An integrated biosensor for the detection of micron-size biological entities using Magnetotactic Bacteria (MTB) being guided under the control of an external magnetic field of a few Gauss is briefly described. The proposed biosensor will be implemented onto a silicon substrate compatible with standard CMOS technologies. To validate the proposed concept, a microfluidic device and a microelectronic chip have been fabricated. Pairs of planar microelectrodes are patterned on the bottom of a microchannel and connected with the microelectronic chip. When a microbead pushed by a single MTB passes between a pair of microelectrodes, a variation in electrical impedance is measured by embedded detection circuits. The paper reviews the proposed microsystem. It emphasizes the sensing principle and describes the technique for implementing the microelectrodes. Finally, the paper reports preliminary experimental results obtained with an integrated circuit implementing the interfacing electronics that confirm the feasibility of turning distinct impedance levels into digital decisions that would reflect the presence of biological entities, especially bacteria or functional microbeads.