Abstract: A method and apparatus for concentration and separation of bacteria in confined geometries such as in thin fluid films and in channels. The bacteria live in an environment of a specified level of pH, so that an increase or decrease of pH levels stimulates the bacteria to avoid areas of unfavorable pH and swim in the direction of the pH gradient. To create the gradient of pH an electric current is transmitted through a thin film or channel containing bacteria. The transmission of the current results in electrolysis in the vicinity of the electrodes and deviation of the pH levels from the bulk values. The bacteria swim away from the electrodes toward the center of the cell, where the pH level is more favorable.