In modern life sciences, a paradigm shift is becoming increasingly evident: life forms are no longer considered to be self-contained units, but instead highly-complex and functionally-interdependent communities of organisms. The exploration of the close links between multi-cellular and especially bacterial life will, in future, be the key to a better understanding of life processes as a whole, and in particular the transition between health and illness. However, how the cooperation and communication of the organisms works in detail is currently still largely unknown. An important step forward in deciphering these multi-organism relationships has now been made by researchers from the Cell and Developmental Biology working group at Kiel University’s Zoological Institute: the scientists, led by Dr. Sebastian Fraune, have been able to prove for the first time that host organisms can control not only the composition of their colonizing bacteria, but also their function. The CAU researchers published their ground-breaking findings – derived from the example of the freshwater polyp Hydra and their specific bacterial symbionts – last Monday in the latest issue of the scientific journal Proceedings of the National Academy of Sciences.