A handheld sensor that can quickly spot contamination by deadly strains of the Escherichia coli bug could help prevent infected food reaching consumers.
In the US alone, 60 people a year die from E. coli, while 73,000 are infected with pathogenic strains. But detecting the bug is a slow process that involves removing whole batches of foodstuffs from production lines while cultures are grown or DNA amplified. Tests can take 24 hours or more, says Peter Wareing, a food safety expert with Leatherhead Food International in Surrey, UK.
Now Raj Mutharasan, an engineer at Drexel University in Philadelphia, Pennsylvania, has developed a cheap, quick and simple detector that just about anyone can use. “It will be as easy to use as a thermometer, giving a result in 10 minutes,” he says.
The device works by detecting how the mass of a few E. coli cells changes the vibration of a miniature glass beam. The prototype sensor comprises a sliver of glass 5 millimetres long and 1 millimetre wide, which is fixed at one end and has a layer of piezoelectric ceramic called lead zirconate titanate (PZT) glued to the other. The glass sliver is then coated with antibodies to E. coli 0157:H7, the strain that causes the illness.
An alternating voltage applied to the piezoelectric layer makes it expand and contract, causing the tiny sliver to vibrate. The vibration is greatest at the sliver’s resonant frequency, and this can be detected by measuring the voltage across the PZT generated by the reverse piezoelectric effect, as it peaks at the resonant frequency. Changes in this resonant frequency as E. coli cells bind to the antibodies give a measure of the concentration of the pathogen.
To make sure only E. coli cells sit on the sliver, the whole operation takes place in moving fluid. A fraction of a millilitre of beef broth, for example, is sloshed back and forth over the sensor during the 10-minute measurement. “The sensitivity is already very high: we can detect E. coli at a concentration of 4 cells per millilitre of sample,” Mutharasan says.
The sensitivity of the sensor, which can also detect bugs such as listeria, has attracted the interest of many US government departments. The Department of Agriculture is developing the sensor further with the Drexel team, with the National Institutes of Health and the Environmental Protection Agency part-funding the work. “And the Department of Homeland Security is interested in its ability to detect other pathogens like anthrax,” Mutharasan says.