Paper-based biosensor offers fast, easy detection of fecal contamination on produce farms

Purdue University researchers are introducing a new biosensor technology to the agricultural industry inspired by advancements achieved during the COVID-19 pandemic.

The system delivered 100% accurate results within an hour of in-field sample collection on a commercial fresh produce farm.

"The approach we've taken is using a fecal indicator called Bacteroidales as a risk marker," said Purdue's Mohit Verma, associate professor of agricultural and biological engineering. Verma and his team of nine postdoctoral researchers, graduate and undergraduate students published their findings in the journal Biosensors and Bioelectronics. Krishi, a startup company where Verma serves as chief technology officer, is licensing the technology through the Purdue Innovates Office of Technology Commercialization.

"The goal has been to estimate what risks might be present in fresh produce operations from nearby animal operations or other wildlife," Verma said. Typically, this is done by measuring pathogens. If pathogens are present on the crop, it is discarded. But detecting pathogens at a low level, in compliance with regulatory requirements governing products with a short shelf life, presents challenges.

The technology Verma uses is called loop-mediated isothermal amplification (LAMP). The team has implemented it on innovative, paper-based devices for rapid results for agricultural uses. Previously, Verma's team developed these tests for bovine respiratory disease and COVID-19.

"To our knowledge, this work represents the first demonstration of a portable LAMP testing platform implemented on a fresh produce farm," the co-authors wrote in their journal article.

The new paper also refines previous results published in the journal Food Microbiology by lead author Jiangshan Wang, who earned his doctorate at Purdue in agricultural and biological engineering this year, Verma and their associates.

Bacteroidales is a fecal organism found in swine, poultry and cattle. Most foodborne pathogens linked to fresh produce, including E. coli and salmonella, originate in the intestines.

The Purdue team tested the system on a commercial lettuce farm in Salinas, California, and in a field next to the Purdue Animal Sciences Research and Education Center in West Lafayette, Indiana. Microbial samples were collected from plastic flags that were placed throughout the testing area.

Small flags made of plastic sheets collected bioaerosol samples for a week in Verma's experiments. The experiments compared testing in the field and in the laboratory. All of the flags were brought to the laboratory for quantitative polymerase chain reaction testing. "That allows you to measure Bacteriodales and therefore the level of fecal contamination," Verma said.

But with the new assay kit, which consists of a drop dispenser, paper-based devices and a heating imager, growers can carry out the entire process in the field. After swabbing the collection flag, they put in it a drop dispenser that is preloaded with liquid to dispense a specific volume.

The liquid dispenses into the paper devices that contain the compounds needed to detect DNA in the samples. The paper device then goes into a heating imager. An hour later, the device reveals how much Bacteriodales is present. The assay provides quantitative test results that confirm what growers, following visual inspection, often intuitively suspect is happening in their fields.