‘Candidatus Midichloria mitochondrii’, formerly IricES1, a symbiont of the tick Ixodes ricinus that resides in the host mitochondria

Abstract

Intracellular bacteria are widespread in nature and may adopt a wide array of life styles. They can be found free in the cytoplasm of their host cells, within host-derived vacuoles, or even in the nucleus. Here, we review current knowledge about ‘Candidatus Midichloria mitochondrii’, an intracellular bacterium that invades the host mitochondria in the ixodid tick Ixodes ricinus. This bacterium was first detected by electron microscopy in two independent studies, published in, 1979 and, 1992, that showed it in the cells of the ovary in adult ticks and also in the cells of the ovarian primordia in larvae and nymphs. This symbiont resides not only in the cytoplasm, but also inside the mitochondria of the ovarian cells, where it appears to penetrate the outer mitochondrial membrane and colonize the intermembrane space. Molecular studies have recently been performed, addressing the phylogenetic position, transmission, and prevalence of this novel bacterium. The functional significance of this symbiotic association has yet to be revealed. Even though ‘Candidatus M. mitochondrii’ seems to behave as a ‘predator’ towards the host mitochondria, this does not appear to interfere with egg development, thus ensuring the vertical transmission of the bacteria to the progeny. The 100% prevalence in the ovaries of females of I. ricinus may indicate a mutualistic association, whereas the peculiar intramitochondrial localization suggests that ‘Candidatus M. mitochondrii’ might be exploiting the energy available in the mitochondrial environment. The possibility that the bacterium is a reproductive parasite should also be considered.