Wax-printed well pads and colorimetric LAMP detection of ApxIA toxin gene

Actinobacillus pleuropneumoniae (A. pleuropneumoniae) produces hemolytic cytotoxins, ApxI, ApxII, ApxIII, and ApxIV, causing a huge loss to the swine industry. We developed the PadLysis protocol for extracting genomic DNA (gDNA) for the LAMP—LeucoCrystal Violet (LCV) colorimetric detection. A paper-based well pad for gDNA extraction was fabricated using a wax printing method. Furthermore, the LAMP amplicons were detected in situ with colorimetric dye, LCV. Wax-printed well pads (WPWPs) have hydrophobic wax barriers by printing a well pattern that allows the establishment of simple and equipment-free PadLysis protocol consisting of the three steps: (1) 2 min lysis, (2) 2 min washing, and (3) 1 min drying. This proposed platform allowed direct LAMP amplification for A. pleuropneumoniae field isolates. We optimized LCV chemical components to enable the accurate LAMP signal readouts. The overall assay takes up to 45 min including 5 min PadLysis, 40 min LAMP amplification, and LCV signal observation with the naked eye. Our WPWPs–LAMP–LCV approach can be used as a rapid screening tool for A. pleuropneumoniae pleuropneumonia (APP) caused by ApxIA toxin in future outbreak areas.