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The Dark Bacillus Crystal


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Toxic "parasporal" crystals of Bacillus thuringiensis. Jim Buckman/ P.R. Johnston. Public domain.

In this photograph are elegant, microscopic agents of death. They are crystals made not of minerals, but of protein, and are found not in vugs, but in guts. Bug guts. They are Cry protein crystals made by the bacterium Bacillus thuringiensis. You may know them better as Bt toxin.

Bt toxin has gotten a lot of negative press, particularly at the hands of those opposed to genetic engineering of crops. My purpose here is not to stoke that debate, but to show you what an amazing thing the Bacillus thuringiensis toxin is in the state of nature.

Bacillus thuringiensis is a soil bacterium, like many of its kin in the the Firmicutes, a group of largely Gram-positive bacteria. Gram-positive bacteria have very thick, naked cell walls made of polymers of sugars and amino acids called peptidoglycan. They seem to be fairly closely related to one another. Mycoplasma, which I wrote about a few weeks ago, are actually Gram-positive bacteria that shed their cell walls after they shacked up with animals. We can tell by their DNA. In contrast, the Gram-negative bacteria are not really a natural grouping, but more of an “everything else”.

The spore-forming bacteria are a subset of the Gram-positive bacteria, and the trick that they perform is incredible indeed. When conditions deteriorate, they can form a tight little tank-like bundle called an endospore. Endospores are dehydrated bacterial survival packets swathed in thick layers of protective proteins and peptidoglycan. Click here for a nice cross-section of the layers.

Endospores of Bacillus anthracis, better known as the agent of anthrax. The endospores are the rounded, bright white objects still found within their parent cells. CDC Public Health Image Library #1893. Public domain.

Endospores differ from normal bacteria in both their content and coating. Enough water is sucked out of the cytoplasm to turn it to a gel, while chemicals are inserted into DNA to fortify it. Removing most of the water helps prevent damage to DNA from heat and light, while wrapping the whole thing in several protective layers helps shield the spore from UV radiation and nasty chemicals.

Endospores probably evolved as a short-term resistance structure to permit survival in dry, hot, nutrient-poor conditions. But these adaptions have also enabled endospores to perform some amazing feats. Boiling, for instance, doesn’t phase them, and even autoclaves — laboratory sterilizers that subject glassware and solutions to high temperature (121C) and pressure — only kill most endospores. And endospores can endure for millions, and perhaps *hundreds* of millions of years under radiation-shielded conditions. Scientists have claimed to have revived bacterial endospores found trapped in salt crystals from the Permian — more than 250 million years ago.

B. thuringiensis endospores have a special feature. Packaged alongside the endospore is a giant crystal — the parasporal body — nearly the size of the endospore itself (see photo here). This is the Cry-protein crystal, and it is a toxin. Here’s a video of the spore and crystal from the University of Azores, along with something the video creator calls an “embedded bodie”. I have no idea what that is.

Technically, the protein is a pro-toxin. It is not lethal until introduced into its host — invariably a larva. In the host’s gut, which, depending on the strain of B. thuringiensis, can include larvae of moths, butterflies, flies, mosquitoes, and beetles, the pro-toxin is cleaved by host enzymes and assumes its lethal form. This protein then binds to the cells lining the larva’s gut, which sets in motion a process that punches holes in the cell membrane. The cell’s vital fluids drain, and then the cell bursts. Result: larva snuffs it.

Why crystals? Why insects? I do not know. Endospore-forming bacteria almost all live in soil; endospores are a good adaptation to soil-living because unlike, say, the ocean, the conditions there are so variable. In the soil, these bacteria focus on eating whatever dead stuff they can find and only infect animals incidentally*. Other species of Bacillus produce antibiotics in an effort to thwart their bacterial competition, so perhaps Bt-toxin is a way to avoid occasional insect annihilation.

________________________________________________

*Including us. Bacillus anthracis endospores cause anthrax.

That’s a guess.

After the discovery of Bt-toxin, it didn’t take long for scientists and agronomists to start imagining what they could do with it. B. thuringiensis endospore preparations have been sprayed directly onto plants as environmentally-friendly herbicides. Here, insects accidentally ingest entire crystal-laden endospores while in the course of normal plant-nomming operations.

Taking things a step further, scientists isolated the gene for the crystal-protein and inserted it directly into the DNA of plants, which then produce only the Bt-toxin. I am not certain if it crystallizes inside their cells, but I doubt that is the case. Although this vastly reduces the amount of toxin needed and eliminates the need for greenhouse gas-burning spraying, this is the point where people’s knickers get into a bunch. I’m staying out of that today.

But one other point about these lethal crystals is worth noting. Apparently, in China and India, cotton plants engineered to produce Bt-toxin have been a little *too* good at their job. As larva-producing insects cannot tolerate the plants, sucking insects like mirid bugs, aphids, and mealybugs have stepped in to take their place.Their young are nymphs that suck juices rather than eat leaves. Bt toxin is useless against them.

Jennifer Frazer About the Author: Jennifer Frazer is a AAAS Science Journalism Award-winning science writer. She has degrees in biology, plant pathology/mycology, and science writing, and has spent many happy hours studying life in situ.
Nature Blog Network
Follow on Twitter @JenniferFrazer.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. ikewinski 4:46 pm 12/20/2012

    I’ll be curious to see if you can avoid the GMO debate in the comments here.

    My understanding is that organic farmers have been using Bt for the last 50 years as an alternative to chemical pesticides. One of the downsides to Bt maize is that not all farmers are planting the required refuge stands of non-GMO maize, and that in time the organic farmers may lose one of their best “natural” pesticides due to evolving resistance (which is the rationale for the refuges).

    Link to this
  2. 2. Charles Hollahan 7:10 pm 12/20/2012

    Bt is so cool that I imagine that only the uneducated would be against it but I’m certain that’s false.

    The Roundup fiasco gives GMO a really bad name, and I grant that the overuse of that herbicide deserves it but GMO has so much potential that being completely and finally against it is a really bad idea.

    What is needed is a less comprehensive ownership over genetically engineered products, something with a definite lief-span like drugs that go the generic route and greater transparency on the owner’s part.

    Perhaps people will realize this before climate change ruins too much arable land for even genetically modified crops.

    Link to this
  3. 3. pabelmont 2:55 pm 12/23/2012

    Mankind seems to be a wildly experimental species, with global warming as the really big unplanned experiment, and various GMO projects (where GMO are widely introduced into nature before long experiments are done) are another.

    Little boys say, “Let’s pull the wings off flies and see what happens.” Big boys have bigger projects. I recall a biologist friend saying, in 1980s, either that GM was OK because nature had been experimenting with mutant genes for a long time and so it was all OK — or maybe he said the opposite (someone was saying the opposite because there are always cautious nay-sayers).

    My guess — they were both guessing. That’s three of us!

    Link to this

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