E coli DSMZ was used as model bacteria E coli

E. coli DSMZ 17076 was used as model bacteria. E. coli are Gram-negative bacillus with average size of 2μm in length and 0.5μm in diameter. The structure of these purchase 4-ap comprises an outer membrane, the periplasm interspace, the inner membrane and the cytosol. Inside the periplasm there is the cell wall, which is mainly composed of peptidoglycan, a negatively charged polymer. Therefore, in electrical terms, these bacteria are constituted of two insulating cell membranes, a conductive cytoplasmatic medium and a more conductive periplasm.
Materials and methods
Results and discussion Fig. 1a and b show Bode plots of the Ab-modified polysilicon IDEs immersed in a solution spiked with E. coli. Measurements were performed at three set incubation times of 0, 15, and 30min. First, the Z magnitude Bode plot exhibits the expected shape with three distinct slopes. At low frequencies (<105Hz) the impedance is dominated by the impedance of the electrode-solution interface. In the case of polysilicon IDEs immersed in aqueous solution, this impedance is related to the electrochemical double layer and the native silicon oxide layer. It is of capacitive nature and therefore produces a slope in the spectrum close to −20db per decade of frequency. At middle frequencies (105 to 106Hz), the impedance is dominated by the resistance of the solution between the IDE electrodes which is independent of frequency and inversely proportional to the solution conductivity, thus yielding a slope close to 0db per decade. At high frequencies (>106) the spectrum slope is also close to −20db per decade as the impedance is dominated by the capacitance of the solution, which is proportional to the solution permittivity. At the highest frequency measured (107Hz), a slight decrease of the slope is appreciated. This can be associated to the resistance of the polysilicon electrodes [16]. Once conductivity variations are compensated, as explained above, the impedance changes produced by the cells captured at the IDE surface are too small to be appreciated in the Bode plots. However, if the relative variation of the impedance module and phase are plotted, a clear change in the spectra is detected, as shown in Fig. 1 c and d, this being the bacteria fingerprint in the impedance spectra due to the interaction with the Ab-modified IDE. The relative variation of the impedance was calculated as the normalized value of the increment of the impedance at a set measured time with respect to that recorded at time=0 (just after the IDE was dipped in the cell solution). In the measured frequency range (103 to 107Hz) there are two clear peaks, one at positive and the other at negative values that increase with time as the density of cells on the IDE surface increases. The increase of impedance at low frequencies (first peak) is consistent with previous results obtained for impedimetric detection of bacteria [8, 9]. The second negative peak indicates a decrease of impedance in the middle frequency range, which suggests that the cells are behaving not as insulating but as conducting particles. This is in agreement with a β-dispersion phenomena observed in cell suspensions [18]. Beyond a critical frequency, fc, currents can penetrate through the cell membranes, and the dielectric properties of the cytoplasm (and in turn of the periplasm in gram-negative cells) have an effect on the measured impedance. In the measurements carried out in the present work, the cells are suspended in a solution having a conductivity of 3·10S/m, which is two orders of magnitude lower than the reported cytoplasm conductivity for E. coli. Therefore, for a frequency higher than fc, a significant decrease in the measured IDE impedance can be expected. In addition, the peaks of differential graphics are shown to duplicate its value from 15 to 30min. These results make evident the ability of this new method to quantify cells as well as specifically distinguish them from other analytes.