## 4 Discussion
Another study reported the detection of _Vibrio parahaemolyticus_ as low as 10 CFU mL\({}^{-1}\), building highly selective aptamers with golden nanoparticles and cyanine dye 3 (Cy3) using the SERS approach on seafood samples [27]. A low detection limit is one of the main advantages of SERS and Raman micro-spectroscopy due to its ability to interrogate one single cell and a relatively short preparation time. Thus, these techniques only isolate cells from clinical or food processing samples [10]. Alternatively, the ORS system used in this study for pathogen detection depends on high concentrations of cells at the stationary phase of growth after 24 h, when the strongest signal is found. Concentration techniques such as centrifugation and resuspension can be used; however, due to its high sensitivity, the Raman signal may change significantly when the cells are instarvation [28]. Previous studies have effectively used a Raman handheld device equipped with an ORS system to test the authenticity of parmesan Reggiano cheese, showing good performance in predicting authentic parmesan cheese from non-authentic parmesan cheese by acquiring the spectra directly from packaged grated samples [29]. The same technology is reported in the present study; using the cell suspension in BHI broth does not require further preparation, such as the fixation on costly slides or other separation steps. Then the preparation time is reduced to media preparation and cell growth, as it requires colony isolation followed by a subculture in broth media. Furthermore, the portability of the Raman handheld allows measurements to be taken in situ, using less space and very few steps of preparation, such as making dilutions. Given that the Raman scanning system relies on the ORS to average the signal spectrum of several cells on suspension, it is essential to assure an axenic subculture in the media broth to guarantee the spectra's homogeneity. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Control BHI** & _Listeria_ & _S. aureus_ & _E. coli_ & _Salmonella_ & **Attribution** \\ \hline
**424** & **426** & **426** & **427** & **426** \\  & & & & 434 \\
437 & **437** & & & \\
446 & & & & \\  & & & 449 & & \\  & & & 497 & & \\  & & & 501 & & \\  & & & & \\
552 & 552 & 552 & & & C–C–C deformation \\  & 584 & & & & \\  & 595 & & & & \\  & & & 618 & Phe \\  & & & & C–C twisting, Phe \\  & & & 657 & & Tyr \\  & & & & C–S stretch \\  & & & 670 & Val \\  & & & 683 & & \\
702 & & & & **694** & \\  & & & & 719 & Adenine; \\  & & & & Peptidoglycan \\
733 & 732 & 731 & 730 & Adenine; Peptidoglycan \\  & 757 & & & Trp \\
**765** & **765** & **764** & **766** & C–C galactose \\  & & 772 & & 772 & \\
779 & & & 782 & \\
785 & 786 & & & 785 & Cytosine, Uracil (ring stretching) \\
**804** & **805** & **806** & **808** & **804** & C–C deformation/O–C–O wagging/C–H vibrations with C–OH \\
819 & & & & 820 & Tyr \\
837 & 838 & & & Tyr \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Peak positions and tentative attributions for Raman spectra of control media broth BHI, _Listeria_, _S. aureus_, _E. coli_, and _Salmonella_ based on similar studies [25; 29; 30; 31; 32; 33; 34; 35]. Distinctive peaks for each strain or control are coloured in green, peaks matching Gram-positive are coloured in blue, Gram-negative are coloured in red, and bolded signals match with all the samples, including controls. Data are expressed in cm\({}^{-1}\); spectral resolution: 12–14 cm\({}^{-1}\). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Control BHI** & _Listeria_ & _S. aureus_ & _E.