Land mines left over from bygone conflicts — or those still being fought — pose silent threats to millions of people around the world. With the help of bacteria that glow in their presence, these hidden hazards may one day be found and safely removed or destroyed.
Researchers at the Hebrew University of Jerusalem have spent a decade developing living land mine sensors using E. coli bacteria. In recent studies, they describe their latest progress. By using genetic engineering, they can turn each bacterium into “a miniature firefly” in the presence of a chemical associated with the explosives, said Shimshon Belkin, the Hebrew University microbiologist leading the research.
In 2019, more than 5,500 people were killed or injured by land mines and explosive remnants of war, and 80 percent of them were civilians, according to the International Campaign to Ban Landmines. Anti-personnel land mines, which can be only a few inches across and easily concealed, are especially dangerous. Estimates vary for the worldwide count of buried land mines, but they are as high as 110 million.
Many strategies have been tried to locate land mines, such as using metal detectors and training detection animals, including an award-winning rat that helped locate 71 land mines before it retired. Each method balances benefits with risks and costs.
The idea of rewiring bacteria to sense land mines originated with Robert Burlage, then at Oak Ridge National Laboratory in Tennessee. In the mid-1990s, Dr. Burlage worked on getting bacteria to light up in response to organic waste and mercury. Looking for a new application for this technique, he got the idea to try targeting land mine chemicals.
Although Dr. Burlage conducted a few small field tests, he was unable to secure more funding and moved on. “My tale of woe,” said Dr. Burlage, now a professor at Concordia University Wisconsin.
Dr. Burlage’s work was an inspiration for the Israeli researchers, and he says he wishes them well in their efforts to advance the technology.
Bacteria are cheap and expendable and can be spread over a lot of ground. And they are relatively quick at reporting back — within hours, or up to a day, they either glow or they don’t.
In studies published in the past year in Current Research in Biotechnology and Microbial Biotechnology, Dr. Belkin and his team describe tinkering with two key components of the E. coli genetic code: pieces of DNA called “promoters” that act as on/off switches for genes, and “reporters” that prompt light-emitting reactions. To produce this effect, researchers borrowed genes from marine bacteria that naturally emit light in the ocean.
Scientists attuned the bacteria to a chemical called 2,4-dinitrotoluene, or DNT, a volatile byproduct of trinitrotoluene, or TNT. Over time, DNT vapor seeps into soil surrounding a land mine, and the bacteria can sniff it out.
Rather than roaming freely, the bacteria are immobilized in tiny gelatinlike beads that feed them while they work. Each bead, about one to three millimeters across, contains about 150,000 active cells.
These latest crops of genetically engineered bacteria are faster to react and more sensitive than bacteria in the group’s earlier field tests, Dr. Belkin said. And the scientists no longer need to use a laser signal to activate the glow.
One key challenge the group is working to overcome is safely locating the bioluminescent bacteria in a real minefield. When they detect land mines, their glow is so faint that light from the moon, stars or nearby cities could drown it out.
To help address this problem, Aharon J. Agranat, a bioengineer at Hebrew University, and other researchers reported in April in the journal Biosensors and Bioelectronics that they had developed a device that shields the bacteria and detects their glow. This sensor system can then report its findings to a nearby computer, but it hasn’t been tested outside a laboratory setting.
The researchers have also recently conducted field tests in Israel, collaborating with the Israeli army to ensure the safety of the experiments, as well as an Israeli defense company. The results of these tests have not been published, but Dr. Belkin called them “generally very successful.”
In the future, the team hopes to use drones to deploy bacteria sensors in a minefield, eliminating the need for humans to get close.
Dr. Burlage came across another issue decades ago that the Hebrew University group grapples with even now: temperature. The Israeli bacteria sensors work only from about 59 to 99 degrees Fahrenheit, meaning researchers will need to figure out how to adapt their systems to more scorching desert conditions.
The Israeli bioengineers also acknowledge that their bacteria sensors could be used for both humanitarian and military purposes. DARPA, the Defense Advanced Research Projects Agency, contributed funding to their research.
Nonetheless, the bacteria sensors for land mines exemplify how the field of synthetic biology has grown “leaps and bounds in the past few decades,” said Dr. Timothy K. Lu, co-founder of Senti Biosciences and biological engineer at the Massachusetts Institute of Technology, who was not involved in these studies.
“It’s super exciting, and I hope to see these sort of applications start migrating out of the lab and into the real world,” Dr. Lu said.
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