## 4 Discussion  
Microbiological quality of seawater is a public health problem that can have serious repercussions in the economy and tourism. The prevention of waterborne diseases relies on microbiological detection methods. Recently, several new analytic methods have been proposed to replace standard procedures that take a few days to provide results. For instance, optical biosensors that use filters and camera enable to detect bacteria in water by fluorescence or hemiluminescence [12; 13; 14; 15]. In the literature, electrochemical sensors are proposed that detect pathogens thanks to the current or potential variations that occur upon pathogen or its biomarker interaction with an electrode [16; 17; 18; 19; 20; 21]. Among various detection methods, the most robust are those based on ELISA immunological tests and on PCR-based molecular tests [22]. Both methods may provide results within 24 h. However, their sensitivity is not sufficient for on-field applications, as an enrichment step is needed prior to detection. The enrichment step is particularly essential for detection of waterborne bacteria that are typically highly diluted. Moreover, PCR-based methods (like qPCR, digital PCR, isothermal methods) are performed on extracted and purified bacterial DNA. Purification of DNA enables to avoid possible inhibitors of the downstream amplification. Multi-steps that include bacterial concentration by filtration, pathogen recovery/enrichment, and DNA extraction/purification significantly increase the cost and time of water analysis. Detection of _E. coli_ is of great importance to the quality control of food and water. The combination of pathogen magnetic pre-concentration and direct PCR strategy proposed in this study enables detection of _E. coli_ without long multi-step DNA extraction. In addition, amplification of DNA sequences by PCR allows detection of VBNC cells that pose problem in conventional culture methods. In our procedure, the enrichment step (that takes from few hours to a few days) was replaced by the magnetic pre-concentration that was performed in less than 1 h. The pre-concentration increased the limit of detection by five orders of magnitude (from \(10^{8}\) CFU/mL to \(10^{3}\) CFU/mL of _E. coli_). Regarding the low concentration of contaminating bacteria in potable, recreational, and bathing waters, we believe that this procedure based on aptamer-coated magnetic beads may successfully replace membrane  
Figure 5: (**a**) Direct PCR detection of _E. coli_ with _yai_O primer set using 20 \(\mu\)L of BP + NaCl medium containing different concentrations of _E. coli_. Line 1: \(10^{8}\) CFU/mL; line 2: \(10^{7}\) CFU/mL; line 3: \(10^{6}\) CFU/mL; line 4: \(10^{5}\) CFU/mL; line 5: \(10^{4}\) CFU/mL; line 6: \(10^{3}\) CFU/mL; line 7: \(10^{2}\) CFU/mL; line 8: \(10^{4}\) CFU/mL; line 9: no template control. (**b**) Direct PCR detection of _E. coli_ with _yai_O primer set using 20 \(\mu\)L of magnet pre-concentrated bacterial cells from 1 mL of BP + NaCl medium containing different concentrations of _E. coli_. Line 1: no template control; line 2: no template control performed with magnetic beads carrying E1 aptamer; line 3: \(10^{8}\) CFU/mL; line 4: \(10^{7}\) CFU/mL; line 5: \(10^{6}\) CFU/mL; line 6: \(10^{5}\) CFU/mL; line 7: \(10^{4}\) CFU/mL; line 8: \(10^{3}\) CFU/mL; line 9: \(10^{2}\) CFU/mL. DNA ladder (1 bp +) was used in all experiments. filtration to concentrate bacteria. It can be performed on-site, as no electricity source is needed nor a specific equipment for sample preparation. A simple magnet enables to collect beads carrying bacterial cells for analysis. As the present work was a proof of principle, a lab PCR was used to verify the feasibility of the system. Portable PCR have already been presented in [14; 18; 23; 24; 25], and the aim of future work is to miniaturize the system for direct PCR detection in order to obtain a transportable system for in-field measurements. Conceptualization, J.V. ; methodology, J.V. ; validation, J.V., D.K. and N.R., formal analysis, Z.K., F.R. and J.V.