Bacillus thuringiensis: A Biological Control Agent...or not?
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Much has been written about Bt and its role in modern pest management. It blurs the lines between biological and chemical control in agriculture and has become vilified because its genes are used in certain GMO crops. It can simultaneously be viewed as wearing both the proverbial white and black hats. It's worth spending some time reading up on it. Make sure you check your sources and read from a variety of perspectives, but definitely include writings from people who know biology and what the term "Mode of Action" means.
Speaking of Mode of Action = how a pesticide kills, typically takes place at the cellular/molecular level, but can be more "whole organism" like a suffocant or desiccant. The Insecticide Resistance Action Committee or IRAC, is an international group dedicated to prolonging the effectiveness of insecticides, acaracides and traits by implementing insecticide resistance management strategies: http://www.irac-online.org/. They know a thing or two about mode of action.
Check it out. |
Above is an image of a single Bt bacterium. From upper left to bottom right are the following important structures: a parasporal body (=crystaline inclusion) made of proteins and an endospore (dark circle).
A collection of Bt protein crystals. These can be formulated into a pesticide and applied as a dust or a spray to places where caterpillars will consume them, ultimately causing death.
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There is a significant difference between a biological control agent and a pesticide. Most can usually agree on that. But what happens when a biological control agent, or part of it, becomes a pesticide? There is a term: biopesticide. Staunch biological control researchers typically draw the line and say that Bt is not a biological control agent because it is applied like a pesticide. There is merit to that stance. But it also blocks out the integration of important methods of overall pest management. There also are problems that come from too loose a terminology. For example, organic certification requires the elimination of synthetic pesticides, yet there is a product that was once a conventional pesticide that is now readily available for organic use. It's now organic because it is a product of the fermentation of another kind of soil dwelling bacteria. That's confusing; so there is a need to clarify the terms, the biology, and how such bacteria and their products are used in food production.
That is the objective of this page. |
The life cycle of naturally occurring Bacillus thuringiensis
2. The process of infection is shown in the diagram above starting with the caterpillar consuming that spore with its associated crystal. Once inside the gut of the insect, the crystal begins to react chemically. The digestive enzymes dissolve the crystal into a slurry of small protein molecules that then become activated (Box B, above). They are referred to as toxins because the proteins bind to receptor sites on the insect's gut wall or epithelium. The receptor sites normally latch onto the nutrients floating in the gut lumen from the digested food that the insect needs for growth and maintenance, and then are actively transported into the body cavity (see insect gut tutorial at right). So these receptor sites have a specific 3-dimensional interface for different kinds of chemicals. The Bt proteins take advantage of these receptors and mimic the 3-dimensionality in order to lock onto the receptor; and then the magic happens. At the receptor site the toxic protein kills the cell and creates a perforation - a hole in the insect gut (Box C, above). Creating the hole in the gut wall allows the spore to enter into the insect's body. Once there, the spore reanimates and the bacterium begins to reproduce and eventually become so abundant that the insect dies from a bacterial infection. In order to complete the life cycle though, the insect's body will disintegrate, spilling the bacteria out on the surface where it died. Some of the bacteria may drop into the soil, or may stay on the leaves. The next step is for those bacteria to go into the spore stage, with their crystal attached, and await the next caterpillar victim to come along and consume them; and then the cycle starts again.
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1. Bacillus thuringiensis bacteria are naturally occurring in soil and on plant surfaces. But they are insect pathogens which means for them to thrive and reproduce they need to be inside of a susceptible insect host. So what occurs outside of a host, in the soil and on plants, is the environmentally resistant spore stage (see diagram at left, box A). The spore is the quiescent stage of the bacterium that can sit and wait for a long time until it gets inside of a host again at which point it reanimates, or germinates, to become a new bacterium. But a Bt spore has a little something extra attached to it, a crystal made up of proteins. The crystal is what helps the spore get back inside of an insect's body so it can come back to life and reproduce.
Sidebar: A quick tutorial on the insect gut, the location where all this Bt business happens:
An insect gut is made up of a thin layer of cells called an epithelium. The gut extends from the mouth to the anus, just like in people. The area most important to the Bt life cycle is in the Ventriculus (above) the area where digestion and nutrient absorption takes place. Inside the gut, digestive enzymes chemically break down food into molecules of amino acids, carbohydrates, lipids, etc. These molecules are then transported across the cells of the gut and dumped into the body cavity of the insect. Insects have an open circulatory system, so there are no veins or arteries to carry nutrients to cells, instead they just get dumped into the body cavity and are moved around passively by the insect blood, called hemolymph, and taken up by cells as needed.
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And now...for the controversy!
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Scary Definitions:
GMO = genetically modified organism. Transgenic = adjective trans·gen·ic \tran(t)s-ˈje-nik\. Being or used to produce an organism or cell of one species into which one or more genes of another species have been incorporated <a transgenic mouse> <transgenic crops>; also : produced by or consisting of transgenic plants or animals (Merriam-Webster). 
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Based on the definitions provided at left, which one best describes the type of organism that is considered to be most controversial and a hot topic in the news all the time?
If you said "GMO", then you are right and wrong. All organisms are the result of genetic modification. Mutations and natural selection create significant genetic modifications in all living organisms; always have. Humans have induced genetic modifications by the simple act of selecting desirable traits in species we've domesticated. Every thing from corn to cattle to carnations have been selectively bred to produce varieties that provide certain qualities that we like or need. But that's not what's controversial. Everyone is good with that kind of genetic modification, but we use the term GMO to describe something that's definitely not okay with lots of people. What causes concern for people is how an organism is genetically modified. That's why I included the definition of transgenic. Transgenic organisms are the ones that have genes from different species inserted into their genome to create what people call "Frankenfood" or glowing mice; and is a more accurate term for these controversial species. |
Why Monsanto, Bt, and transgenics form the perfect storm for controversy.
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What you see in the photo to the right is a corn earworm larva feeding heavily on an ear of corn; the husks have been pulled back so that you can see the damage taking place. For sweet corn, this is a very unappetizing outcome. Thus, the need for pesticides to kill corn earworm. And if you think about how much sweet corn is grown in just the US, it doesn't take much to imagine the staggering amount of accumulated pesticides that have been sprayed year in and year out for decades.
Wouldn't it be great to kill corn earworm without spraying pesticides and protect a cherished US commodity and the environment at the same time? If you said "yes", then you've just given tacit acceptance of Monsanto's plan...please read on. Bacteria biology is what allows the question above to be answered in the affirmative and the concept to come to fruition in the fields. In the Bt DNA are genes that code for the proteins that make up the crystal described above. We have the ability to replicate those genes, and in the very early stages of this technology we used another naturally occurring bacterium, Agrobacterium tumifaciens, the causative agent of crown gall (pictured below), to insert those genes into the cells of a plant - in this case corn. Click the file below to read the hard science version of how Agrobacterium works:
The trunk of a tree near the soil line (the crown) is swollen and deformed with a disease call crown gall. Agrobacterium tumifaciens bacteria attack the cells of the plant and insert their genes into plant cell, hijacking the cell's DNA to create a tumor that mass produces more Agrobaterium. What you see above is a transgenic plant - a natural transgenic plant.
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So what Monsanto did, was take a gene from Bt that produces a protein that reacts with a particular kind of receptor on a caterpillar gut wall cell and inserted it into a corn plant cell. Instead of using Agrobacterium, now they just use a gene gun and blast the gene into the plant cell - saves a lot of time. Then, using tissue culture techniques, the cell is grown into a full-fledged plant that produces seeds. Because the plant was grown from a single cell, that means every cell in the plant carries the Bt gene and every cell subsequently produces the Bt protein. And the seeds from that plant carry the Bt protein gene. That means when those transgenic corn seeds are planted, every cell in the daughter plants express the Bt protein.
When a caterpillar munches on one of the plants, it gets a mouthful of corn plant, but also a lot of that Bt protein. The protein then reacts with the receptors on many, if not all the the cells in its gut, killing most of them and basically melting its gut wall (see diagram above). The insect stops feeding and eventually dies from starvation or from septicemia (image below), but not from the Bt endospores, rather just the naturally occurring bacteria present on the surfaces of the plants upon which it was feeding. 
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So, what's the big deal? Where's the controversy? Well, there are several differences between the natural life cycle of Bt explained earlier and the transgenic situation described above.
1. In the natural life cycle of Bt, there are usually only just a few bacteria swallowed by a caterpillar while it's feeding. So those few Bt crystal/endospore combinations that do manage to successfully find their way into a caterpillar gut then perforate just a few of the cells and in so doing may take quite awhile to develop into a bacterial infection and the caterpillar keeps right on feeding. In fact, older caterpillars have been shown to have resistance to the effects of Bt; so it's really only the younger caterpillars that are susceptible.
2. Eating the transgenic corn floods the caterpillar gut with its amplified proteins that affect most, if not all, the cells of the gut. The first symptom is cessation of feeding by the caterpillar. This is, obviously, a good thing for pest management - stops the problem immediately. But since there is no endospore, the bacteria don't regenerate and proliferate. The pathogen itself has been removed from the equation. So the protein toxins eventually just disintegrate along with the dead caterpillar.
3. In naturally occurring Bt, they have genes that produce a myriad of similar, but slightly different proteins (referred to as Cry toxins, if you're looking to google this for your self). The naturally occurring Bt have evolved a way to avoid having the caterpillars developing resistance to them by having a slew of differently shaped proteins in their crystals hoping that at least one of them will be three dimensionally correct and fit into a cell's receptor site and kill the cell creating a hole for the endospore to pass through. When we engineer a transgenic plant we are inserting a gene that produces just one type of protein, nowadays two at most.
Monsanto took a different path to avoid having the insects develop resistance to their plan. If a grower purchases transgenic seed from Monsanto, they enter into a contractual agreement to use that seed for one year only and return any unused seed. Then the next year, Monsanto provides seeds with a different gene. And so it goes, year in and year out. By not using the same gene over and over, Monsanto hopes to avoid the caterpillars developing resistance to any one type of protein.
It's a smart move on their part because as anyone knows, developing resistance to such things is incredibly easy. Just look at antibiotic resistance in humans. Smart? Yes. Great business model? Yes. But does it work? For the most part yes, but we are starting to see some in-the-field resistance to the Bt transgenic plants. Monsanto has a plan for this too. It's called refuge planting, look it up. Newsflash: It too, is not without controversy, just click on the previous antibiotic resistance link to watch the inevitable.
4. Are you eating a pesticide when you eat a GMO plant? Yes. When you eat transgenic corn, you too are consuming the Bt proteins. But...are your cells going to have a reaction to the protein, causing perforations in your gut? Short answer: NO! The proteins have evolved to operate in a gut that is more basic than acidic. Our stomachs have amazingly low pH - around 2 or so. The lepidopteran larval gut pH ranges from 8 to 12 - very basic. The pH environment affects how the proteins unravel from the crystal and how they interact with the cellular receptors. In other words, the basic gut in insects "activates" the proteins making them "toxic". That won't happen in our stomachs. Also, there are no receptors on our cells for these proteins to interact with. And our gut is lined with a thick coating of mucous. For humans, eating the transgenic corn means consuming one additional type of protein that is digested and broken down into amino acids, just like all the other proteins we eat. You may read online where some groups are attempting to debunk the acid vs. base idea, but you don't have to have much in the way of critical thinking skills to see that they can't actually debunk it.
Is there any reason to worry? Well, like any type of protein we ingest, some are broken down more easily than others and a certain small segment of the population is going to have sensitivities or maybe even allergies. Think nut allergies. So, it's not all black and white. As with most things biological, there is no 100%. Except, it's pretty much 100% certain you've eaten some GMOs at some point in your life and you didn't have a choice in that...so there's your controversy.
1. In the natural life cycle of Bt, there are usually only just a few bacteria swallowed by a caterpillar while it's feeding. So those few Bt crystal/endospore combinations that do manage to successfully find their way into a caterpillar gut then perforate just a few of the cells and in so doing may take quite awhile to develop into a bacterial infection and the caterpillar keeps right on feeding. In fact, older caterpillars have been shown to have resistance to the effects of Bt; so it's really only the younger caterpillars that are susceptible.
2. Eating the transgenic corn floods the caterpillar gut with its amplified proteins that affect most, if not all, the cells of the gut. The first symptom is cessation of feeding by the caterpillar. This is, obviously, a good thing for pest management - stops the problem immediately. But since there is no endospore, the bacteria don't regenerate and proliferate. The pathogen itself has been removed from the equation. So the protein toxins eventually just disintegrate along with the dead caterpillar.
3. In naturally occurring Bt, they have genes that produce a myriad of similar, but slightly different proteins (referred to as Cry toxins, if you're looking to google this for your self). The naturally occurring Bt have evolved a way to avoid having the caterpillars developing resistance to them by having a slew of differently shaped proteins in their crystals hoping that at least one of them will be three dimensionally correct and fit into a cell's receptor site and kill the cell creating a hole for the endospore to pass through. When we engineer a transgenic plant we are inserting a gene that produces just one type of protein, nowadays two at most.
Monsanto took a different path to avoid having the insects develop resistance to their plan. If a grower purchases transgenic seed from Monsanto, they enter into a contractual agreement to use that seed for one year only and return any unused seed. Then the next year, Monsanto provides seeds with a different gene. And so it goes, year in and year out. By not using the same gene over and over, Monsanto hopes to avoid the caterpillars developing resistance to any one type of protein.
It's a smart move on their part because as anyone knows, developing resistance to such things is incredibly easy. Just look at antibiotic resistance in humans. Smart? Yes. Great business model? Yes. But does it work? For the most part yes, but we are starting to see some in-the-field resistance to the Bt transgenic plants. Monsanto has a plan for this too. It's called refuge planting, look it up. Newsflash: It too, is not without controversy, just click on the previous antibiotic resistance link to watch the inevitable.
4. Are you eating a pesticide when you eat a GMO plant? Yes. When you eat transgenic corn, you too are consuming the Bt proteins. But...are your cells going to have a reaction to the protein, causing perforations in your gut? Short answer: NO! The proteins have evolved to operate in a gut that is more basic than acidic. Our stomachs have amazingly low pH - around 2 or so. The lepidopteran larval gut pH ranges from 8 to 12 - very basic. The pH environment affects how the proteins unravel from the crystal and how they interact with the cellular receptors. In other words, the basic gut in insects "activates" the proteins making them "toxic". That won't happen in our stomachs. Also, there are no receptors on our cells for these proteins to interact with. And our gut is lined with a thick coating of mucous. For humans, eating the transgenic corn means consuming one additional type of protein that is digested and broken down into amino acids, just like all the other proteins we eat. You may read online where some groups are attempting to debunk the acid vs. base idea, but you don't have to have much in the way of critical thinking skills to see that they can't actually debunk it.
Is there any reason to worry? Well, like any type of protein we ingest, some are broken down more easily than others and a certain small segment of the population is going to have sensitivities or maybe even allergies. Think nut allergies. So, it's not all black and white. As with most things biological, there is no 100%. Except, it's pretty much 100% certain you've eaten some GMOs at some point in your life and you didn't have a choice in that...so there's your controversy.
