A method for collecting a microbiological substance utilizes a micro fabricated biochip having a collection chamber. A fluid sample containing a microbiological entity of interest is delivered to the collection chamber in the biochip. Then a non-uniform electric field is generated in the collection chamber, to retain the microbiological entity of interest in the collection chamber. The microbiological entity is retained through dielectrophoresis induced by the energization of the electrodes by a periodically applied, alternating current.