Currently diagnosing common bacterial infections requires growing cultures in a laboratory over a period of days, but diagnosis could be greatly speeded by a number of new sensors based on various nanomaterials that are being developed for ultrasensitive, rapid DNA detection. The new instrument would take from 15 minutes to two hours for a diagnosis and could be used in doctor's offices, hospitals, and homes.
Glow of hairpin-shaped DNA nanosensors.
Each sensor is a hairpin-shaped strand of DNA, complementary to the genetic sequence being targeted, that is fixed on a gold film. Gold quenches the glow of a fluorescent molecule attached to one end of the DNA. The DNA stays folded over until a target genetic sequence links to it. Its unfolding results in the fluorescent molecule moving away from the gold film and glowing, which can be seen under a fluorescent microscope.
A blood or urine sample to be tested would be placed directly on the device. The device will be a lab-on-a-chip, with rapid, miniaturized ways to prepare the sample for testing. "In the device there are steps for cleaning up samples, that is, extracting material you're interested in and amplifying the [bacterial] DNA," Miller says. The device will then be placed in a small portable instrument that does the fluorescence imaging and analysis. Each device should cost a few dollars, Miller says.
By attaching different DNA strands on the gold film, the same device could screen for multiple pathogens, Miller says. So far, the researchers have made a sensor to detect antibiotic-resistant staph bacteria that cause skin infections. They are now working on detecting bacteria responsible for common urinary-tract infections. The sensors could also be used to quickly spot bacteria in food or bioterror agents in water supplies, or even to screen for genetic disorders or cancer.
In a newer version of the sensor, Miller and colleagues stick DNA strands on silver nanoparticles. The silver nanoparticles make the fluorescent signal 10 times brighter. Plus, because thin layers of silver nanoparticles are transparent, the sensor could be coated on glass and optical fibers to make new types of detecting instruments, Miller says.