In the last years, Protists (Protozoa) gained a significant attention from the scientific community because of their role as Trojan horses respect to opportunistic pathogens of humans and animals. Although this role is nowadays well established and accepted for some Protists like Acanthamoeba, little is known for other groups of possible hosts. Recently, members of the coordinating unit of Pisa got evidences that many of the natural occurring bacterial symbionts of ciliates have a strong phylogenetic affiliation to human and animal (especially fish) pathogens like Rickettsia (Vannini et al., 2005; Ferrantini et al., 2009; Ferrantini et al., in prep) and Francisella (Schrallhammer et al., in prep.). These observations raised the question whether ciliates may also represent natural reservoirs for potentially pathogenic bacteria especially in tropical regions where the environmental conditions favor microbial growth and fast evolution. Furthermore tropical countries are predicted as the Hot Spot for emerging infectious diseases and zoonosis due to hygienic standards lower than in Western countries and the living together of humans with their farmed animals. Aim of the present project is to set up an international research network that, starting from the collection of novel isolates especially from tropical regions, performs a complete ecological, functional and evolutionary genomic investigation on symbiotic bacteria retrieved from ciliates that shows phylogenetic affinities to pathogenic ones. Units from the proposed network posses expertise that ranges from the traditional morphological investigation to advanced ultrastructure, ecology, functional biology, molecular phylogeny, comparative genomics and postgenomics. Aim of the research network will be to coordinate the research activities of the involved units into joined projects and to transfer know-how to the younger researcher to allow them to re-establish the acquired competences in their home laboratories.