This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
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.
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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.
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*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.