When bacteria such as Salmonella or Yersinia cause fever, diarrhea or abdominal pain, tiny “injection needles” are at work: their type 3 secretion system, or T3SS for short, shoots bacterial virulence proteins directly into the eukaryotic host cells. Researchers have thought of using bacterial injection devices to introduce proteins into eukaryotic cells. A research team has now succeeded in controlling the injection system optogenetically, i.e. with light. In the future this will enable to use the system in biotechnological or medical applications.