Abstract: Transient molecules in the gastrointestinal (GI) tract, such as nitric oxide and hydrogen sulfide, are important signals and mediators of inflammatory bowel disease (IBD). Because these molecules may be short-lived in the body, they are difficult to detect. To track these reactive molecules in the GI tract, a miniaturized device has been developed that integrates genetically engineered probiotic biosensors with a custom-designed photodetector and readout chip. Leveraging the molecular specificity of living sensors, bacteria were genetically encoded to respond to IBD-associated molecules by luminescing. Low-power electronic readout circuits (e.g., using nanowatt power) integrated into the device convert the light from just 1 ?L of bacterial culture into a wireless signal. Biosensor monitoring was demonstrated in the GI tract of small and large animal models and integration of all components into a sub-1.4 cm3 ingestible form factor capable of supporting wireless communication.