The foodborne pathogen Escherichia coli O157 causes an estimated 73,000 illnesses and 60 deaths every year in the United States. Better safety tests could help avoid some of the illnesses caused by this strain of E. coli and other harmful bacteria.
A new test was developed that is based on a type of liquid droplet that can bind to bacterial proteins. This interaction, which can be detected by either the naked eye or a smartphone, could offer a much faster and cheaper alternative to existing food safety tests.
Two years ago, MIT researchers developed a way to easily make complex droplets including droplets called Janus emulsions that consist of two equally sized hemispheres: one made of a fluorocarbon and one made of a hydrocarbon. Fluorocarbon is denser than hydrocarbon, so when the droplets sit on a surface, the fluorocarbon half is always at the bottom.
The researchers decided to explore using these droplets as sensors because of their unique optical properties. In their natural state, the Janus droplets are transparent when viewed from above, but they appear opaque if viewed from the side because of the way light bends as it travels through the droplets. To turn the droplets into sensors, a surfactant molecule was designed containing mannose sugar to self-assemble at the hydrocarbon-water interface, which makes up the top half of the droplet surface. These molecules can bind to a protein called lectin that is found on the surface of some strains of E. coli. When E. coli is present, the droplets attach to the proteins and become clumped together. This knocks the particles off balance, so light hitting them scatters in many directions, and the droplets become opaque when viewed from above.
To demonstrate how the droplets could be used for sensing, they were placed into a Petri dish atop a QR code that can be scanned with a smartphone. When E. coli are present, the droplets clump together and the QR code can’t be read.
Current food safety testing often involves placing food samples in a culture dish to see if harmful bacterial colonies form, but that process takes two to three days. More rapid techniques based on bacterial DNA amplification or antibody-bacteria interactions are expensive and require special instruments. The MIT team hopes to adapt its new technology into arrays of small wells, each containing droplets customized to detect a different pathogen, and linked to a different QR code. This could enable rapid, inexpensive detection of contamination using only a smartphone, allowing factory personnel to scan and test food to make sure it’s safe before shipping.