A Spinning Disc Spots Spoiled Food

To speed up and simplify the identification of foods tainted with Salmonella bacteria, researchers have built a microfluidic disc that can detect the pathogens’ DNA in product samples. As the device spins, it performs DNA extraction, amplification, and detection in less than 30 minutes (Anal. Chem. 2014, DOI: 10.1021/ac403971h).

Salmonella causes an estimated 1.2 million illnesses and 450 deaths in the U.S. each year. The standard test for foodborne pathogens is to grow cultures and check for dangerous strains of microbes. This can take days. “If we had a quick and easy test it could save a lot of money,” says Yoon-Kyoung Cho of Ulsan National Institute of Science & Technology, in South Korea. Other methods, such as DNA amplification with polymerase chain reaction are faster, but may require expensive equipment and trained personnel to run the tests.

To create a fast way to check for Salmonella, Cho developed a so-called lab-on-a-disc, which is similar to a microfluidic chip, except it’s shaped like a music CD. Her team previously built one for detecting viral DNA in blood (Lab Chip 2011, DOI: 10.1039/C0LC00205D).

Cho’s Salmonella-detecting disc is made up of six identical slices, each capable of performing the same test. The slices contain a network of channels and chambers that carry out individual steps in the pathogen detection process. After a sample is added to the center of the device, the rotation of the disc forces the solution outward into the channels of each slice.

To test the device, researchers spiked milk and buffer with known amounts of Salmonella enteritidis colonies. Before loading samples of either liquid onto the disc, the researchers concentrated the bacteria by capturing them with magnetic beads coated in anti-Salmonella antibodies. They loaded the beads suspended in water onto the disc and inserted the disc into a machine with a computer-controlled rotor and laser.

In the first on-disc step, the laser shines on the chamber containing the beads, lysing the cells and releasing their DNA. The disc then rotates so that the laser light hits a wax valve, which dissolves due to the heat, allowing the sample to flow into the next chamber. There, triggered by the heat of the laser, the DNA is amplified via a reaction called recombinase polymerase amplification using primers specific to a known Salmonella gene. The primers are decorated with two molecules that allow for detection of the DNA using reagents from a kit made by TwistDx.

With the obliteration of a few more wax valves, the DNA enters a channel containing these reagents and a sensor strip. As the DNA migrates up the strip, it hits a detection area, where the reagents mixed with the DNA cause a visible band of color to form. The researchers found they could detect as few as 100 colony-forming units of Salmonella in milk and 10 in buffer. This isn’t perfect, as even one cell can cause disease.

But the detection limit is pretty good, says Steven A. Soper of the University of North Carolina, Chapel Hill. He’s impressed by the range of operations performed on the disc but says the device would be most useful if the concentration step were done on the disc too. Cho says the magnetic separation is fast and easy, though she agrees that integrating this step onto the disc would be an improvement.