Rapid detection of Salmonella in food matrices by photonic PCR based on the photothermal effect of Fe3O4

Abstract

Salmonella causes most deaths from diarrheal disease worldwide. Therefore, Salmonella must be accurately and quickly detected, even in complex food matrices, which is difficult to achieve using conventional culture methods. Here we propose a novel photonic polymerase chain reaction (PCR) method based on ferroferric oxide (Fe₃O₄) for the detection of Salmonella typhimurium in complex samples. Owing to the great photothermal conversion performance of Fe₃O₄, rapid thermal cycling could be accomplished. Our optimized photonic PCR system specifically detected Salmonella typhimurium in complex food matrices within 50 min. Quantitative data showed a limit of detection up to 10² CFU/mL in food samples. This method is suitable for the detection of all pathogenic microorganisms and is universal.

Keywords: Fe3O4; Food; Photonic PCR; Rapid detection; Salmonella typhimurium.

Detection of Salmonella typhimurium in food matrices by photonic PCR

Scheme 1

Schematic illustration of the photonic PCR method based on the photothermal effect of Fe₃O₄.

Fig. 1

Optimization of the number of thermal cycles (a). 1, DNA ladder; 2, conventional PCR, 40 cycles; 3–6, photonic PCR, 30, 35, 40 and 45 cycles. Optimization of the denaturation temperature (b). 1, DNA ladder; 2, conventional PCR, 95℃; 3–5, photonic PCR, 75–80℃, 80–85℃ and 85–90℃. Optimization of the time required for denaturation (c). 1, DNA ladder; 2, conventional PCR, 5 s; 3–6, photonic PCR, 0, 5, 10 and 15 s. Optimization of the time required for annealing (d). 1, DNA ladder; 2, conventional PCR, 10 s; 3–5, photonic PCR, 5, 10 and 15 s. Optimization of the annealing temperature (e). 1, DNA ladder; 2, conventional PCR, 60℃; 3–5, photonic PCR, 50–55℃, 55–60℃ and 60–65℃.

Fig. 2

The temperature profile of different concentrations of Fe₃O₄ for one thermocycle (a). Optimization of the concentrations of Fe₃O₄ by PCR (b). 1, DNA ladder; 2, conventional PCR, 10 mg/mL; 3–8, photonic PCR, 2–12 mg/mL.

Fig. 3

The temperature profile for the 40 thermocycles (a). Characterization of ramp rate for heating and cooling (b). Temperature execution profile of the photothermal device (c). Thermal images of Fe₃O₄ solution upon infrared laser irradiation (d).

Fig. 4

Standard curve of photonic PCR for Salmonella typhimurium in ultrapure water (a). Photonic PCR in real samples (b). 1, DNA ladder; 2–3, tap water, 10⁶ CFU/mL, 10² CFU/mL; 4–5, juice, 10⁶ CFU/mL, 10² CFU/mL; 6–7, pork, 10⁶ CFU/mL, 10² CFU/mL; 8–9, egg white, 10⁶ CFU/mL, 10² CFU/mL; 10–11, milk, 10⁶ CFU/mL, 10² CFU/mL. The fluorescence intensity of photonic PCR in the presence of different common bacteria (10⁶ CFU/mL) (c). The selectivity of the detection was shown by agarose gel electrophoresis (d). 1, DNA ladder; 2–6, Salmonella typhimurium, Salmonella enteritidis, Listeria monocytogenes, Escherichia coli, Staphylococcus aureus.