteria (kingdom); Bacteria (domain); Firmicutes (phylum); Bacilli (class); Bacillales (order); Bacillaceae (family); Bacillus (genus); Bacillus cereus group
Species
Bacillus thuringiensis
Description and significance
B. thuringiensis (Bt) is gram-positive, soil-dwelling,spore-forming, and rod-shaped. This bacteria grows optimally around 37 degrees Celsius, adapting well to body temperature. Furthermore, it is used very prominently in the field of agriculture as a pesticide. In fact, it is the most common one in use and is the building block of 90% of the others in the same market used currently. This is due to the fact that unlike many other kinds of farming predator repellents, Bt is very harmless to organisms that are not the target of its use, such as humans, plants, etc. The reason for this is in the manner in which it functions: the bacteria releases diamond-shaped crystal spores that are lethal to the insects that digest them. These crystals attach to specific receptors in the gut of the host to carry out its purpose. In the organisms not meant to the prey of these spores, the receptors are not present, so that the required bonding is disallowed. Moreover, there exists various forms of the microbe; this suggests to researchers that effectiveness against the target insects will become more complex and grow stronger. All the studied variants are still toxic to all its directed hosts. The aforementioned variants were able to be studied because recent research was able to map out B. thuringiensis' complete genome. It was determined that B. cereus is its closest relative. Even though they share very close genotypes, they differ in phenotypes mainly due to Bt's ability to translate Cry proteins that make crystal spores. research states that if not for this feature, the two would have been classified as one species.
Genome structure
B. thuringiensis was sequenced in total in February of 2007. Researchers were able to observe the many variant strains of the bacteria. These variations lead to increased effectiveness of the function of Bt to fight off insects that are harmful to crops in the field of agriculture. In fact, crops that are genetically modified with B. thuringiensis efficiently kill off many pests that try to attack the plants to serve as their source of food. As a reference, it was found that the GC content within the circular shaped chromosome of Bt is about 35%, counted out of a possible 5,800 bases. The number of plasmids are great in number also. Most of the pathogen's genome contributes toward endotoxins which are part of its main arsenal in attacking the insects that prey on a farmer's crops.
Cell structure and metabolism
Its accentuation of a thick peptidoglycan cell wall comes from the fact that Bt is gram-positive in nature. this means that the components of the outer wall protecting the organism is composed of much amino acid polypeptide and sugar. Even though the cell wall is known to bring rigidity and structure to the inner contents, the actual main support for this function is something called the periplasmic space, which is located between the cell wall and the cell's plasma membrane. This area serves as a location for many chemical reactions to take place that help build proteins to aid in protection purposes.
The most prominent feature of this organism has to do with the production of crystal spores. Therefore, protein synthesis for Cry is very key to Bt's function. Various protein secretions are all composed of different domains. All the domains can be broken down into components consisting of different combinations of alpha helices and beta sheets. These toxins get inside the gastrointestinal tract of host organisms. This is done only through the process of digestion; the target insect will have to take in the spores by eating the crystals. These crystals then get to work: they strip off their protective coating, attach to specific receptors, and form pores on the inside of the lumen. The host soon dies from starving to death due to improper digestion and maintenance of food.
Ecology
B. thuringiensis is known to be one of the safest forms of pesticides in the market today. This is because of the fact that only insects have the proper receptors within their guts to be vulnerable to the function of these microbes. No negative effects will be able to be seen when doing experiments with genetically modified crops and various test subjects like hens, fish, humans, or on the sprayed plant crop themselves. Also, it washes away with rain, of even excessive and constant exposure to sunlight. Since it already exists as a soil bacterium, there is no unnatural introduction of the organism into a new environment that may lead to unexpected evolutionary changes. Not only does the fact that it rids itself in a short time under natural conditions make it desirable, this characteristic is also beneficial to farmers in that it avoids a prolonged exposure to the target insects; this is a positive characteristic because of the needed conditions for the host to develop resistance to the pathogen. Since it attacks its victims in short outbursts, the host is unable to build up an immunity towards it, and will then be constantly vulnerable to attack whenever the insecticide is sprayed again.
Pathology
It seems almost as if these microbes are specifically designed to help in the field of agriculture as an aid to farmers that grow crops. Bt is pathogenic only towards certain insects such as beetles, wasps, moths, black-flies, mosquitoes, bees, and nematodes. These target hosts are usually in the orders of Lepidoptera, Coleptera, and Diptera. All these are in the category of invertebrates, which is due to the fact that the pesticide has no toxic effects toward vertebrates, including humans. The Cry protein with B. thuringiensis releases an endotoxin that gets digested by a target insect. Once inside the gut of the host, the outer protective coat is shed. This is thought to happen when the toxin comes into contact with a higher environmental pH level, which successively causes it to attach to specific host receptors. The bond then immediately begins to tear holes from inside the gut to the outer spaces causes leaks to spring up. The crystal further begins producing more spores which reinforce the damage and make it more widespread within the gut. The many pores starve the host which immediately weakens and kills the insect fairly quickly. After observing the many strains of the bacteria, researchers expect the effectiveness of Bt to increase as the toxin gets more flexible. One important thing to note is that the translation of the Cry gene and the protein is conserved among the various strains. Therefore, we can conclude that this feature is a basic and foundational key to the purpose of the bacteria.
Application to Biotechnology
In use in science, researchers were able to extract the crystal spores and use them to mass produce other pesticides that would not require the presence of these organisms to fight off crop-preying insects. All these manufactured goods progress in the same manner as the microbe-produced insecticide and maintain the same (non)hazardous characteristics to various organisms that come into contact with the crystal spores. Since this is so convenient, in a general manner, approximately 90% of all insecticides use Bt from which to build. This eliminates the use of chemicals that may or may not produce very negative effects on humans and even desirable insects.
Current Research
Cattaneo, Manda G. et al. "Farm-scale evaluation of the impacts of transgenic cotton on biodiversity, pesticide use, and yield." Proceedings of the National Academy of Sciences of the USA 103(2006): 7571-7576.
There exists many forms of insecticides today. This study performed a comparative experiment observing the effects of genetically modified and the use of Bacillus thuringiensis on crops as with nontreated crops. However, the two groups were not left alone after being separated as such. The tested measure was that of the amount of herbicides and insecticides after planting that it took to maintain sufficient crop yields. It was found that more supplemental treatment was needed for the untreated group than for the transgenic modified group. In the first year, an average 6.6 insecticide applications were needed for the control group while only 3.4 applications was needed for the Bt group. In the following year, 6.8 applications were needed for the non transgenic group while 5.1 applications were necessary for the Bt experimental group. This shows that the genetically modified transgenic group showed more efficient crop production than the normal control group.
Rosi-Marshall, E.J.. "Toxins in transgenic crop byproducts may affect headwater stream ecosystems." Proceedings of the National Academy of Sciences of the USA 104(2007): 16204-16208.
Much research has been done to prove the effectiveness of the popular insecticide, Bt. This is based on the functions of the microbe for which it gets its name, Bacillus thuringiensis. However not many studies focus on the effect of this product on the environment. Rosi-Marshall led a study to get a bigger picture of its effect on the water nearby to where the pesticide is being used. What was discovered was that stray airborne treated pollen was flying over into the water supplies near the farms. This was measured using various methods that estimated the ratio of transgenic crop traces in the sediments beneath the surrounding bodies of water. Furthermore, the distance of travel of these pollen was directly related to the velocity of the bodies of water through which it would travel. There was no correlation between the use of the pesticide and travel distance as compared with the control non adjusted group; both travelled the same average distance, and under no particular favor. Here, far from the farms that depends on these pesticides for optimum growth, are a completely different type of ecological system. Therefore, the drifted pollen caused the deaths of many various insects that associate within this new environment.
Wei, Jun-Zhi. "Bacillus thuringiensis crystal proteins that target nematodes." Proceedings of the National Academy of Sciences of the USA 100(2003): 2760-2765.
The microbe Bacillus thuringiensis has been researched to have the characteristic of being soil-dwelling. Therefore, the question comes up that asks why this organism would develop such lethal effects against organisms that rarely spend time feeding in the soil. This research study then aims to study the effect of this insecticide on organisms that actually ive in the soil, one such as nematodes. Since the organism uses released spores made by the Cry protein, the study used seven variants of the crystal protein to test on five different species of nematodes: C. elegans, P. pacificus, D. veechi, P. redivivus, and Acrobeloides. Many different combinations of spore to host were attempted and the many different results were recorded. It was found that in very few cases, some organisms were found to be resistant to the spore and produced no negative effect to ingesting it.