NOVELTY - The sensor comprises an anolyte space (4) for containing a micro-organism and an anolyte (8), an anode (12) placed in the anolyte space for receiving electrons originating from the micro-organism, a catholyte space (6), and a biocathode (14) placed in the catholyte space. The anode and the cathode are operatively connected such that a current and/or current difference is measurable as a result of an applied vibration or an electric and/or magnetic field. The anolyte space comprises a mediator for receiving the electrons from the micro-organism and relinquishing them to the anode.    USE - The sensor is useful in a microbial fuel cell for measuring effects of vibrations and/or fields on a micro-organism in order to influence a micro-organism such as Baker's yeast, where the vibrations are acoustic vibrations, an electric field and/or a magnetic field.    ADVANTAGE - The sensor is capable of quickly measuring effects of vibrations and/or fields on a micro-organism with low investment and labor costs.    DETAILED DESCRIPTION - The sensor comprises an anolyte space (4) for containing a micro-organism and an anolyte (8), an anode (12) placed in the anolyte space for receiving electrons originating from the micro-organism, a catholyte space (6), and a biocathode (14) placed in the catholyte space. The anode and the cathode are operatively connected such that a current and/or current difference is measurable as a result of an applied vibration or an electric and/or magnetic field. The anolyte space comprises a mediator for receiving the electrons from the micro-organism and relinquishing them to the anode. The sensor uses a redox couple comprising a ferricyanide/ferrocyanide couple. Circulation means are provided for circulating anolyte through a separate space. The anolyte comprises diluted juice concentrate, such as apple juice. A membrane is provided as separation between the anolyte space and the catholyte space. An absorbing element such as a cotton cloth is provided between the anolyte space and the cathode continuously or periodically with anolyte. INDEPENDENT CLAIMS are included for:    (1) a bioreactor for producing and/or selecting a microorganism; and    (2) a method for influencing a micro-organism.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic representation of the operation of a sensor    Anolyte space (4)    Catholyte space (6)    Anolyte (8)    Anode (12)    Biocathode. (14)