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General schematic of whole-cell bacterial biosensors in the gut. • The human gut microbiota interacts extensively with the host and is intricately linked to health and disease. • Non-invasive tools to monitor microbial states and disease biomarkers in the gut are lacking. • Engineered bacteria sense and respond to external signals and can function as reporters of the gut environment. • Emerging technologies will help create diagnostic bacteria that can monitor and respond to multiple signals in vivo. Advances in synthetic biology and microbiology have enabled the creation of engineered bacteria which can sense and report on intracellular and extracellular signals. When deployed in vivo these whole-cell bacterial biosensors can act as sentinels to monitor biomolecules of interest in human health and disease settings. This is particularly interesting in the context of the gut microbiota, which interacts extensively with the human host throughout time and transit of the gut and can be accessed from feces without requiring invasive collection. Leveraging rational engineering approaches for genetic circuits as well as an expanding catalog of disease-associated biomarkers, bacterial biosensors can act as non-invasive and easy-to-monitor reporters of the gut. Here, we summarize recent engineering approaches applied in vivo in animal models and then highlight promising technologies for designing the next generation of bacterial biosensors. [ABSTRACT FROM AUTHOR]