A drop dispenser for simplifying on-farm detection of foodborne pathogens

Rapid nucleic-acid biosensors are useful for on-farm detection of foodborne pathogens on fresh produce during pre-season and pre-harvest stages. Such tools aim to be user-friendly so that a producer could operate them in a few simple steps and detect multiple targets. Currently, an easy-to-use device for on-farm applications does not exist commercially. One of the bottlenecks is the delivery of a prescribed amount of sample to the reaction sites of the biosensor using a simple and precise approach. Here, we developed drop dispensers using 3D printing and a hydrophilic surface chemistry treatment to generate precise drops (DNA/bacterial samples) of a few micro-liters ([~]20 to [~]33 {micro}L). We tested multiple copies of these devices over time of repeated application to estimate their shelf-lives which is about one month. In addition to drop generation tests, we employed these devices in nucleic-acid testing. The tests used loop-mediated isothermal amplification (LAMP) to detect DNA or whole cells of Shiga-toxin-producing Escherichia coli O157:H7. These tests were performed to simulate the on-farm sample collection (using collection flags that we previously designed) and delivery using the drop dispensers. Our results showed that these devices performed similarly to standard commercial pipettors in LAMP assays, providing a limit of detection of 7.8x106 cell/mL for whole-cell detection. This drop dispenser will eventually be part of a user-friendly consumable kit that will enable performing LAMP assays by non-specialist users for a cost of USD 4 per test.