Rapid, sensitive, point-of-care detection of bacteria is extremely important in food safety. To address this requirement, we developed a new surface-enhanced Raman scattering (SERS)-based lateral flow (LF) strip biosensor combined with recombinase polymerase amplification (RPA) for simultaneous detection of Listeria monocytogenes and Salmonella enterica serotype Enteritidis. Au@Ag core-shell nanoparticles were used in this SERS-LF. Highly sensitive quantitative detection is achieved by measuring the characteristic peak intensities of SERS tags. Under optimal conditions, the SERS intensities of MBA at 1077 cm on test lines are used to measure S. Enteritidis (y = 1980.6x - 539.3, R = 0.9834) and L. monocytogenes (y = 1696.0x - 844, R = 0.9889), respectively. The limit of detection is 27 CFU/mL for S. Enteritidis and 19 CFU/mL for L. monocytogenes. Significantly, this SERS-LF has high specificity and applicability in the detection of L. monocytogenes and S. Enteritidis in food samples. Therefore, the SERS-LF is a feasible method for the rapid and quantitative detection of a broad range of bacterial pathogens in real food samples.

The efficient and timely detection of pathogens is a major concern worldwide. The aim of this study was to establish a rapid detection method for Salmonella bacteria in food samples to facilitate timely treatment. Widely used detection methods currently include culture-based methods and PCR-based methods. The former are time consuming, requiring 2 to 3 d, whereas the latter have higher accuracy but are typically complicated, requiring expertise and expensive instruments. In this study, a sensitive and rapid approach for the visual and point-of-use detection of Salmonella bacteria based on recombinase polymerase amplification (RPA) and a lateral-flow (LF) nucleic acid strip was established. We designed a pair of primers according to the invA gene of Salmonella bacteria: one was modified with digoxin, and the other was modified with biotin. In the presence of the biotin- and digoxin-modified primers and target DNA, the RPA produced a substantial amount of duplex DNA attached to biotin and digoxin. The products were detected using LF strips through immunoreaction: anti-digoxin antibodies on the gold nanoparticles, digoxin on the duplex, streptavidin on the LF test line, and biotin on the duplex. The developed RPA-LF assay allowed detection of Salmonella genomic DNA in less than 20 min with simple water bath equipment or portable thermal equipment. In addition, the RPA-LF assay was highly sensitive, with a detection limit as low as 20 fg of target DNA or 1.05 × 10 cfu of bacteria in pure culture, and highly specific, exhibiting no cross-reaction with Staphylococcus aureus, Escherichia coli, Listeria monocytogenes, Shigella, Enterobacter aerogenes, or Campylobacter jejuni. Importantly, Salmonella could be detected in milk and chicken breast at concentrations as low as 1.05 × 10 cfu/mL or 1.05 × 10 cfu/g after enrichment for 2 h and in eggs at 1.05 × 10 cfu/g after enrichment for 4 h. Furthermore, RPA was more sensitive than PCR, which requires a thermal cycling device. In summary, this study describes a sensitive, simple, and point-of-use detection method for Salmonella bacteria.

Recombinase polymerase amplification (RPA) can amplify target DNA at 37 to 42 °C without a thermal cycler. Lateral flow (LF) strips are portable, cheap and easy to operate. RPA combined with LF strips to detect Listeria monocytogenes can be widely used in remote areas.

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