Outwitting planarians antibacterial defence mechanisms: Rickettsiales bacterial trans-infection from Paramecium multimicronucleatum to planarians

Most of the microorganisms belonging to genera responsible for vector-borne diseases (VBD) have hematophagous arthropods as vector/reservoir. Recently, many new species of microorganisms phylogenetically related to agents of VBD were found in a variety of aquatic eukaryotic hosts, in particular, numerous new bacterial species related to the genus Rickettsia (Alphaproteobacteria, Rickettsiales) were discovered in protist ciliates and other unicellular eukaryotes. Although their pathogenicity for humans and terrestrial animals is not known, these bacteria might act as etiological agents of possible VBD of aquatic organisms, with protist as vectors. In the present study, we characterized a novel strain of the Rickettsia-Like Organism (RLO) endosymbiont \"Candidatus (Ca.) Trichorickettsia mobilis\" in the macronucleus of the ciliate Paramecium multimicronucleatum through Fluorescence In Situ Hybridization (FISH) and molecular analyses. Ultrastructural investigations on the presence of flagella confirmed previous studies on the same bacterial species. The potential trans-infection per os of this bacterium to planarians (Dugesia japonica), a widely used model system able to eliminate a wide range of bacteria pathogenic to humans and other Metazoa, was further verified. Ciliate mass cultures were set up, and trans-infection experiments were performed by adding homogenized paramecia to food of antibiotic-treated planarians, performed. Treated and non-treated (i.e. control) planarians were investigated at day 1, 3, and 7 after feeding for endosymbiont presence by means of PCR and ultrastructural analyses. Obtained results were fully concordant and suggest that this RLO endosymbiont can be transferred from ciliates to metazoans, being detected up to day 7 in treated planarian enterocytes inside and, possibly, outside phagosomes.