Cartoon of a typical cell membrane

The device Pillai and Beskok are designing for NASA will use positively-charged electrodes to attract bacteria. The membrane "bag" that separates a bacterium's innards from the outside world is made from a kind of fat molecule. Embedded in this membrane are a wide variety of larger, carbohydrate and protein molecules that control the microbe's interaction with the outside world. These embedded molecules each have a distinctive pattern of positively and negatively charged regions on their surfaces. For the pH levels (i.e., acidity) typical of drinking water, the net charge from these embedded molecules is usually slightly negative, so they will be drawn to a positively charged electrode.