Hi and thanks for visiting. Join me on a journey of middle-aged distance learning about toxic substances as well as technology.
For my first ever blog post, I’ll introduce you to Bacillus thuringiensis (Bt). Bt is a soil-dwelling bacterium that is used as a microbial pesticide/insecticide. Briefly, this microbe holds appeal as an insecticide because it is selectively toxic to certain insects and not mammals. This strain of bacteria was discovered in the Free State of Thuringia in central Germany in 1915.
Structure discussion here (discuss microscopic findings)
Back to our topic, Bacillus thuringiensis (Bt). If, like me, this was a new word to you, I’ve attached a link with pronunciation of this microorganism.
https://youtu.be/q9pixg7-SXY
The main benefit of using a biopesticide such as Bt is the select toxicity of this agent to certain insects (predominantly worms and caterpillars, not beneficial insects such as ladybugs, earthworms and honey bees) and not off-target mammalian species due to the mechanism of action. Here’s an interesting website from some other school that discusses this.
How to control invasive pests while protecting pollinators and other beneficial insects
Here’s a video that not only provides a great summary of how Bt works, it also makes the microbe look adorable.
Link: Summary of Bt
Bt would be used topically on plants as a pesticide (insecticide), but it can also be incorporated in plants using genetic modification. Plants, such as corn, cotton, potato and tobacco can be genetically modified to produce the Bt insecticidal Cry/Cyt proteins.
Bt containing products are available over-the-counter as well as online and do not require specialized personal protective equipment or training to use.
Link: https://www.bonide.com/products/garden-naturals/view/802/thuricide-bt-conc
Toxicokinetics of Bt if ingested or inhaled by mammalian species:
Mammal ingestion: Bt remains in the digestive system and it is not absorbed systemically into the body when ingested. The Bt bacteria does not reproduce inside the body of the mammal that has ingested it. The Cry/Cyt protein that is produced by Bt is digested in the acidic gastric fluid of mammals (versus this protein being soluble in the alkaline solution in the midgut of sensitive insects).
Link to pH of gastric fluid discussion: Safety and Advantages of Bacillus thuringiensis-Protected Plants to Control Insect Pests
Mammal inhalation: When inhaled, Bt can be absorbed via the lungs, blood and lymphatic system and the kidneys but is then removed by the immune system resulting in a decrease in Bt levels systemically within one day. Link: http://npic.orst.edu/factsheets/BTgen.pdf
Carcinogenicity of Bt: This substance has been studied in humans, and was not noted to be associated with an increased risk of cancer. http://npic.orst.edu/factsheets/BTgen.pdf
Mechanism of Action of Cry/Cyt toxin that is produced by Bt: The feeding insect ingests the Bt bacterium and once inside the insect, the crystallized Cry/Cyt toxin is digested and processed to produce an endotoxin. This endotoxin binds to epithelial cell receptors in the midgut of the insect digestive system and results in pore formation. This alters potassium flux across these damaged cells as well as an increase in intracellular pH. The midgut epithelial cells are destroyed by pH and osmotic lysis leading to starvation and septicemia of the insect. Here’s a nice (silent) video that goes over the mechanism of action in detail: Bt toxin method of action
This is an in-depth article that discusses the mode of action of Bt in lysing the midgut epithelial cells of the insect. Mode of action of Bacillus thuringiensis Cry and Cyt toxins and their potential for insect control
Target organ(s):
Insects: in the midgut of the digestive (alimentary) system of the insect.
Humans:
- Gastrointestinal system if ingested
- Respiratory system if inhaled
- Skin if topical exposure
Signs and symptoms of toxicity:
Insects-starvation, death.
Link: http://www.bt.ucsd.edu/how_bt_work.html
Humans: Given the insect specificity of the method of action of Bt, this substance is considered to be of low toxicity to mammals. Some potential symptoms that may be reported by individuals that are exposed to a Bt-containing insecticide include:
- Eye irritation
- Skin irritation
- Rarely, people with hay fever reported symptoms such as difficulty with sleep and concentration, stomach upset, nose and throat irritation but seasonal (pollen) factors may contribute to this report
- Farm workers with exposure of > 4 month duration may develop an immune response and potential skin allergies
http://npic.orst.edu/factsheets/BTgen.pdf
Genetic susceptibility or heritable traits:
Human genetic susceptibility not reported.
Insects are developing some mutations that provide some resistance to Cry proteins, so interventions to reduce the development of resistance to this pesticide are being researched.
Historical or unique exposures reported: A historical concern about using Bt crops and monarch butterflies was noted historically but a peer-reviewed study showed
“that monarch butterflies exposed to Bt corn in the environment are not subjected to any significant risk.”
Treatment:
- Human ingestion exposure:
- Human inhalational exposure:
- Human dermal exposure:
Fact sheet: General Fact Sheet
Biomarkers: Since this substance is digested in the mammalian digestive tract a biomarker could not be found, but there is a method (biomarker called CaDPA) to observe germination of spores of Bacillus thuringiensis. The utility of this evaluation would be in food safety/public health settings.
In summary, I found that I have learned quite a bit about toxic substances over the past few weeks and I’m excited to be participating.
Stacy