A: An app alone can't test water, but a sensor connected to it can. And the key bacterium to check for is E. coli. That's because if any pathogens are present, it's highly likely that E. coli is too, says Sushanta Mitra, a mechanical engineer at York University in Canada. Detecting E. coli usually requires days of culturing in a lab. Mitra's prototype, the Mobile Water Kit, detects E. coli in hours with chemicals that change the water color when the bacterium is there. It's currently being field-tested.