Most microbial particles have a negatively charged surface and in this work, we describe a water quality monitoring application of a cationic polythiophene derivative (AH-35) for the rapid assessment of microbial contamination of water. Using E. coli as a prototype microbial particle, we demonstrate that the AH-35 polymer can provide a qualitative assessment of water if exposed to more than 500 CFU/mL, thereby paving the way to a new family of biosensors potentially useful for monitoring drinking water distribution systems.