When the National Science Foundation put out
a call to researchers last year to develop sensor technologies that
could track the origin and movement of explosives used in terrorist
bombings, Paul Millard figured that he and his two close collaborators
at the University of Maine were positioned ideally to address the
problem. Representing three different, yet complementary fields —
biological engineering; electrical and computer engineering; and
biochemistry, microbiology and molecular biology — Millard, Mauricio
Pereira da Cunha and John Singer worked as a team to develop a unique
multidisciplinary project proposal.
For the last few years, engineers Millard and
Pereira da Cunha have conducted basic and applied research in molecular
sensors to detect pathogenic bacteria, as well as viruses currently
threatening fish raised in Maine's aquaculture industry. With NSF
support, they then expanded their research into the human-safety realm,
merging DNA-recognition strategies with sensor technology to detect
bacterial undesirables, such as salmonella, Vibrio cholerae, and
enteropathogenic E. coli O157:H7 in the environment.
To Millard and his colleagues, much of that work
seemed to provide a logical lead-up to the NSF's newest challenge:
Devise a method that law enforcement officials could use while
investigating a terrorist bombing to determine where the explosive
materials were manufactured and the route they traveled to the scene of
Unique identifying markers incorporated into
individual lots of explosives are called taggants. One of the most
common explosives taggants in use today consists of tiny multicolored
plastic chips bonded to a magnetic material, acting as a bar code of
sorts to identify the factory that made the explosive material and on
what date, in what batch, as well as its distributor and the places that
When a material is detonated, the taggants are
released and can be collected from the area with magnets. But the
technology has its drawbacks. Someone conceivably could fabricate
counterfeit taggants, thereby subverting the identification process.
DNA can also be valuable as a taggant, but it, too,
has its limitations. One of the biggest flaws of current DNA taggant
systems is that the equipment necessary to read the genetic code is
fairly expensive, is not portable and requires skilled technicians to
operate. So instead of being able to read the code quickly and easily in
the field, officials have to send the genetic taggants to a laboratory
for analysis that could take days to complete. Besides that, unprotected
exposed DNA is susceptible to degradation caused by environmental
Millard believes he and his colleagues have hit on a method that
will allow them to overcome the limitations of the DNA-recognition
method. Instead of using naked, vulnerable DNA, the team plans instead
to make use of one of nature's most ingenious little survival mechanisms
— bacterial endospores.
NSF has provided nearly $400,000 for the novel
three-year explosive tracking project being developed by the UMaine
research team, which now includes a multidisciplinary cross section of
graduate and undergraduate students.
Bacteria such as bacillus and clostridium are able
to ensure their own survival during periods of environmental stress by
producing endospores, which are dormant, tough, nonreproductive
structures in which the bacterial DNA can be safely stored until
conditions become favorable again for growth.
"The endospore is a cell reduced to its minimum —
more like a seed — that can withstand extreme heat, desiccation,
radiation and other harsh environmental conditions, which makes it an
ideal container for DNA," says Millard. "Before us, no one had proposed
using endospores as nucleic acid taggants."
The clever safeguarding mechanism is a bit like
backing up critical system data from a computer onto a flash drive.
Provided, of course, that the flash drive is then placed inside a brick,
But what is most critical to the making of foolproof
explosives taggants that can't be stolen, replicated or manipulated by
terrorists is the genetic material locked in that endospore. That's
where bioengineering comes into play, and where the team will rely on
Singer's expertise in bacterial genetics. The team will use geobacillus,
a thermophilic bacterium that is widely distributed in soil, hot springs
and sediments. Though it is known to cause spoilage in food, it is
nontoxic to humans and animals.
Millard says the project will involve the generation
of a number of genetically modified spores, each with a unique DNA
sequence or sequences spliced into its genome. In other words, a code so
biologically well disguised that terrorists looking to hide their tracks
after a bombing could never crack it without the key. They wouldn't even
know where to begin to look.
In a real-world application, the DNA-bearing endospores might be
mixed in with a batch of explosive material to uniquely differentiate it
and its manufacturer. Should terrorists get their hands on some of that
batch and detonate it, the taggants — like their plastic-microchip
cousins — would be strewn about the attack site. But unlike nonliving
taggants, each endospore would have the capacity to give rise to an
unlimited number of growing bacteria, greatly enhancing the potential
sensitivity of the method.
As the headlines of 2001 made so chillingly clear,
Bacillus anthracis, the bacterium that causes the anthrax disease, can
be weaponized because its endospores germinate at body temperature into
toxin-producing pathogens. When enough of them are inhaled or ingested,
they can multiply rapidly, eventually sickening or killing their hosts.
Geobacillus endospores, on the other hand, favor
temperatures closer to 200 degrees Fahrenheit to trigger the
reactivation process; 160 degrees F for the nonpathogenic bacteria to
Pereira da Cunha, who recently developed a UMaine-patented
sensor for health monitoring of Air Force jet engines, is now working on
a new surface acoustic wave (SAW) device that will not only detect the
endospores on site, but also create the conditions necessary to end
their hibernation, producing dividing bacteria from which DNA can be
isolated and screened.
The SAW device, which will be developed and
manufactured in UMaine's Laboratory for Surface Science and Technology,
is intended to be an all-purpose tool for law enforcement. Through the
recognition of the coded DNA taggants, it will be able to identify the
source and transit routes of seized contraband explosives. The complete
microsystem will be able to sample the taggants at a bomb site, and then
process them for subsequent DNA analysis.
UMaine's research should give rise to a new class of
devices that will serve as a platform for the processing and detection
of environmental samples, and provide portability for the sensing
microsystem, says Pereira da Cunha.
"That's the beauty of this system," says Millard.
"You don't have to send anything back to a lab. You can collect, process
and analyze on-site with a single, cheap, handheld device."
While helping to stem terrorist activity is the
primary objective of this research, Millard says, the fundamental
advances in bioengineering and sensor science expected to emerge could
also be of significant benefit in first-responders services, healthcare,
food safety and antipollution efforts.
"This technology is being developed for a specific
niche, but the research should also answer some interesting scientific
questions along the way."
by Tom Weber
January - February, 2009
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