Electrochemical Biosensor for the Monitoring of Phages of in Milk-Based Samples

Lactococcus lactis (LL) bacteriophage infections in milk prevent proper lactic fermentation, leading to the production of unsaleable low-quality products and great economic losses in the dairy industry. In this work, we present an innovative biosensing approach for the cost-effective detection of LL phages (PGs) in milk-based samples through electrochemical impedance spectroscopy (EIS). The detection is based on the evident parametric shifts in the charge transfer resistance and the impedance phase at 100 Hz caused by different bacteria proliferation due to PG activity. The EIS results are compared with optical absorbance measurements at 600 nm, in order to validate the proposed method. Preliminary experimental tests with filtered milk-based samples confirm the sensor capability to detect PGs at different concentrations in milk-based solutions in less than 4 h. In order to reach a higher sensitivity, we propose a new milk pretreatment adding calcium chloride (CaCl2) to the samples. The EIS results with CaCl2-treated milk evidence an enhanced PG activity, which leads to a much larger parametric shift. Lastly, the sensor is tested with CaCl2-treated milk-based samples at different PG concentrations, obtaining an enhanced performance reaching the limit of detection (LOD) of 103 PFU/mL.