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Electrostatic finite element modelling software and an ac equivalent circuit model have been used to investigate an impediometric microelectrode array biosensor for the detection of bacteria. The electrostatic model showed the capacitance of the biosensor to decrease with increasing numbers of bacteria trapped on the sensor's surface in a suspension of relatively high dielectric permittivity. Optimization of the model suggests that reducing the spatial wavelength of the biosensor's electrodes either through a decrease in electrode width or gap will improve the sensor's sensitivity. In addition, the model confirmed that the permittivity of the external medium had a significant effect on detection efficiency. Increased sensitivity in suspensions of lower relative dielectric permittivity was observed. The equivalent circuit model (ECM) was used to analyze the effect of high levels of immobilized bacteria at fixed signal frequencies (100 Hz and 1 MHz). It has been shown that the ECM discussed in this paper is able to successfully model the experimental data for the actual sensor in the low frequency ranges, allowing prediction of the sensor response and analysis of its performance. Overall, the modelling results obtained in the present paper are in general agreement with those from other published data and can be used in the development and optimization of impediometric biosensors for rapid and reliable detection of pathogenic bacteria.