Interdigitated microelectrode (IME) impedance sensor for the detection of viable Salmonella typhimurium

Interdigitated microelectrodes (IMEs) were used as impedance sensors for rapid detection of viable Salmonella typhimurium in a selective medium and milk samples. The impedance growth curves, impedance against bacterial growth time, were recorded at four frequencies (10 Hz, 100 Hz, I kHz, and 10 kHz) during the growth of S. typhimurium. The impedance did not change until the cell number reached 10(5) - 10(6) CFU ml(-1). The greatest change in impedance was observed at 10 Hz. To better understand the mechanism of the IME impedance sensor, an equivalent electrical circuit, consisting of double layer capacitors, a dielectric capacitor, and a medium resistor, was introduced and used for interpreting the change in impedance during bacterial growth. Bacterial attachment to the electrode surface was observed with scanning electron microscopy, and it had effect on the impedance measurement. The detection time, t(D), defined as the time for the impedance to start change, was obtained from the impedance growth curve at 10 Hz and had a linear relationship with the logarithmic value of the initial cell number of S. typhimurium in the medium and milk samples. The regression equations for the cell numbers between 4.8 and 5.4 x 10(5) CFU ml(-1) were t(D) = -1.38 log N + 10.18 with R-2 = 0.99 in the pure medium and t(D) = -1.54 log N + 11.33 with R-2 = 0.98 in milk samples, respectively. The detection times for 4.8 and 5.4 x 10(5) CFU ml(-1) initial cell numbers were 9.3 and 2.2 h, respectively, and the detection limit could be as low as I cell in a sample. (C) 2003 Elsevier B.V. All rights reserved.