7 February 2012
Bioanalytics: Device detects intact, live viruses within minutes
A DNA-based biosensor is the first to distinguish live virus particles from dead, disintegrated ones, its developers say (Anal. Chem., DOI:10.1021/ac203412m). The new test could find use detecting, in just minutes, viable bioweapons or viruses that kill cancer cells, say the researchers.
The vaccinia virus, which is commonly used to immunize people againstsmall pox, also can selectively kill cancer cells. Typically, when researchers grow a batch of the viruses, they test it to see how many viable virus it contains. The test often involves adding a sample of the viruses to a cell culture and watching over several days how many cells die.
Now Maxim Berezovski of the University of Ottawa, in Ontario, and his colleagues have built a DNA-based biosensor to determine virus viability in about 15 minutes. The sensor uses DNA strands called aptamers, which can bind specific proteins on the surface of the virus.
To find these DNA sequences, the team set up an iterative screen. They started with a pool of 60 trillion DNA sequences and mixed them with live vaccinia virus particles. After incubating the mixture, they centrifuged it to form a pellet of live virus particles. Then the chemists amplified the bound DNA in the pellet using polymerase chain reaction and added the result back to live viruses. The team ran the selection process 12 times, each time narrowing in on sequences that had high affinity for live viruses.
In the middle of the process, after eight rounds, to remove any sequences that also had high affinity for dead viruses, the scientists ran a few selection rounds in which they spiked the mixture with heat-treated virus particles. These virus fragments couldn’t form a pellet with the larger live viruses. As a result, any DNA sequences that bound the dead viruses didn’t get amplified.
In the end, the researchers found 15 winning sequences. Berezovski says the same selection process could find aptamers that bind other targets such as spores produced by the bacteria that cause anthrax.
To build the sensor, the team attached the seven strongest binding aptamer sequences to gold electrodes on a chip. When the chip’s aptamers captured virus particles, the resistance of the electrode would drop in proportion to the number of bound particles. So by measuring the resistance of a circuit containing the electrodes, the researchers could detect and quantify live viruses. The researchers tested their sensor with a mixture of live and dead viruses and found they could detect as little as 60 live virus particles in 1 µL of solution.
Beate Strehlitz, of the Helmholtz Center for Environmental Research, in Germany, says the aptamers are the first that can distinguish between live and dead viruses. The sensor’s portability and speed make it ideal for use in the field, especially for spotting bioweapons, she adds.
But biosensor expert Anthony Turner, of Linköping University, in Sweden, says he’s left wondering how the sensor works: Since live and dead viruses could have similar protein coats, the big question is how the aptamers distinguish between them, he says.