Integrating microbial cells on Si nano-forest is a new structure combines whole cell biosensors with a functional nanostructured electrode. Such structure produces a unique electrochemical impedance spectrum in response to charged immobilized microbes. That signal, which is observed by both Nyquist diagram and the Bode plot was investigate in the current research. Microbial biosensors are widely used for toxicity measurements, food monitoring e.g. sensing of alcohol, glucose, or fatty acids, environmental monitoring, monitoring of microbial growth rate, and biocide measurements. Microbial cells can generate a widespread selective responses to chemical and biological reactions. Utilization of this advantage can yield specific sensors and allay one of the major challenges of chemical and biological sensor design. Moreover, the use of lives cells allows for label-less, non-destructive real-time monitoring of the biological reactions as they develop, with no require for preparatory and analytical steps such as staining or hybridization. Bacterial cells were genetically engineered to display ZZ protein on the surface of their cell (a kind gift from Prof. Benhar lab). Z domain is a small engineered protein of 58 amino acids, analog of Protein A B domain, originally developed as an affinity-purification ligand. ZZ fragments exhibit high affinity (~10-8M) to Fc region of human IgGs, higher than the affinity of a single Z domain. Each Z domain interact with two distinct sites of the Fc region. Expression and display of Z domain in E.coli was demonstrated before. The combination of nanostructures, such as Si nanowires (SiNWs), with bacterial cells offers new and unique opportunities to develop novel biosensors. Usually in biosensors, microbial cell immobilized to a planar electrode, consequently part of the displayed charge on the surface of the cell is screened by the electrode itself. This screening effect is decreasing while using SiNWs structure to bind the microbial cells. We present a study of highly dense, disordered, and randomly oriented ensemble, i.e. Si nano-forest. This Si nanostructured was deposited using relatively low cost and scalable techniques, such as Plasma Enhanced Chemical Vapor Deposition (PECVD), at low temperature (below 350°C); this enables deposition on various substrates including flexible polymeric substrates. Si nano-forest were functionalized using rabbit anti mouse IgG as binding receptor for ZZ protein. EIS measurements were performed, at the range of 0.1 Hz to 200 kHz, in phosphate buffer saline (PBS) solution using potentiostat/galvanostat (VSP®, BioLogic science instruments Ltd.). Modeling was preformed using EC-LAB software. Our results confirm that Si nano-forest electrodes can serve as a sensitive and specific whole cell biosensor.