You get an infection, you are given penicillin—and then you could get hemorrhagic diarrhea. This rare but extremely unpleasant side reaction can be related to the enterotoxin tilivalline produced by a regular intestinal bacterium. Austrian scientists have now scrutinized the toxin’s biosynthetic pathway and presented the results in the journal Angewandte Chemie. Their findings give important insights in the pathobiology of antibiotic side reactions and unveil the multifunctionality of bacterial toxins.