An autonomous synthetic DNA machine for ultrasensitive detection of Salmonella typhimurium based on bidirectional primers exchange reaction cascades.

Statistics show that food poisoning caused by Salmonella typhimurium (S. Typhimurium) often tops the list of bacterial food poisoning types in countries around the world. However, detecting traces of S. Typhimurium in real samples remains challenging. In recent years, primer exchange reaction (PER), a new isothermal amplification strategy, has rapidly attracted the attention of researchers in the field of biosensing. In this work, We developed a nanostructure called DNA arch bridge (DAB) and combined the DAB with cascade PER technology to construct a novel bidirectional PER (B-PER) for ultra-sensitive detection of pathogenic bacteria as a novel fluorescent biosensor. This strategy relies on the B-PER reaction mediated by binding of the target and adaptor, which occurs with the assistance of Klenow Fragment (KF) (3'-5'exo) polymerase and produces a good deal of G-quadruplex sequences that generate a fluorescent signal by embedding fluorescent dyes. Under the best conditions, the biosensor achieves ultrasensitive detection of S. Typhimurium, and the detection limit of the strategy is 9.3 cfu mL-1 over the linear detection scope of 101-105 cfu mL-1. The method has the merits of facile operation, rapid response, and high sensitivity. Furthermore, the biosensor is expected to achieve ultrasensitive detection of various small molecules through recognizing different target and primer sequences. Therefore, our proposed strategy provides an efficient, stable, universal, and practical sensing platform for pathogen and other small molecules detection.