On a tiny square: 25 tests, 5 positive results (Image: Harvard's Wyss Institute)

Discs of souped-up filter paper could change how we diagnose infections. To demonstrate the power of the approach, which involves embedding DNA from jellyfish and other organisms into paper, its developers have successfully used it to identify two strains of Ebola.

Although far from being ready for testing in west Africa, paper discs that detect the virus are being developed for potential use as a cheap, simple method to identify infected people.

“It’s a platform for a new class of diagnostics, and a very clear and important practical extension of synthetic biology, opening up a whole generation of new technologies for diagnosis,” says Jim Collins of the Wyss Institute for Biologically Inspired Engineering at Harvard University in Boston, Massachusetts.


The key to the technology is the ability to print sequences of DNA on paper, then freeze-dry and store the discs at room temperature. The DNA is reactivated by adding water. Once active, it enables the paper to change colour if a chosen target – such as a segment of Ebola viral RNA – is present in the water.

Test the rainbow

The target fragment binds to a gene switch in the DNA, which triggers the production of a colourful substance such as the protein that gives jellyfish a green glow under ultraviolet light, or proteins from bacteria that produce colour changes visible to the naked eye. The colour the paper changes to indicates which of the target pathogens has been detected.

“We’re extending the concept of litmus paper to biochemical reactions, putting the power of molecular biology onto paper,” says Collins. “It’s actually very easy.”

Keith Pardee, a colleague of Collins, improved the test by inserting tailor-made gene switches that prevent any colour change happening unless a very specific target molecule is present. Called toeholds, the synthetic switches enabled the paper to simultaneously test for 24 distinct regions of viral RNA – many more than would have been possible using only naturally occurring DNA sequences. This provided a means to distinguish between a synthetic version of the Zaire strain of Ebola – the one responsible for the west African epidemic – and a synthetic version of another known as the Sudan strain.

The test identified the strains within 30 minutes, rivalling the speed of more expensive and complex tests that use antibodies. Collins estimates it cost just $21 to develop the litmus sensor – the cost of buying the sequences of DNA that detect the viral RNA. The whole thing took just 12 hours to assemble.

Printing molecules

By making the DNA circuits “in house” instead, the cost could come down to just 2 to 4 cents per sensor. “It’s the order of pennies,” says Collins. “The advantages we’re bringing include low cost, no refrigeration needed and fast output,” he says.

More work is needed to refine the tests so that they can detect tinier amounts of target molecule, and ensure the rates of false positives and negatives meet the standards required for diagnostic tests.

“The beauty of it is that it’s going back to a biological version of litmus,” says Paul Freemont of Imperial College London. He is head of a team currently developing a similar type of test that identifies Pseudomonas aeruginosa, a bacterium that often causes infections in people with cystic fibrosis. “The work on Ebola is really cool, and will open up the opportunities for developing these biosensors,” he says.

“Potentially, it’s wonderful, but it’s one thing to do it in the lab and quite another to manufacture it up to the standards required for it to work in real situations,” says Ruth McNerney of the London School of Hygiene and Tropical Medicine. She points out that the test would have to have a very low rate of false positives to avoid putting healthy people at risk of picking up diseases from other, genuinely infected people.

As well as identifying the Ebola strains, Collins and his colleagues have made other tests that detect genes from antibiotic-resistant bacteria. And because the molecular circuitry can also be printed into fabrics, there is potential to add these to bandages and clothing worn by medical staff, which would change colour on contact with superbugs.

Journal reference: Cell, DOI: 10.1016/j.cell.2014.10.004 and Cell, DOI: 10.1016/j.cell.2014.10.002