Genetically engineered Escherichia coli can sense and report on conditions in the mouse gut, highlighting the potential for development of 'living diagnostics'. Mice were administered bacteria containing a memory system—a 'trigger element' (the lambda Cro gene transcribed from a tetracycline-inducible promoter) and a 'memory element' (from the phage lambda cl/Cro region). Faeces from anhydrotetracycline-treated mice contained only bacteria in the Cro state; faeces from untreated mice contained bacteria in the cl state.