TNT-Detecting Bacteria Could Illuminate Landmines

A hidden and indiscriminate threat, landmines injure and kill soldiers, civilians, and even inhabitants of now-peaceful regions every day. It’s impossible to know how many landmines are buried worldwide, but most estimates place the number somewhere between 100 million and 200 million devices. Once planted, landmines remain a threat until they are detected and detonated, a process that can take decades or longer if it is not a high priority in the region. Even when it is a priority, detecting these mines is slow and risky work.

A pile of decommissioned land mines at a war museum in Cambodia
Image Credit: Neil Ricka. (CC BY 2.0)

Most landmine searches are done one step at a time by people with handheld probes. Often these are metal detectors, but modern mines are made of plastic and require other detection methods. Detection by handheld probes leads to a high number of false positives (i.e. detecting scrap metal rather than a landmine) and false negatives, and comes with a high risk of personal injury or even death for those willing to take on the job. Landmine-detecting dogs can search an area faster, but the cost is high and human follow-up is usually still required. In an effort to design a new system that can safely detect landmines remotely, a team of researchers from Hebrew University of Jerusalem led by Shimshon Belkin and Aharon Agranat is employing not people or dogs, but bacteria and lasers.

Scientists are developing a method to remotely detect buried landmines using a bacterial sensor and a laser-based scanning system.
Image Credit: Hebrew University.

Published last week in the journal Nature Biotechnology, this new method is based on the fact that all landmines leak small amounts of explosive vapors into the soil around them. Just as some dogs can be trained to sniff out explosives, some bacteria can be genetically engineered to detect trace amounts of chemicals, including TNT. While a dog might bark or exhibit other specific behavior when it smells a target, the Hebrew University researchers engineered bacteria so that when they detect TNT (or the related chemical DNT), they glow a fluorescent green color under a laser.

The driving idea is this: Imagine spreading bacteria over the ground in an area that may contain landmines. A laser system, perhaps attached to a drone, vehicle, or low-flying plane, then scans the area and records the fluorescent signals produced by the bacteria. This information maps the locations of landmines and unexploded weapons (such as grenades), which can then be fenced off or detonated. Done remotely, the detection could be done quickly and without risking human lives.

The technique is not ready for deployment yet, but results are in from a small-scale field test. For the test, the researchers encapsulated the bacteria into small beads. This keeps the bacteria viable for several hours while also letting TNT molecules diffuse in. The beads were distributed across an area approximately 4m x 1m that contained 18 targets: DNT flakes, TNT flakes, and anti-personnel landmines (with the triggering mechanisms removed), as well as some dummy mines to act as controls. Each target was buried under either garden soil or one of two types of sand.

To search for signs of landmines, the team used a scanning laser system they built previously. The system has a blue laser that illuminates the ground as well as a detector that measures fluorescent light returned from the illuminated area. If the laser light is absorbed by bacteria that have detected TNT, they fluoresce green. If no TNT is detected, the bacteria don’t emit any light.

After scanning the area from 20m away, the system identified and mapped all of the targets buried in sand. However, it was not able to detect the targets buried in the soil. The researchers suggest that this could be because the TNT wasn’t buried in the soil for long enough (it was buried for five days) or because the vapor may have adhered to the soil particles.

This technique shows promise, say the researchers, but more follow-up is needed to explore how the system works with different types of landmines, weather conditions, and burial times, depths, and soils. If things go well, the focus will turn to making the scanning system more portable, increasing the stability and sensitivity of the bacteria, and determining how best to remove or inactivate the bacteria after the scanning is complete.

Scientists are working on other possible detection methods as well, such as ground-penetrating radar systems and detectors that mimic a dog’s nose. The wide range of terrain and weather in the more than 70 countries with landmines suggests that the best options are hardy, inexpensive, and remotely operated. Detecting millions of landmines is a huge project to tackle, but the status quo is a dangerous, heartbreaking, and costly situation. It’s also one that won’t go away unless we do something about it.

Kendra Redmond

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