An intracellular bacterium with the unique ability to enter mitochondria exists in the European vector of Lyme disease, the hard tick Ixodes ricinus. Previous phylogenetic analyses based on 16S rRNA gene sequences suggested that the bacterium formed a divergent lineage within the Rickettsiales (Alphaproteobacteria). Here, we present additional phylogenetic evidence, based on the gyrB gene sequence, that confirms the phylogenetic position of the bacterium. Based on these data, as well as electron microscopy (EM), in situ hybridization and other observations, we propose the name 'Candidatus Midichloria mitochondrii' for this bacterium. The symbiont appears to be ubiquitous in females of I. ricinus across the tick's distribution, while lower prevalence is observed in males (44%). Based on EM and in situ hybridization studies, the presence of 'Candidatus M. mitochondrii' in females appears to be restricted to ovarian cells. The bacterium was found to be localized both in the cytoplasm and in the intermembrane space of the mitochondria of ovarian cells. 'Candidatus M. mitochondrii' is the first bacterium to be identified that resides within animal mitochondria.

Biology Centre Institute of Parasitology Academy of Sciences of the Czech Republic Branišovská 31 370 05 České Budějovice Czech Republic

Dipartimento di Biologia Animale Università di Pavia Piazza Botta 9 27100 Pavia Italy

Dipartimento di Patologia Animale Igiene e Sanità Pubblica Veterinaria Università degli studi di Milano 20133 Milan Italy

School of Biological Sciences The University of Sydney New South Wales 2006 Australia

Sezione Diagnostica di Pavia Istituto Zooprofilattico Sperimentale della Lombardia e dell'Emilia Romagna Bruno Ubertini 27100 Pavia Italy

References provided by Crossref.org

Deep sequencing of 16 Ixodes ricinus ticks unveils insights into their interactions with endosymbionts

Gene Transfer Agents in Bacterial Endosymbionts of Microbial Eukaryotes

Ixodes ricinus ticks have a functional association with Midichloria mitochondrii

A dual endosymbiosis supports nutritional adaptation to hematophagy in the invasive tick Hyalomma marginatum

Life Cycle, Ultrastructure, and Phylogeny of New Diplonemids and Their Endosymbiotic Bacteria

Tick-Pathogen Interactions and Vector Competence: Identification of Molecular Drivers for Tick-Borne Diseases

Emerging horizons for tick-borne pathogens: from the 'one pathogen-one disease' vision to the pathobiome paradigm

Hard ticks and their bacterial endosymbionts (or would be pathogens)

Multiple origins of endosymbiosis within the Enterobacteriaceae (γ-Proteobacteria): convergence of complex phylogenetic approaches

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