Intramitochondrial bacteria in ticks and host-microbe interactions

Ticks are amongst the most important vectors of infectious diseases of medical and veterinary importance, being present from tropical to temperate areas and being capable of transmitting viruses, bacteria, protozoa and even nematodes. PCR using universal 16S rRNAprimers followed by cloning and sequencing led to detection of a novel alpha-proteobacterium in the hard tick Ixodes ricinus, which has recently been named Midichloria mitochondrii. Electron microscopy showed that M. mitochondrii resides in the mitochondria of infected tick cells. The discovery of a bacterium residing in mitochondria opens the question of whether Midichloria causes damage to the host. The damage, if any, is probably not very important. Indeed, the prevalence of M. mitochondrii in wild populations of I. ricinus is (or is very close to) 100% in females, which suggests that the fitness of infected females is certainly not seriously reduced. However, electron microscopy clearly shows that M. mitochondrii leads to the consumption of infected mitochondria. In addition, there is evidence that bacterial load and prevalence decrease along generations in lab colonies of I. ricinus. Finally, infection prevalence in adult male ticks collected in the wild is around 50%, with low bacterial load in infected male individuals. The kind of the interaction between M. mitochondrii and its host is probably not easy to classify. Low prevalence in males seems to suggest that the bacterium is not useful to individuals of this sex.