Borrowing concepts from medical diagnostic devices, researchers have created a simple, inexpensive set of handheld tests that can detect the presence of many water- or food-borne pathogens. If applied in the field, such tests could greatly reduce the number of expensive follow-up tests needed to keep the food supply safe from fecal contamination.
Researchers targeted a broad class of bacteria known as fecal indicator bacteria (FIB), which cause the highest number of hospitalizations and deaths from food poisoning. It commonly enters the food supply through contaminated water used to irrigate green vegetables like alfalfa sprouts, spinach, and lettuce. While federal regulations require frequent testing of fruits and vegetables for contamination with fecal matter, existing processes are still lacking. Common techniques like immunoassays and poly-merase chain reactions (PCR) work reasonably well, but they require expensive equipment to perform, and can lead to false positives. The gold standard for bacterial detection is a lab culture, but this can take up to 48 hours to complete.
In this work, two types of tests were made that detect an enzyme associated with the FIB bacteria. The first is a small strip of paper treated with a substrate molecule that changes color when it contacts the bacterial enzyme, similar to the way a home pregnancy test works. The researchers envision that a smartphone app could be coupled with the paper test, which uses about 2 cents worth of material. The second test consists of screen-printed carbon electrodes on transparent sheets that are inserted into a reader, similar to the way a home glucometer works.
Tests of contaminated water from an open-air lagoon were conducted, as well as water contaminated with E. coli and Enterococcus faecalis that was used to wash clean alfalfa sprouts. Both tests detected the harmful bacteria within four to 12 hours.
The new tests can't tell exactly which bacteria are present, but they can detect the broad class of FIB bacteria that is usually responsible for foodborne illness outbreaks. In contrast, PCR tests for bacteria currently in use are more specific, but are slower and more expensive. A simple pre-test like this one could save money and time by cutting back on the overall number of food safety tests needed.
As a next step, the team wants to build a mobile computing platform for their tests. They are working on a Raspberry Pi-based system that could perform kinetic measurements to detect changes in the bacteria levels over time, and automatically transmit the information to a cloud platform.
The project has included the perspective and expertise of microbiologists, who would be most likely to use the devices in the field.