The multiplexed digital polymerase chain reaction (PCR) is widely used in molecular diagnosis owing to its high sensitivity and throughput for multiple target detection compared with the single-plexed digital PCR; however, current multiplexed digital PCR technologies lack efficient coding strategies that do not compromise the sensitivity and signal-to-noise (S/N) ratio. Hence, we propose a fluorescent-encoded bead-based multiplexed droplet digital PCR method for ultra-high coding capacity, along with the creative design of universal sequences (primer and fluorescent TaqMan probe) for ultra-sensitivity and high S/N ratios. First, pre-amplification is used to introduce universal primers and universal fluorescent TaqMan probes to reduce primer interference and background noise, as well as to enrich regions of interest in targeted analytes. Second, fluorescent-encoded beads (FEBs), coupled with the corresponding target sequence-specific capture probes through streptavidin-biotin conjugation, are used to partition amplicons via hybridization according to the Poisson distribution. Finally, FEBs mixed with digital PCR mixes are isolated into droplets generated via Sapphire chips (Naica Crystal Digital PCR system) to complete the digital PCR and result analysis. For proof of concept, we demonstrate that this method achieves high S/N ratios in a 5-plexed assay for influenza viruses and SARS-CoV-2 at concentrations below 10 copies and even close to a single molecule per reaction without cross-reaction, further verifying the possibility of clinical actual sample detection with 100% accuracy, which paves the way for the realization of digital PCR with ultrahigh coding capacity and ultra-sensitivity.
Keywords: SARS-CoV-2; bead-based multiplexed droplet digital PCR; high signal-to-noise ratio; influenza virus; ultrasensitive; universal prime and probe.