Inflammation in the gut, whether cartier bracelet caused by infection or autoimmunity, is challenging to detect, monitor, and treat. Riglar et al. engineered a strain of Escherichia coli obtained from the gut of a mouse to record exposure to tetrathionate, a downstream product of reactive oxygen species generated official website during inflammation. They used these programmed bacteria to sense in situ levels of tetrathionate. The bracelets bugs effectively monitored inflammation during Salmonella-induced colitis in mice and responded in a mouse model of inflammatory bowel disease. They exhibited long-term genetic stability, and the synthetic genetic circuits continued to function as intended in bacteria colonizing the mammalian gut. Thus, engineered bacteria have the potential to stably and reliably probe pathophysiological processes for which traditional diagnostics may not be feasible or cost-effective.