2.7. Culture preparation and target DNA extraction DNA was extracted from four E. coli strains, Listeria monocytogenes and Bacillus cereus. Two E. coli strains contained the stx1 gene (O157 -ATCC35150 and O103) while the other two strains did not contain this gene (O157 -NCTC12900 and O91 -09_A_15_1_1). Briefly, the cultures were grown up from culture collection stocks (Teagasc Food Research Centre Ashtown) which were stored on protective beads at -80°C. A single bead of each isolate was streaked on Tryptone Soy Agar (Oxoid, Fisher Scientific Ireland) and incubated overnight at 37 °C. An isolated colony was then placed in Tryptone Soy Broth and incubated overnight at 37 °C. DNA was extracted from the overnight culture using a Qiagen DNeasy Blood and Tissue kit (Qiagen, Manchester, UK), and in accordance with the manufacturer's instructions. The DNA concentration was measured using a Qubit dsDNA BR Assay Kit (Thermo Fisher Scientific, Ireland) on a Qubit 4.0 fluorometer. Before electrochemical measurements, DNA samples were diluted 5 times in 0.1 M PBS buffer, pH 7.2, and heated to 95 °C for 5 minutes to denature dsDNA. Subsequently, the samples were cooled down on ice and incubated on prepared sensors for 30 minutes at room temperature followed by electrochemical measurements.