New detector promises earlier detection of viral infections
8 July 2009
A chemist and a biomedical engineer at Vanderbilt University in the
US have teamed up to develop a respiratory virus detector that is
sensitive enough to detect an infection at an early stage, takes only a
few minutes to return a result and is simple enough to be performed in a
Writing in The Analyst, a journal published by the Royal
Society of Chemistry, the developers report that their technique, which
uses DNA hairpins attached to gold filaments, can detect the presence of
respiratory syncytial virus (RSV) — a leading cause of respiratory
infections in infants and young children — at substantially lower levels
than the standard laboratory assay.
“We hope that our research will help us break out of the catch-22
that is holding back major advances in the treatment of respiratory
viruses,” says Associate Professor of Chemistry David Wright, who is
working with Professor of Biomedical Engineering Frederick “Rick”
Haselton on the new detection method.
According to the chemist, major pharmaceutical companies are not
investing in the development of antiviral drugs for RSV and the other
major respiratory viruses because there is no way to detect the
infections early enough for the drugs to work effectively without
harmful side-effects. “There are antiviral compounds out there — we have
discovered some of them in my lab — that would work if we can detect the
virus early enough, before there is too much virus in the system,” he
In addition, the lack of a reliable early detection system adds to
the growing problem of antibiotic resistance. The symptoms of
respiratory infections caused by viral agents are nearly identical to
those caused by bacteria. As a result, antibiotics, which target
bacteria, are often incorrectly prescribed for viral infections. Not
only is this ineffective, but it also increases the number of
Currently, there are several standard tests for RSV including
culturing the virus, polymerase chain reaction (PCR) and the
enzyme-linked immunosorbent assay (ELISA). To have any of these tests
done, doctors must send a mucous sample from a patient to a special
When combined with delivery times, backlogs and other delays, it
frequently takes a day or more to get the results. Unfortunately,
respiratory viruses multiply so rapidly that this can be too late for
antiviral drugs to work, Wright says.
By contrast, “our system could easily be packaged in a disposable
device about the size of a ballpoint pen,” says Haselton. To perform a
test, all that would be required is to pull off a cap that will expose a
length of gold wire, dip the wire in the sample, pull the wire through
the device and put the exposed wire into a fluorescence scanner. If it
lights up, then the virus is present.
The new detector design is a combination of two existing
technologies. One is the filament-based antibody recognition assay
(FARA) developed several years ago by Haselton and patented by
FARA uses antibodies — special proteins produced by the immune system
that binds to specific foreign substances — that are coated on the
surface of a polyester filament. When the coated filament is exposed to
a sample, if it contains any of the target molecules, they stick to the
antibodies, forming complexes that can be detected with fluorescent
One advantage of this approach is that a sample can be put through
different processing steps simply by pulling the filament through a
series of small chambers. In the RSV detection application, the chambers
contain washing solutions that remove non-specific binding molecules.
“Originally we thought that we would have to put special seals
between the chambers but we found that if we make the openings small
enough, then the solutions in the chambers stay in place as we pull the
wire through,” says Haselton.
The second technology is based on molecular beacon probes, an
approach often used in PCR. The probes consist of short lengths of
single-strand DNA that normally form a hairpin shape but straighten out
when they are bound to a target molecule.
A fluorescent dye molecule is attached to one leg of the hairpin and
a molecule that quenches its fluorescence is attached to the other. When
the probe is in its hairpin configuration, the dye and quencher
molecules lay side by side so the probe does not fluoresce. When it is
bound to a target, such as a piece of viral RNA, the ends spring apart,
turning on the probe’s fluorescence.
The Vanderbilt researchers realized that if they attached molecular
beacons to a gold-coated filament, the gold could theoretically replace
the quencher molecule and inhibit the beacon’s fluorescence. However,
they had to find a linking molecule — the molecule that attaches the
beacon to the wire — that was just the right length to make it work.
Once they solved this problem, the researchers tested the sensitivity
of the new system. They found that it could detect the presence of RSV
virus particles at levels that are 200 times below the minimum detection
level of the standard ELISA method. This extreme sensitivity combined
with the basic simplicity of the approach makes it “attractive for
further development as a viral detection platform,” the scientists write
in the Analyst article, which was published online May 15 2009.
According to Haselton, there are two areas where further development
is needed. One is sample preparation. Commercial RNA sample preparation
kits are available, but they are more expensive and complex than
desirable. The team is currently examining the design of a simple
pull-through RNA isolation chamber.
The team is also exploring ways to reduce false detections. There are
a lot of other molecules in mucous besides viral RNA that can bind to
some extent with the molecular beacons. However, the researchers argue
that it should be possible to reduce the number of false positives
significantly by adding a heating step that is calibrated to drive off
the molecules that are less strongly bound to the beacons than the viral
The next major step in the development process is to see how the
device performs with real patient samples.
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