A biosensor that employs a bacterium in a biofilm present on one of two electrodes of an electrochemical cell. The bacterium is genetically modified by having deleted at least one native gene that encodes for the enzymatic transformation of a molecular moiety, and having substituted for the deleted native gene a different gene having a transcription factor that is under the control of an inducible promoter in conjunction with an inducer molecule. When the molecular moiety is present in a specimen of interest that is in contact with the biofilm, an electric current is generated in response to the presence of the inducer molecule. In the absence of the inducer molecule, no electricity is generated. The electric signal that is generated can be analyzed to determine the presence and the quantity of the inducer molecule in the specimen of interest.