On-chip impedimetric detection of bacteriophages in dairy samples

Bacteriophage infection of starter cultures constitutes a major problem in the dairy fermentation industry, which may bring about important economic losses. In this study, a rapid detection method of bacteriophages was developed based on analysis of impedance changes occurred upon infection of a host-biofilm established onto metal microelectrodes. Bacteriophage PhiX174 and Escherichia coli WG5 were chosen as models for bacteriophage and host strain, respectively, because of their easiness of manipulation. Impedimetric changes occurring at the microelectrode surface, caused by bacteriophage infection and subsequent lysis of the host strain, were monitored over a 6-h period after the initial inoculation of phages by non-faradic impedance spectroscopy (IS) in PBS and milk samples. Analysis of data was performed by two different approaches: (1) the equivalent circuit modelling theory, where a decrease in the magnitude of both the double layer and the biofilm capacitances due to the bacteriophage infection process was recorded, and (2) analysis of the impedance value, specially the impedance imaginary component (Z(i)) at selected frequencies. Z(i) is related to the capacitance of the circuit and also showed a decrease with respect to the control sample (without bacteriophages). The simplicity of the assay and the possibility of miniaturization of the system as well as its wide application, being able of detecting any bacteriophage as long as a suitable bacterial host is available, increase the number of applications to which this system could be used for.