September 2010


Detecting Explosives with Nematodes

This post was chosen as an Editor's Selection for Nematodes, i.e. the humble roundworm, are the most common multicellular animals on the planet. You can find them in just about every ecosystem.

Many people think of nematodes as dangerous or pesky parasites (e.g. the cause of heartworm infections in dogs). However, some nematode species are very useful to scientists.

An example is Caenorhabditis elegans. They are one of the simplest animals that nevertheless possess a nervous system, they can "come back to life" after they've been frozen, and it's straightforward to disrupt a specific gene in their DNA (enabling one to determine the function of the gene).

Stephen Trowell (Commonwealth Scientific and Industrial Research Organization Entomology and Food Futures Flagship, Australia) and coworkers have reported a new use for these rugged critters. They have worked towards using nematodes to detect explosives.

Why use nematodes?

Dogs may be one of the best explosive sniffers around. In some cases, they can detect explosives at a concentration of 100 parts per quadrillion.

This is equivalent to 0.5% of one drop of water in a full Olympic-sized swimming pool. There's no arguing that dogs are cheap for this line of work.

Even so, dogs can't tell you what kind of explosives have been detected. For this, chemical and instrumental detection is needed, e.g. molecular sponges for detecting TNT (more specifically, a legally-mandated contaminant in TNT).

However, the cost of developing a fundamentally new sensor for various explosives, and purchasing the necessary instrumentation, is formidable. Nematodes offer the possibility of detecting a diverse set of chemicals based on chemotaxis, e.g. movement to or from a chemical signal.

In fact, nematodes can detect over one hundred volatile carbon-based molecules (e.g. aldehydes and alcohols), and possess over one thousand potential chemical sensors that remain uncharacterized. To date, nematodes are not known to detect volatile chemicals associated with explosives; this is what Trowell and coworkers set out to investigate.

Nematode attraction to cyclohexanone.

The scientists screened Caenorhabditis elegans' chemotaxis response to 17 chemicals which are associated with explosives (trials on two different days, and four to eight repeats). This includes explosives themselves (e.g. RDX and nitroglycerine) and their associated solvents, contaminants, and degradation products (e.g. ethyl hexanol and hexamine).

Screened at 1 microliter drops of a 0.1% dilution, the scientists found statistically significant positive responses to several chemicals (e.g. potassium nitrate). However, for a number of reasons (e.g. variability in the data) that rendered their data inconclusive, they decided to focus their efforts on cyclohexanone; this chemical is present in some explosives.

Detection (e.g. nematode movement towards the source of the chemical attractant) was at least 3 parts per million in air, but is likely lower (better) than this due to experimental limitations. Largely similar results were observed for benzaldehyde and diacetyl.

What is the biological basis of cyclohexanone detection? Through genetic mutations, the scientists found that a pair of neurons known as the AWC neuron pair (important in nematode olfaction) is largely responsible for nematodes' positive chemotaxis response to cyclohexanone.

Not every similar chemical induces a similar chemotaxis response. The scientists found that nematodes adapted to cyclohexanone partially lost their response to only benzaldehyde (64% loss) and cyclohexanol (33% loss), two of seven chemicals tested which are chemically similar to cyclohexanone.

Overall evaluation.

With further development, does this research does show promise for practical use? My opinion is "maybe."

What does this development have in its favor? I can envision scientists genetically engineering a nematode that responds ideally to cyclohexanone (a contaminant in explosives), and developing instrumentation that both reads out the chemotaxis response and reports it in a way that tells the user whether cyclohexanone is present.

Furthermore, as previously noted, nematodes are rugged and require minimal maintenance. However, a limitation is that the nematodes seem to need an hour to complete a measurable chemotaxis response (a long time).

This limitation certainly needs to be fixed if nematodes are to be used to detect explosives in real time. Furthermore, scientists need to figure out if chemicals mixed in with cyclohexanone affects the results (cyclohexanone will be present in a mixture, not in a pure form); if it does, this development is unlikely to get off the ground.

This is still interesting research, and should be further pursued as an early-stage compliment to explosives sensors currently in use.

NOTE: The scientists' research was funded by the National Security Science and Technology Unit and Emergency Management Australia and by the Commonwealth Scientific and Industrial Research Organization. for more information:
Liao, C., Gock, A., Michie, M., Morton, B., Anderson, A., & Trowell, S. (2010). Behavioural and Genetic Evidence for C. elegans' Ability to Detect Volatile Chemicals Associated with Explosives PLoS ONE, 5 (9) DOI: 10.1371/journal.pone.0012615