Development of an immunomagnetic separation–propidium monoazide–polymerase chain reaction assay with internal amplification control for rapid and sensitive detection of viable Escherichia coli O157:H7 in milk

A lab-on-chip device for the sample-in-result-out detection of viable Salmonella using loop-mediated isothermal amplification and real-time turbidity monitoring

Rapid screening of foodborne pathogens is key to prevent food poisoning. In this study, a lab-on-chip device was developed for rapid, automatic and sensitive detection of viable Salmonella typhimurium using loop-mediated isothermal amplification (LAMP) and smartphone real-time turbidity monitoring. First, magnetic nanoparticles (MNPs) coated with anti-Salmonella capture antibodies in propidium monoazide (PMA) were fully mixed with bacterial samples using two active magnetic stirring mixers at reverse rotating directions, and incubated in the serpentine channel with 470 nm blue light exposure, allowing specific formation of magnetic bacteria and sufficient PMA pretreatment of the DNA of dead bacteria. Then, the PMA-treated magnetic bacteria were separated in the separation chamber using the magnetic field and their genomic DNA templates were extracted using lysis buffer at 70 °C. Finally, the viable bacteria's DNA was amplified using LAMP in the detection chamber preloaded with the lyophilized LAMP reagents at 67.5 °C after blocking with paraffin oil to avoid aerosol cross contamination. Finally, the turbidity of the LAMP reaction system was monitored in a real-time manner for the quantitative detection of viable bacteria. The experimental results demonstrated that this device was able to automatically detect viable Salmonella as low as 14 CFU mL-1 in spiked chicken meat supernatants within 1.5 h. This device is very promising to provide a sample-in-result-out solution for the in-field detection of Salmonella and could be easily extended for other foodborne pathogens.

Quantitative PCR coupled with sodium dodecyl sulfate and propidium monoazide for detection of viable Staphylococcus aureus in milk

Conventional quantitative PCR (qPCR) are unable to differentiate DNA of viable Staphylococcus aureus cells from dead ones. The aim of this study was to use sodium dodecyl sulfate (SDS) and propidium monoazide (PMA) coupled with lysostaphin to detect viable Staph. aureus. The cell suspensions were treated with SDS and PMA before DNA extraction. The SDS is an anionic surfactant, which can increase the permeability of dead cells to PMA without compromising the viability of live cells. The lysostaphin was applied to improve the effectiveness of DNA extraction. The reliability and specificity of this method were further determined by the detection of Staph. aureus in spiked milk. The results showed that there were significant differences between the SDS-PMA-qPCR and qPCR when a final concentration of 200 μg/mL of lysostaphin was added in DNA extraction. The viable Staph. aureus could be effectively detected when SDS and PMA concentrations were 100 µg/mL and 40 μM, respectively. Compared with conventional qPCR, the SDS-PMA-qPCR assay coupled with lysostaphin was more specific and sensitive. Therefore, this method could accurately detect the number of viable Staph. aureus cells.

Biotin-exposure-based immunomagnetic separation coupled with nucleic acid lateral flow biosensor for visibly detecting viable Listeria monocytogenes

Infectious diseases caused by Listeria monocytogenes pose a great threat to public health worldwide. Therefore, a rapid and efficient method for L. monocytogenes detection is needed. In this study, a biotin-exposure-based immunomagnetic separation (IMS) method was developed. That is, biotinylated antibody was first targeted to L. monocytogenes. Then, streptavidin-functionalized magnetic nanoparticles were added and anchored onto L. monocytogenes cells indirectly through the strong noncovalent interaction between streptavidin and biotin. Biotin-exposure-based IMS exhibited an excellent capability to enrich L. monocytogenes. Specifically, more than 90% of L. monocytogenes was captured when the bacterial concentration was lower than 10⁴ colony-forming units (CFU)/mL. Importantly, the antibody dosage was reduced by 10 times of that in our previous study, which used antibody direct-conjugated magnetic nanoparticles. Propidium monoazide (PMA) treatment prior to PCR amplification could eliminate the false-positive results from dead bacteria and detected viable L. monocytogenes sensitively and specifically. For viable L.monocytogenes detection, enriched L. monocytogenes was treated with PMA prior to asymmetric PCR amplification. The detection limits of the combined IMS with nucleic acid lateral flow (NALF) biosensor for viable L. monocytogenes detection were 3.5 × 10³ CFU/mL in phosphate buffer solution and 3.5 × 10⁴ CFU/g in lettuce samples. The whole assay process of recognizing viable L. monocytogenes was completed within 6 h. The proposed biotin-exposure-mediated IMS combined with a disposable NALF biosensor platform posed no health risk to the end user, and possessed potential applications in the rapid screening and identification of foodborne pathogens.

Detection of viable but non-culturable Escherichia coli O157:H7 by PCR in combination with propidium monoazide

The aim of this study was to evaluate the applicability of the conventional PCR detection method combined with propidium monoazide (PMA) treatment for the detection of viable but non-culturable (VBNC) state Escherichia coli O157:H7 in ground beef meatballs. Under low temperature, E. coli O157:H7 cells were induced into the VBNC state in ground beef meatballs at - 20 °C after 152 days. The optimal PMA concentration of 5 µg/mL was obtained in beef meatball samples, which could completely inhibit the DNA amplification on dead cells (10⁶ cells/mL) but with no inhibition on viable cells. The established PMA-PCR assay revealed that the VBNC counts exceeded 10⁷ CFU/mL in artificial contamination beef samples, which could be used for semi-quantitative detection of VBNC cells in beef meatball samples. This study indicated that the PMA-PCR assay might be a potential method for detection of VBNC state E. coli O157:H7 cells in food products.

Rapid detection of Enterobacter cloacae by immunomagnetic separation and a colloidal gold-based immunochromatographic assay

This is the first study of detection of Enterobacter cloacae via combined immunomagnetic separation and a colloidal gold-based immunochromatographic assay.