Bt

There’s a lot of interesting and sometimes conflicting things about Bt out there. Most people know that the Bt gene originally came from Bacillus thuringiensis bacteria, which are common in soil, but there’s a lot more to know! Here, I’ll discuss the differences and similarities between Bt from bacteria and Bt which is found in genetically engineered crops, as well as the way these similarities and differences are considered for purposes of toxicology testing.

A field is a complex place, even without consideration of the different types of Bt. Before we get too far into the “weeds”, let’s look at Life in a standard and in Bt maize field, a video produced for the European MOBITAG project. Discussion of Bt begins at 16:40.

Many forms of Bt

Bt as found in B. thuringiensis bacteria is not quite the same protein as that which is used in transgenic crops. There are 2 key differences:

1) The DNA sequence has been converted from the language (aka codon specificity or codon preference) recognized by bacterial DNA transcription/translation machinery to the language recognized by plant machinery. While the gene sequences are different, this does not change the actual sequence of the resulting protein.

2) The DNA sequence from the bacteria was shortened before being used to transform plants. The native protein crystallizes while the altered one does not. Both proteins must be activated by specific enzymes in the basic pH of an insect gut, and this active form of the protein then binds to specific receptors in the membrane of cells in the insect’s gut, creating holes in the gut, which kills the insect. Since the protein only works when the receptor is present, only certain types of insects are susceptible to the Bt toxin.

The part of the Bt protein that binds to receptors in the insect’s gut can be altered to become more or less specific. If the receptor is the key, and the Bt is the lock – we can make the lock fit a set of similar keys or change it to fit a different key. The DNA sequence of the original Bt has been changed by humans to create a series of Bt proteins that affect different insects. It’s not just people that have altered Bt. Good old mother nature has been modifying the Bt protein as well – there have been close to 200 different types of Bt identified in different strains of B. thuringiensis.

All natural B. thuringiensis based pesticide.

The different forms of the protein are given different names. While the common name of the group of proteins is Bt (and that name works well enough for most purposes), when we want to be specific, the proteins are called Cry (for crystalline protein) with a combination of letters and numbers, such as Cry1A. Another way to know what type of Bt is used is to look at the crop variety. For example, MON810 expresses Cry1A(b). Note that various forms of Bt are not just used in engineered crops – the proteins are also expressed in bacteria and used as whole bacteria or Bt extracts in sprays in organic and conventional farming, including many of the same Bt proteins that are used in biotech crops.

What about safety?

Because Bt is a pesticide (insecticide, to be exact), in the United States, the EPA (Environmental Protection Agency) is responsible for determining its safety before use is approved. The EPA has approved 12 different Bt proteins for use in corn and/or cotton since 1995, some of which are no longer in use. The links here are from the Biosafety Clearing-House, set up by the Cartagena Protocol on Biosafety.

EDIT: For specific information on Bt traits currently on the market in the US, check out the Handy Bt Trait Table from University of Wisconsin Extension.

The EPA has a very nuanced stance on testing of Bt for human health, which they discuss in the Biopesticides Registration Action Document for Bt.

Some testing has been done with protein either extracted from various strains of B. thuringiensis or extracted from E. coli engineered to express Bt (or E. coli engineered to express the plant language version of Bt). These early tests were useful, and can to some degree be extrapolated to the more recent forms of Bt that have been engineered to be more specific to different insects. The proteins are very similar to one another, break down in similar ways, etc. However, we can’t assume that they’re exactly the same – they are at least slightly different protein sequences after all! Each new version of Bt has undergone new testing for allergenicity and toxicity both by the company wanting to sell crops expressing the new protein and by independent agencies all around the world.

The basic premise relied on for the toxicology assessment is the fact that all the Bt plant-incorporated protectants are proteins. Proteins are commonly found in the diet and, except for a few well described phenomena, present little risk as a mammalian hazard. In addition, for the majority of Bt proteins currently registered, the source bacterium has been a registered microbial pesticide which has been approved for use on food crops without specific restrictions. Because of their use as microbial pesticides, a long history of safe use is associated with many Bt products.

Does that mean that the EPA considers Bt to be GRAS (generally recognized as safe)? Definitely not. The EPA collects 3 types of data for each new type of Bt ”to provide a reasonable certainty that no harm will result from the aggregate exposure to these proteins”. The tests are intended to show that the Bt protein:

  • Behaves as would be expected of a dietary protein, as determined with an in vitro digestion assay.
  • Is not structurally related to any known food allergen or protein toxin, as determined with amino acid sequence homology comparisons, and
  • The Bt protein does not display any oral toxicity when administered at maximum hazard dose using purified protein of the plant incorporated protectant as a test substance. Due to limitations of obtaining sufficient quantities of pure protein test substance from the plant itself, an alternative production source of the protein is often used such as the Bacillus thuringiensis source organism or an industrial fermentation microbe.

Note that they aren’t testing oral toxicity of any old Bt protein, they are looking at the specific Bt protein in question, albeit expressed in a bacteria or yeast instead of in the crop itself – because the crop doesn’t produce enough of the protein to result in a toxic response.

EPA believes that protein instability in digestive fluids and the lack of adverse effects using the maximum hazard dose approach in general eliminate the need for longer-term testing of Bt protein plant-incorporated protectants. Dosing of these animals with the maximum hazard dose, along with the product characterization data should identify potential toxins and allergens, and provide an effective means to determine the safety of these protein.

Despite the fact that the EPA and various scientific bodies find it unnecessary to preform additional toxicity testing of Bt proteins, scientists do it anyway. Numerous tests have been conducted feeding Bt crops (mostly corn, but also cotton and potato) to livestock, other animals such as quail, and yes, even monarch butterfly. Few have found significant differences between Bt and non-Bt feed, and those that have found differences often have methodological or statistical errors that have been well covered elsewhere.

Ironically, crops expressing Bt have reduced toxin exposure all over the world. Corn expressing Bt has dramatically reduced incidence of fungal infection and of potentially deadly mycotoxins (toxins produced by fungus). Why? The Bt corn has fewer insect bite marks, which is how the fungus enters the kernels to colonize the ear. There are other ways to prevent fungus growth, but none are better than Bt, especially in tropical and sub-tropical areas such as southern Africa and Central America where maize aka corn is a staple. In addition, use of Bt has allowed farmers to use fewer broad spectrum insecticides, letting more non-pest insects live and reducing exposure to farmers and neighbors during pesticide application. Finally, because Bt protects corn from insect damage, Bt corn has higher yields so less land is needed to grow the same amount of food.

Delicious sweet corn! (Although this isn’t Bt corn, I was lucky to get some non wormy ears!)

I love corn, and knowing that Bt is a safe protein for animal (and human!) consumption, and knowing the risks of mycotoxins (as well as how gross earworm can be!) I would be happy to eat Bt corn, whether in field corn ground for tortillas, masa, or other foods, or in delicious sweet corn*.

Further reading

* To date, while Bt sweet corn varieties do exist, they aren’t widely used.

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Anastasia is a Board Member of Biology Fortified, Inc. and the Co-Executive Editor of the Biofortified Blog. She has a PhD in genetics with a minor in sustainable agriculture from Iowa State University. Her favorite produce is artichokes! Learn more about Anastasia at about.me. Disclaimer: Anastasia's words are her own and views expressed do not necessarily represent the views of her employer(s). She is not paid to blog or conduct any social media activities. Any mention of a specific company or product does not indicate endorsement of that company or product.


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17 comments to Bt

  • This is a spectacular summary and I’m really glad you posted it. One point should be added: how do we manage insects developing resistance? It’s pretty similar to how you prevent any organism from changing in a way inconvenient to us — make sure the trait we want to survive (susceptibility to a pesticide) survives. For Bt corn and soy, the current protocols have various planting patterns that leave refuges. If a particular insect survives the Bt-toxin on the GM crop, then it is likely to mate with one from a refuge plant and produce still susceptible offspring.

    Considering one of the many worries about GM crops is “what happens when the plant/insect becomes resistant?!” I think it’s important to point out that we manage it in a way very similar to conventional pesticides.

  • MikeB

    It’s articles like this that keep a total non-expert like me coming back. Thanks.

  • Thanks for your kind comments, Rachel and Mike!

    The resistance issue is a strange one. Resistance can be managed quite well, if we want to. It seems the folks concerned about weed resistance to herbicides and insect resistance to insecticides (including Bt) should be strong advocates for integrated pest management, but I don’t think I’ve heard anyone claiming to care about resistance mention IPM. Strange.

  • Anastasia, I’m typing up a post right now on Bt sweet corn (which I ate several ears of over Memorial Day weekend) and this post has given me some great info to link too! Thanks!

    I’ll be sure to tweet the post to you when it’s done!

  • Daws

    One thing I always wondered, where is the bt gene expressed in the GM plants? Is it just expressed in the leaves or is it every part of the plant, or does it vary depending on the plant/strain designed?

    Are we able to specify yet to this degree?

    • This is a great question! Genetic engineers can target expression of a gene pretty well using different promoters. For example, in my own research, I used a promoter from a seed storage protein, with the goal of expressing my protein of interest in just the endosperm (starchy part) of corn seeds. There are also promoters that are leaf specific, root specific, etc. If I recall correctly, early examples of genetic engineering used an ubiquitous promoter that expressed it in the whole plant, but more recent ones have been using root or leaf promoters depending on what is needed – like Bt being expressed with root specific promoters when the goal is to combat rootworm. One nice thing about this approach is that the protein that you are expressing can be expressed where it is needed but not in the edible part of the plant.

      • Daws

        Ah perfect, that’s nearly the nail in the coffin for my debating certain anti-gmo peeps. So glad they can do this. Is there anyway to tell if the ubiquitous promoters have been discontinued entirely? (how long ago did they start specifying to just the roots or leaves?) I do recall the video in the article above saying that all parts of the corn seem to get attacked, so maybe not with corn but perhaps with other plants? Can anyone mention specifics on which plants may have which promotion?

    • MON810 uses CaMVe35S promoter (so is constitutive)
      MON88017 uses a 35S promoter (so is constitutive)
      Bollgard also uses a constitutive promoter – all the info is in the patents, it’s just a case of finding the darned patent – best way to do this is by event name as far as I’ve seen (and I just happen to always remember MON810 and MON88017 off the top of my head – Bollgard took a bit longer (MON15985) mainly because I’ve never worked on cotton.

      • Ewan, say it isn’t so! Please tell me Monsanto is using at least some tissue specific promoters! It’s de rigueur in academic labs, you know.

        • I’m just reporting what is in the commercial material that I looked up this am – I can’t comment on what promoters we utilize in testing beyond saying that it isn’t all constitutive (it is however, somewhat more complicated than academic useage, as intellectual property law can be a pain in the rear end in the world of industry).

          MON863 appears to be constitutive also.

          It seems that the bulk of commercially utilized events are constitutively expressed – I don’t have the time to track down everything at the moment however.

      • Daws

        For the layperson (ie me ;) what’s “constitutive” mean? That it’s expressed everywhere?

        Also how can one (me again ;) find the patents to read for themselves to find out this stuff?

        • Ewan R

          Yeah, constitutive means everywhere.

          Google scholar is my source I just put in the event name (MON810 for instance) and add patent to the search term (probably not the most elegant way to do it)

          It may take a bit more digging to figure out which promoter does what – I just know that CaMV and 35S are constitutive from experience (although it shouldnt be overly hard to figure it out with a bit of googling, or simply asking here)

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