Upconversion-Linked Immunoassay for the Diagnosis of Honeybee Disease American Foulbrood

American foulbrood (AFB) caused by the bacterium Paenibacillus larvae is the most destructive disease of the honeybee brood. Therefore, rapid and sensitive detection methods are required to limit spreading of this pathogen, which has a major impact on agriculture and biodiversity. While P. larvae is typically detected by microbial cultivation or polymerase chain reaction, antibody-based detection represents a viable alternative. Here, we prepared an antibody specific for P. larvae and used it for the development of an upconversion-linked immunosorbent assay (ULISA). Photon-upconversion nanoparticles (UCNP) were conjugated to streptavidin via a PEG-linker using copper-catalyzed click chemistry to replace the enzyme label in conventional enzyme-linked immunosorbent assay (ELISA). The ULISA showed low cross-reactivity and provided a limit of detection of 2.9 × 10
³
CFU/mL, representing a 22-fold improvement compared to the ELISA. This level is within the bacterial loads present in honeybee larvae during an AFB infection. The assay was successfully applied to the analysis of spiked samples of bees, larvae, and hive debris.