Microbes in our gut produce enzymes that help with our metabolism and influence our health. For example, bile salt hydrolases (BSHs) cleave parts of bile acids, leading to the formation of secondary bile acids that are involved in fat digestion. Elena A. Goun of the University of Missouri and coworkers have developed a bioluminescent probe to measure BSH activity in a variety of settings, including in bacteria, fecal samples, and live mice (Sci. Adv. 2021, DOI: 10.1126/sciadv.aaz9857). The probes (example shown) are made of bile acids conjugated to luciferin via an amide bond. When BSH cleaves the amide and liberates the luciferin (red), the enzyme luciferase can react with the freed molecule to produce light. Using the probes, Goun and coworkers showed that some prebiotics—dietary fibers that are digestible by microbes but not by humans—increase BSH activity. They also showed that BSH activity is reduced in fecal samples from people with inflammatory bowel disease compared with people without the disease. The probe could provide a way for researchers to further understand microbes’ role in our gut health.