Indagine volta alla comprensione del ruolo biologico dei simbionti batterici della zecca dura Ixodes Ricinus

Analysis to clarify the biological role of the bacterial symbionts from hard tick Ixodes ricinus Midichloria mitochondrii is a bacterial endosymbiont of hard ticks. In Ixodes ricinus, this bacterium has been observed both in the cytoplasm and in the mitochondria of the infected cells. Electron microscopy shows that these bacteria multiply inside the tick mitochondria. We have performed a PCR screening for Midichloria-related bacteria in DNA extracts from Ixodidae ticks. A total of seven species from five genera were found positive for bacteria closely related to M. mitochondrii. Phylogenetic analyses performed on the 16S rDNA sequences obtained from the different tick species revealed that the bacteria form a monophyletic group within the order Rickettsiales. In order to monitor the growth of these bacteria we developed a real time PCR method for their quantification. Our results show that Midichloria bacterial load is very high during all of the stages of tick development. A reduction of the bacterial load was observed after the hatching of the eggs and after each molt. This bacterial diminution was followed by cycles of bacterial increase after each blood meal. This increase in the bacterial load suggest that Midichloria requires for its multiplication some factor present in the blood. We are currently using the whole genome shotgun technique with the goal of obtaining the complete genome sequence of M. mitochondrii. The partial results obtained include 280 thousand pyrosequencing reads and 500 Sanger sequences. The sequencing of the genome will allow us to understand the different metabolic routes of this symbiont, as well as possible peculiar functions. In particular it will help to understand which kind of relationship is occuring between M. mitochondrii and its host I. ricinus.