Three-dimensional images reveal the impact of the endosymbiont Midichloria mitochondrii on the host mitochondria

Abstract

The hard tick, Ixodes ricinus, a main Lyme disease vector, harbors an intracellular bacterial endosymbiont. Midichloria mitochondrii is maternally inherited and resides in the mitochondria of I. ricinus oocytes, but the consequences of this endosymbiosis are not well understood. Here, we provide 3D images of wild-type and aposymbiotic I. ricinus oocytes generated with focused ion beam-scanning electron microscopy. Quantitative image analyses of endosymbionts and oocyte mitochondria at different maturation stages show that the populations of both mitochondrion-associated bacteria and bacterium-hosting mitochondria increase upon vitellogenisation, and that mitochondria can host multiple bacteria in later stages. Three-dimensional reconstructions show symbiosis-dependent morphologies of mitochondria and demonstrate complete M. mitochondrii inclusion inside a mitochondrion. Cytoplasmic endosymbiont located close to mitochondria are not oriented towards the mitochondria, suggesting that bacterial recolonization is unlikely. We further demonstrate individual globular-shaped mitochondria in the wild type oocytes, while aposymbiotic oocytes only contain a mitochondrial network. In summary, our study suggests that M. mitochondrii modulates mitochondrial fragmentation in oogenesis possibly affecting organelle function and ensuring its presence over generations.

Ticks and Tick-Borne Pathogens Circulating in Peri-Domestic Areas in Mainland Portugal

“…Additionally, it was also detected in D. marginatus, in agreement with a previous study [68]. A recent report showed that this mitochondrial-residing bacterium affects oogenesis [69], but much remains to be known regarding the impact of its presence in ticks. Although this bacterium can be transmitted through tick bites [64], another study showed no clinical signs in people exclusively infected with M. mitochondrii; however, although it may not be harmful to humans, it could play a role in TBP transmission to mammalian hosts [62], since endosymbionts like M. mitochondrii often maintain a delicate microbial balance in ticks, hindering the presence of other microbes, blocking related pathogens or even enhancing their presence.…”

Meeting report ‘Microbiology 2023: from single cell to microbiome and host’, an international interacademy conference in Würzburg

“…Davide Sassera (University of Pavia, Italy) described advances in research on Midichloria , a genus of tick-borne mutualist bacteria, found to reside within the intermembrane space of different tick species (Stavru et al 2020 ). Advanced 2D and 3D electron microscopy suggested that Midichloria bacteria influence mitochondrial network fragmentation (Uzum et al 2023 ). Additionally, comparative genomics revealed a possible connection between the bacterium’s ability to colonize mitochondria and the presence of specific genetic traits, including type IV secretion system and flagellum (Floriano et al 2023 ).…”