In situ determination of enteric pathogen bacteria is essential in the prevention of a number of illnesses associated to contaminated water consumption. Current methods for this mostly depend on long incubation times (i.e. Colibert test) or poorly selective/ sensitive protocols (amperometric sensors). Based on the Colilert test, this manuscript presents a low-cost and small-size photonic lab-on-a-chip (PhLoC) containing discrete auxiliary optical elements (i.e. light emitting diodes, LEDs, and detectors) and an external electronic control system for environmental light correction. Preliminary results demonstrate the suitability of the present technology for the sensitive and selective determination of coliform bacteria. These architectures and measurement protocols open the possibility for fast in situ determination of microbial contaminants in drinking and wastewaters.