Recombinase Polymerase Amplification Combined with Lateral Flow Strip for Listeria monocytogenes Detection in Food.

Listeria monocytogenes is an important food-borne pathogenic bacterium that causes human disease, resulting in economic losses worldwide. The current detection methods for L. monocytogenes are not well suited for direct field testing because they involve complicated, time-consuming operations. A simple, efficient method is vital for L. monocytogenes detection. In this study, we combined isothermal recombinase polymerase amplification (RPA) with a lateral flow (LF) strip to rapidly and reliably detect L. monocytogenes. In the presence of biotin- and digoxin-modified primers, RPA produced numerous digoxin- and biotin-attached duplex DNA products. These products were detected on an LF strip via dual immunoreactions (digoxin on the duplex DNA reacted with the anti-digoxin antibody on the gold nanoparticle (Au-NP) and the biotin on the duplex DNA captured by the streptavidin on the LF test zone). The accumulation of Au-NPs produced characteristic bands, enabling the visual detection of L. monocytogenes without instrumentation. This assay could be used to detect L. monocytogenes within 15 min, including DNA amplification with RPA for 10 min at 39 °C and visualization of the amplicons by LF strips for 5 min. Experiments confirmed a detection limit as low as 300 fg of DNA and 1.5 × 101 CFU in pure cultures. Furthermore, RPA-LF exhibited no cross-reactions with pathogens. Evaluation of the method with food samples indicated that the detection limit was substantially improved to 1.5 × 10° CFU for the original bacterial content in 25 g/mL samples after enrichment for 6 hr. RPA-LF can be used as a sensitive and rapid detection technique for L. monocytogenes. PRACTICAL APPLICATION: 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.

SERS-Based Lateral Flow Strip Biosensor for Simultaneous Detection of Listeria monocytogenes and Salmonella enterica Serotype Enteritidis.

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. AuMBA@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-1 on test lines are used to measure S. Enteritidis (y = 1980.6x - 539.3, R2 = 0.9834) and L. monocytogenes (y = 1696.0x - 844, R2 = 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.

Development of an isothermal amplification-based assay for the rapid visual detection of Salmonella bacteria.

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 × 101 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 × 100 cfu/mL or 1.05 × 100 cfu/g after enrichment for 2 h and in eggs at 1.05 × 100 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.

[Effect of enhanced green fluorescent protein fusion on Ano1 physiological feature].

The aim of the present study was to investigate whether the physiological features of Ano1 were affected by enhanced green fluorescent protein (EGFP) fusing at Ano1 C-terminal. The eukaryotic expression vectors of Ano1 and EGFP-Ano1 were constructed, and these plasmids were transfected into Fischer rat thyroid follicular epithelial (FRT) cells using liposome. The expression and location of Ano1 were examined by using inverted fluorescence microscope. The ability of Ano1 to transport iodide was detected by kinetics experiment of fluorescence quenching. The results showed that both Ano1 and EGFP-Ano1 were expressed on FRT cell membrane and could be activated by Ca(2+). There was no significant difference of the ability to transport iodide between Ano1 and EGFP-Ano1. These results suggest Ano1 and EGFP-Ano1 have similar physiological feature.