cell surface, Escherichia coli, food vacuole, holobiont,
Protozoa, especially ciliates and amoeba, have developed many endosymbiotic interactions with bacteria and algae during evolution. Generally the well-described systems are evolutionarily old. A long co-evolution of the partners allowed the establishment of stable symbioses with a variety of mutualistic dependencies. Often the dependency has led to the inability especially of the bacterial partner to grow axenically outside its host. These systems have been studied in depth at the morphological and ultrastructural level. However, they are not ideally suited for studying physiological, genetic or molecular aspects of endocytobiosis formation. These functional approaches require versatile experimental systems including the option for genetic manipulation at least of the bacterial partner. Towards these intentions, we have started to provide experimental tools for analysing de novo established synthetic endobioses. We focus on Tetrahymena pyriformis as the protozoan and on Escherichia coli as bacterial partner. As the establishment of endocytobiosis is inevitably linked to many recognition and communication processes between the partners, we set out to analyse the interactions directly at the cytostome and between bacteria and the vacuolar membrane in food vacuoles. Intentional physicochemical modifications of surface properties of the bacteria, assisted by employing synthetic food particles with chemically defined surfaces, provide many experimental approaches in this direction. We propose this system as a versatile laboratory model for studying especially the early steps during endobiosis formation and wish to strongly encourage the community to join this field of research, in order to understand the emergence and establishment of holobionts in the protozoan world.