Ultrasensitive and rapid count of Escherichia coli using magnetic nanoparticle probe under dark-field microscope

Escherichia coli (E. coli) is one of the best-known zoonotic bacterial species, which pathogenic strain can cause infections in humans and animals. However, existing technologies or methods are deficient for quickly on-site identifying infection of E. coli before they breakout. Herein, we present an ultrasensitive and on-site method for counting E. coli using magnetic nanoparticle (MNP) probe under a dark-field in 30 min. The antibodies functionalized MNP, binding to E. coli to form a golden ring-like structure under a dark-field microscope, allowing for counting E. coli. This method via counting MNP-conjugated E. coli under dark-field microscope demonstrated the sensitivity of 6 CFU/μL for E. coli detection. Importantly, due to the advantages such as time-saving (only 30 min) and almost free of instrument (only require a portable microscope), our MNP-labeled dark-field counting strategy has the potential of being a universal tool for on-site quantifying a variety of pathogens with size ranges from a few hundreds of nanometers to a few micrometers. In summary, the MNP-labeled dark-field counting strategy is a rapid, simple, sensitive as well as low-cost assay strategy, which has the potential of being a universal tool for on-site quantification of micrometer-size pathogens like E. coli.